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  • 1.
    Alm, Love
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Li, Bin
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Statistical altitude distribution of the auroral density cavity2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 2, p. 996-1006Article in journal (Refereed)
    Abstract [en]

    The statistical altitude distribution of auroral density cavities located between 3.0 and 6.5 R-E is investigated using in situ observations from flux tubes exhibiting auroral acceleration. The locations of the observations are described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellite. The upper edge of the auroral acceleration region is observed between 4.375 and 5.625 R-E. Above 6.125 R-E, none of the events exhibit precipitating inverted V electrons, though the upward ion beam can be observed. This indicates that the satellites are located inside the same flux tube as, but above, the auroral acceleration region. The electron density decreases as we move higher into the acceleration region. The spacecraft potential continues to decrease once above the acceleration region, indicating that the density cavity extends above the acceleration region. From 3.0 to 4.375 R-E the pseudo altitude increases by 0.20 per R-E, consistent with a distributed parallel electric field. Between 4.375 and 5.625 R-E the pseudo altitude increases weakly, by 0.01 per R-E, due to an increasing number of events per altitude bin, which are occurring above the acceleration region. Above 5.625 R-E the pseudo altitude increases by 0.28 per R-E, due to a rapid increase in the number of events per altitude bin occurring above the acceleration region, indicating that the remaining parallel potential drop is concentrated in a narrow region at the upper edge of the acceleration region, rather than in a distributed parallel electric field.

  • 2.
    Alm, Love
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Electron density and parallel electric field distribution of the auroral density cavity2015In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 120, no 11, p. 9428-9441Article in journal (Refereed)
    Abstract [en]

    We present an event study in which Cluster satellites C1 and C3 encounters the flux tube of a stable auroral arc in the pre-midnight sector. C1 observes the mid cavity, while C3 enters the flux tube of the auroral arc at an altitude which is below the acceleration region, before crossing into the top half of the acceleration region. This allows us to study the boundary between the ionosphere and the density cavity, as well as large portion of the upper density cavity. The position of the two satellites, in relation to the acceleration region, is described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellites.The electron density exhibits an anti-correlation with the pseudo altitude, indicating that the lowest electron densities are found near the top of the density cavity. Over the entire pseudo altitude range, the electron density distribution is similar to a planar sheath, formed out of a plasma sheet dominated electron distribution, in response to the parallel electric field of the acceleration region. This indicates that the parallel electric fields on the ionosphere-cavity boundary, as well as the mid cavity parallel electric fields, are part of one unified structure rather than two discrete entities.The results highlight the strong connection between the auroral density cavity and auroral acceleration as well as the necessity of studying them in a unified fashion.

  • 3.
    Alm, Love
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran T.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    In situ observations of density cavities extending above the auroral acceleration region2014In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 7, p. 5286-5294Article in journal (Refereed)
    Abstract [en]

    The uppermost part of a stable potential structure in the auroral acceleration region was studied using simultaneous observations of Cluster satellites C1 and C3. Both satellites observe a monotonically decreasing electron density as they ascend through the auroral acceleration region. As C1 exits the top of the auroral acceleration region, the electron densities continue to decrease, and the minimum electron density is reached 14 km above the upper edge of the auroral acceleration region. The electron density does not return to noncavity values until the spacecraft exits the potential structure's flux tube. The data indicate that the auroral density cavity is not confined by the potential structure and may extend above the auroral acceleration region.

  • 4.
    Alm, Love
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran T.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Masson, A.
    Pseudo altitude: A new perspective on the auroral density cavity2013In: Journal of Geophysical Research A: Space Physics, ISSN 2169-9380, Vol. 118, no 7, p. 4341-4351Article in journal (Refereed)
    Abstract [en]

    Studying the density distribution inside the auroral density cavity is complicated by the difficulties in achieving simultaneous measurements within the same flux tube at different altitudes. Comparisons between different events are complicated by variations in both the location of the density cavity and the location of the related potential structure. Describing the spacecraft's location inside the density cavity relative to the potential structure instead of the Earth offers a more practical and consistent frame of reference, a pseudo altitude. The pseudo altitude is determined by comparing the potential drop above the spacecraft, as determined from the characteristic energy of the downward electrons, with the parallel potential drop below the spacecraft, determined from the characteristic energy of the upward ions. A pseudo altitude of 0 corresponds to the bottom of the potential structure and a pseudo altitude of 1 to the top of the structure. Seven events from 2008 were selected, each of which corresponds to a Cluster crossing of a mainly quasi-static potential structure. All of the events exhibit a consistent anticorrelation between the pseudo altitude and the electron density. No upper limit of the density cavity can be observed, while all cavities have a lower limit above a pseudo altitude of 0.33. These observations show that the auroral density cavity is predominately concentrated to the upper parts of the quasi-static potential structure.

  • 5.
    Blomberg, Lars G.
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Eriksson, Stefan
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Cumnock, Judy A.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Yamauchi, M.
    Clemmons, J. H.
    Marklund, Göran T.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, R.
    Solar windmagnetosphere-ionosphere coupling: an event study based on Freja data2004In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 66, no 5, p. 375-380Article in journal (Refereed)
    Abstract [en]

    Freja data are used to study the relative contributions from the high-latitude (reconnection/direct entry) and low-latitude (viscous interaction) dynamos to the cross-polar potential drop. Convection streamlines which are connected to the high-latitude dynamo may be identified from dispersed magnetosheath ions not only in the cusp/cleft region itself but also several degrees poleward of it. This fact, together with Freja's orbital geometry allows us to infer the potential drop from the high-latitude dynamo as well as to obtain a lower limit to the potential drop from the low-latitude dynamo for dayside Freja passes. All cases studied here are for active magnetospheric conditions. The Freja data suggest that under these conditions at least one third of the potential is generated in the low-latitude dynamo. These observations are consistent with earlier observations of the potential across the low-latitude boundary layer if we assume that the low-latitude dynamo region extends over several tens of Earth radii in the antisunward direction along the tail flanks, and that the majority of the potential drop derives from the sun-aligned component of the electric field rather than from its cross-boundary component, or equivalently, that the centre of the dynamo region is located quite far down tail. A possible dynamo geometry is illustrated.

  • 6.
    Blomberg, Lars G.
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran T.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Primdahl, F.
    Brauer, P.
    Bylander, Lars
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, Superseded Departments, Alfvén Laboratory.
    Cumnock, Judy
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, Superseded Departments, Alfvén Laboratory.
    Eriksson, Stefan
    KTH, Superseded Departments, Alfvén Laboratory.
    Ivchenko, Nickolay V.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, Superseded Departments, Alfvén Laboratory.
    Merayo, J. M. G.
    Pedersen, E. B.
    Petersen, J. R.
    EMMA - the electric and magnetic monitor of the aurora on Astrid-22004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 1, p. 115-123Article in journal (Refereed)
    Abstract [en]

    The Astrid-2 mission has dual primary objectives. First, it is an orbiting instrument platform for studying auroral electrodynamics. Second, it is a technology demonstration of the feasibility of using micro-satellites for innovative space plasma physics research. The EMMA instrument, which we discuss in the present paper, is designed to provide simultaneous sampling of two electric and three magnetic field components up to about 1 kHz. The spin plane components of the electric field are measured by two pairs of opposing probes extended by wire booms with a separation distance of 6.7 m. The probes have titanium nitride (TiN) surfaces. which has proved to be a material with excellent properties for providing good electrical contact between probe and plasma. The wire booms are of a new design in which the booms in the stowed position are wound around the exterior of the spacecraft body. The boom system was flown for the first time on this mission and worked flawlessly. The magnetic field is measured by a tri-axial fluxgate sensor located at the tip of a rigid. hinged boom extended along the spacecraft spin axis and facing away from the Sun. The new advanced-design fluxgate magnetometer uses digital signal processors for detection and feedback, thereby reducing the analogue circuitry to a minimum. The instrument characteristics as well as a brief review of the science accomplished and planned are presented.

  • 7.
    Blomberg, Lars
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory.
    Lindqvist, Per-Arne
    KTH, Superseded Departments, Alfvén Laboratory.
    Primdahl, F.
    Brauer, P.
    Bylander, Lars
    KTH, Superseded Departments, Alfvén Laboratory.
    Cumnock, Judy
    KTH, Superseded Departments, Alfvén Laboratory.
    Eriksson, S.
    Ivchenko, Nickolay
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Kullen, Anita
    KTH, Superseded Departments, Alfvén Laboratory.
    Merayo, J. M. G.
    Pedersen, E. B.
    Petersen, J. R.
    The EMMA Instrument on the Astrid-2 Micro-Satellite2003Report (Other academic)
  • 8.
    Cumnock, Judy A.
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Blomberg, Lars G.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sundberg, K. Å. Torbjörn
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Small-scale characteristics of extremely high latitude aurora2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 9, p. 3335-3347Article in journal (Refereed)
    Abstract [en]

    We examine 14 cases of an interesting type of extremely high latitude aurora as identified in the precipitating particles measured by the DMSP F13 satellite. In particular we investigate structures within large-scale arcs for which the particle signatures are made up of a group of multiple distinct thin arcs. These cases are chosen without regard to IMF orientation and are part of a group of 87 events where DMSP F13 SSJ/4 measures emissions which occur near the noon-midnight meridian and are spatially separated from both the dawnside and duskside auroral ovals by wide regions with precipitating particles typical of the polar cap. For 73 of these events the high-latitude aurora consists of a continuous region of precipitating particles. We focus on the remaining 14 of these events where the particle signatures show multiple distinct thin arcs. These events occur during northward or weakly southward IMF conditions and follow a change in IMF B-y. Correlations are seen between the field-aligned currents and plasma flows associated with the arcs, implying local closure of the FACs. Strong correlations are seen only in the sunlit hemisphere. The convection associated with the multiple thin arcs is localized and has little influence on the large-scale convection. This also implies that the sunward flow along the arcs is unrelated to the overall ionospheric convection.

  • 9. Eriksson, A. I.
    et al.
    Engelhardt, I. A. D.
    Andre, M.
    Bostrom, R.
    Edberg, N. J. T.
    Johansson, F. L.
    Odelstad, E.
    Vigren, E.
    Wahlund, J. -E
    Henri, P.
    Lebreton, J. -P
    Miloch, W. J.
    Paulsson, J. J. P.
    Wedlund, C. Simon
    Yang, L.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Jarvinen, R.
    Broiles, T.
    Mandt, K.
    Carr, C. M.
    Galand, M.
    Nilsson, H.
    Norberg, C.
    Cold and warm electrons at comet 67P/Churyumov-Gerasimenko2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A15Article in journal (Refereed)
    Abstract [en]

    Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P/Churyumov-Gerasimenko and show filamentation of this plasma. Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas. Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5-10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order -10 V. Conclusions. The warm (5-10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

  • 10.
    Eriksson, Stefan
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Blomberg, Lars G.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ivchenko, Nickolay V.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran T.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Magnetospheric response to the solar wind as indicated by the cross-polar potential drop and the low-latitude asymmetric disturbance field2001In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, no 6, p. 649-653Article in journal (Refereed)
    Abstract [en]

    The cross-polar potential drop Phi (pc), and the low-latitude asymmetric geomagnetic disturbance field, as indicated by the mid-latitude ASY-H magnetic index, are used to study the average magnetospheric response to the solar wind forcing for southward interplanetary magnetic field conditions. The state of the solar wind is monitored by the ACE spacecraft and the ionospheric convection is measured by the double probe electric field instrument on the Astrid-2 satellite. The solar wind-magnetosphere coupling is examined for 77 cases in February and from mid-May to mid-June 1999 by using the interplanetary magnetic field B-z component and the reconnection electric field. Our results show that the maximum correlation between Phi (pc) and the reconnection electric field is obtained approximately 25 min after the solar wind has reached a distance of II R-E from the Earth, which is the assumed average position of the magnetopause. The corresponding correlation for ASY-H shows two separate responses to the reconnection electric field, delayed by about 35 and 65 min, respectively. We suggest that the combination of the occurrence of a large magnetic storm on 18 February 1999 and the enhanced level of geomagnetic activity which peaks at Kp = 7(-) may explain the fast direct response of ASY-H to the solar wind at 35 min, as well as the lack of any clear secondary responses of Phi (pc) to the driving solar wind at time delays longer than 25 min.

  • 11.
    Figueiredo, Sonia
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory.
    Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 3, p. 923-934Article in journal (Refereed)
    Abstract [en]

    Based on Astrid-2 satellite data, results are presented from a statistical study on subauroral ion drift (SAID) occurrence. SAID is a subauroral phenomenon characterized by a westward ionospheric ion drift with velocity greater than 1000 m/s, or equivalently, by a poleward-directed electric field with intensity greater than 30 mV/m. SAID events occur predominantly in the premidnight sector, with a maximum probability located within the 20:00 to 23:00 MLT sector, where the most rapid SAID events are also found. They are substorm related, and show first an increase in intensity and a decrease in latitudinal width during the expansion phase, followed by a weakening and widening of the SAID structures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID events were calculated. The results reveal that the strongest SAID electric field peaks are associated with the lowest Pedersen conductivity minimum values. Clear modifications are seen in the ionospheric Pedersen conductivity distribution associated with the SAID structure as time evolves: the SAID peak is located on the poleward side of the corresponding region of reduced Pedersen conductivity; the shape of the regions of reduced conductivity is asymmetric, with a steeper poleward edge and a more rounded equatorward edge; the SAID structure becomes less intense and widens with evolution of the substorm recovery phase. From the analysis of the SAID occurrence relative to the mid-latitude trough position, SAID peaks are seen to occur relatively close to the corresponding mid-latitude trough minimum. Both these features show a similar response to magnetospheric disturbances, but on different time scales - with increasing magnetic activity, the SAID structure shows a faster movement towards lower latitudes than that of the mid-latitude trough. From the combined analysis of these results, we conclude that the SAID generation mechanism cannot be regarded either as a pure voltage generator or as a pure current generator, applied to the ionosphere. While the anti-correlation between the width and the peak intensity of the SAID structures with substorm evolution indicates a magnetospheric source acting as a constant voltage generator, the ionospheric modifications and, in particular the reduction in the conductivity for intense SAID structures, are indicative of a constant current system closing through the ionosphere. The ionospheric feedback mechanisms are seen to be of major importance for sustaining and regulating the SAID structures.

  • 12.
    Figueiredo, Sonia
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Johansson, Tommy
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Ebihara, Y
    Ejiri, M
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H
    Fazakerley, A
    Temporal and spatial evolution of discrete auroral arcs as seen by Cluster2005In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 7, p. 2531-2557Article in journal (Refereed)
    Abstract [en]

    Two event studies are presented in this paper where intense convergent electric fields, with mapped intensities up to 1350 mV/m, are measured in the auroral upward current region by the Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both events are from May 2003, Southern Hemisphere, with equatorward crossings by the Cluster spacecraft of the pre-midnight auroral oval. Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about similar to 10 kV, and upflowing field-aligned currents with densities of 3 mu A/m(2) (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude. Event 2 occurs during a more active period associated with the expansion phase of a moderate substorm. Images from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft show that the Cluster spacecraft crossed the horn region of a surge-type aurora. Conjugated with the Cluster spacecraft crossing above the surge horn, the South Pole All Sky Imager recorded the motion and the temporal evolution of an east-west aligned auroral arc, 30 to 50 km wide. Intense electric field variations are measured by the Cluster spacecraft when crossing above the auroral arc structure, collocated with the density gradient at the PS poleward boundary, and coupled to intense upflowing field-aligned currents with mapped densities of up to 20 mu A/m(2). The surge horn consists of multiple arc structures which later merge into one structure and intensify at the PS poleward boundary. The surge horn and the associated PS region moved poleward with a velocity at the ionospheric level of 0.5 km/s, following the large-scale poleward expansion of the auroral oval associated with the substorm expansion phase.

  • 13. Forsyth, C.
    et al.
    Fazakerley, A. N.
    Rae, I. J.
    Watt, C. E. J.
    Murphy, K.
    Wild, J. A.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Mutel, R.
    Owen, C. J.
    Ergun, R.
    Masson, A.
    Berthomier, M.
    Donovan, E.
    Frey, H. U.
    Matzka, J.
    Stolle, C.
    Zhang, Y.
    In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge2014In: Journal of Geophysical Research: Space Physics, ISSN 2169-9380, Vol. 119, no 2, p. 927-946Article in journal (Refereed)
    Abstract [en]

    The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100km at altitudes of 4000-7000km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120-240s after Cluster 4 at 1300-2000km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven wedgelets. Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW.

  • 14. Frey, H. U.
    et al.
    Amm, O.
    Chaston, C. C.
    Fu, S.
    Haerendel, G.
    Juusola, L.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lanchester, B.
    Nakamura, R.
    Ostgaard, N.
    Sakanoi, T.
    Seran, E.
    Whiter, D.
    Weygand, J.
    Asamura, K.
    Hirahara, M.
    Small and meso-scale properties of a substorm onset auroral arc2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, p. A10209-Article in journal (Refereed)
    Abstract [en]

    We present small and meso-scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground-based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth-phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted-V structure together with a strong Alfven wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field-aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso-scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope-like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfven resonator that can drive structuring in luminosity over the range of scales observed.

  • 15. Gunell, H.
    et al.
    Nilsson, H.
    Stenberg, G.
    Hamrin, M.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Maggiolo, R.
    Andre, M.
    Lundin, R.
    Dandouras, I.
    Plasma penetration of the dayside magnetopause2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 7, p. 072906-Article in journal (Refereed)
    Abstract [en]

    Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%-0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux.

  • 16. Gunell, H.
    et al.
    Wieser, G. Stenberg
    Mella, M.
    Maggiolo, R.
    Nilsson, H.
    Darrouzet, F.
    Hamrin, M.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Keyser, J.
    Andre, M.
    Dandouras, I.
    Waves in high-speed plasmoids in the magnetosheath and at the magnetopause2014In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 32, no 8, p. 991-1009Article in journal (Refereed)
    Abstract [en]

    Plasmoids, defined here as plasma entities with a higher anti-sunward velocity component than the surrounding plasma, have been observed in the magnetosheath in recent years. During the month of March 2007 the Cluster spacecraft crossed the magnetopause near the subsolar point 13 times. Plasmoids with larger velocities than the surrounding magnetosheath were found on seven of these 13 occasions. The plasmoids approach the magnetopause and interact with it. Both whistler mode waves and waves in the lower hybrid frequency range appear in these plasmoids, and the energy density of the waves inside the plasmoids is higher than the average wave energy density in the magnetosheath. When the spacecraft are in the magnetosphere, Alfvenic waves are observed. Cold ions of ionospheric origin are seen in connection with these waves, when the wave electric and magnetic fields combine with the Earth's dc magnetic field to yield an E x B/B-2 drift speed that is large enough to give the ions energies above the detection threshold.

  • 17. Gustafsson, Georg
    et al.
    Andre, M.
    Carozzi, T.
    Eriksson, A. I.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Grard, R.
    Holmgren, G.
    Holtet, J. A.
    Ivchenko, Nickolay V.
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Khotyaintsev, Y.
    Klimov, S.
    Laakso, H.
    Lindqvist, Per-Arne
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Lybekk, B.
    Marklund, Göran T.
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Mozer, F.
    Mursula, K.
    Pedersen, A.
    Popielawska, B.
    Savin, S.
    Stasiewicz, K.
    Tanskanen, P.
    Vaivads, Andris
    Wahlund, J. E.
    First results of electric field and density observations by Cluster EFW based on initial months of operation2001In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, no 12-okt, p. 1219-1240Article in journal (Refereed)
    Abstract [en]

    Highlights are presented from studies of the electric field data from various regions along the CLUSTER orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from I mHz to 9 kHz. This presents completely new opportunities to study spatial-temporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.

  • 18. Hamrin, M.
    et al.
    Andre, M.
    Ganguli, G.
    Gavrishchaka, V. V.
    Koepke, M. E.
    Zintl, M. W.
    Ivchenko, Nickolay V.
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Clemmons, J. H.
    Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study2001In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 106, no A6, p. 10803-10816Article in journal (Refereed)
    Abstract [en]

    We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current-driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.

  • 19. Hamrin, M.
    et al.
    Norqvist, P.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    Fu, H. S.
    Buchert, S.
    André, M.
    Marghitu, O.
    Pitkänen, T.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    The evolution of flux pileup regions in the plasma sheet: Cluster observations2013In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, no 10, p. 6279-6290Article in journal (Refereed)
    Abstract [en]

    Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, EJ<0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, EJ>0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R(E). Our observations are compared to recent simulation results and previous observations.

  • 20. Hamrin, M.
    et al.
    Pitkänen, T.
    Norqvist, P.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    André, M.
    Buchert, S.
    Vaivads, Andris
    Marghitu, O.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    Evidence for the braking of flow bursts as they propagate toward the Earth2014In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 11, p. 9004-9018Article in journal (Refereed)
    Abstract [en]

    In this article we use energy conversion arguments to investigate the possible braking of flow bursts as they propagate toward the Earth. By using EJ data (E and J are the electric field and the current density) observed by Cluster in the magnetotail plasma sheet, we find indications of a plasma deceleration in the region -20 R-E < X < - 15 R-E. Our results suggest a braking mechanism where compressed magnetic flux tubes in so-called dipolarization fronts (DFs) can decelerate incoming flow bursts. Our results also show that energy conversion arguments can be used for studying flow braking and that the position of the flow velocity peak with respect to the DF can be used as a single-spacecraft proxy when determining energy conversion properties. Such a single-spacecraft proxy is invaluable whenever multispacecraft data are not available. In a superposed epoch study, we find that a flow burst with the velocity peak behind the DF is likely to decelerate and transfer energy from the particles to the fields. For flow bursts with the peak flow at or ahead of the DF we see no indications of braking, but instead we find an energy transfer from the fields to the particles. From our results we obtain an estimate of the magnitude of the deceleration of the flow bursts, and we find that it is consistent with previous investigations.

  • 21.
    Johansson, Tommy
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Figueiredo, Sonia
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory.
    Fazakerley, Andrew
    Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking.
    Buchert, Stephan
    Swedish Institute of Space Physics, Uppsala.
    Lindqvist, Per-Arne
    KTH, Superseded Departments, Alfvén Laboratory.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Intense high-altitude auroral electric fields: temporal and spatial characteristics2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 7, p. 2485-2495Article in journal (Refereed)
    Abstract [en]

    Cluster electric field, magnetic field, and energetic electron data are analyzed for two events of intense auroral electric field variations, both encountered in the Plasma Sheet Boundary Layer (PSBL), in the evening local time sector, and at approximately 5 R-E geocentric distance. The most intense electric fields (peaking at 450 and 1600 mV/m, respectively) were found to be quasi-static, unipolar, relatively stable on the time scale of at least half a minute, and associated with moving downward FAC sheets (peaking at similar to10 muA/m(2)), downward Poynting flux (peaking at similar to35 mW/m(2)), and upward electron beams with characteristic energies consistent with the perpendicular potentials (all values being mapped to 1 R-E geocentric distance). For these two events in the return current region, quasi-static electric field structures and associated FACs were found to dominate the upward acceleration of electrons, as well as the energy transport between the ionosphere and the magnetosphere, although Alfven waves clearly also contributed to these processes.

  • 22.
    Johansson, Tommy
    et al.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Figueiredo, Sonia
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Buchert, Stephan
    Swedish Institute of Space Physics, Uppsala.
    A statistical study of intense electric fields at 4-7 R-E geocentric distance using Cluster2005In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 7, p. 2579-2588Article in journal (Refereed)
    Abstract [en]

    Intense high-latitude electric fields (> 150 mV/m mapped to ionospheric altitude) at 4-7 R-E geocentric distance have been investigated in a statistical study, using data from the Cluster satellites. The orbit of the Cluster satellites limits the data collection at these altitudes to high latitudes, including the poleward part of the auroral oval. The occurrence and distribution of the selected events have been used to characterize the intense electric fields and to investigate their dependance on parameters such as MLT, CGLat, altitude, and also K-p. Peaks in the local time distribution are found in the evening to morning sectors but also in the noon sector, corresponding to cusp events. The electric field intensities decrease with increasing latitude in the region investigated (above 60 CGLat). A dependence on geomagnetic activity is indicated since the probability of finding an event increases up to K-p=5-6. The scales sizes are in the range up to 10 km (mapped to ionospheric altitude) with a maximum around 4-5 km, consistent with earlier findings at lower altitudes and Cluster event studies. The magnitudes of the electric fields are inversely proportional to the scale sizes. The type of electric field structure (convergent or divergent) is consistent with the FAC direction for a subset of events with electric field intensities in the range 500-1000 mV/m and with clear bipolar signatures. The FAC directions are also consistent with the Region 1 and NBZ current systems, the latter of which prevail only during northward IMF conditions. For scale sizes less than 2 km the majority of the events were divergent electric field structures. Both converging and diverging electric fields were found throughout the investigated altitude range (4-7 RE geocentric distance).

  • 23.
    Johansson, Tommy
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liléo, Sonia
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marchaudon, Aurelie
    LPCE, CNRS, Orleans Cedex.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Fazakerley, Andrew
    Mullard Space Science Laboratory, University College, London.
    On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 6, p. 1713-1723Article in journal (Refereed)
    Abstract [en]

    The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5-1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.

  • 24.
    Johansson, Tommy
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liléo, Sonia
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna.
    Buchert, Stephan
    Swedish Institute of Space Physics, Uppsala.
    Scale sizes of intense auroral electric fields observed by Cluster2007In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 25, no 11, p. 2413-2425Article in journal (Refereed)
    Abstract [en]

    The scale sizes of intense (>0.15 V/m, mapped to the ionosphere), high-altitude (4-7 R-E geocentric distance) auroral electric fields (measured by the Cluster EFW instrument) have been determined in a statistical study. Monopolar and bipolar electric fields, and converging and diverging events, are separated. The relations between the scale size, the intensity and the potential variation are investigated.

    The electric field scale sizes are further compared with the scale sizes and widths of the associated field-aligned currents (FACs). The influence of, or relation between, other parameters (proton gyroradius, plasma density gradients, and geomagnetic activity), and the electric field scale sizes are considered.

    The median scale sizes of these auroral electric field structures are found to be similar to the median scale sizes of the associated FACs and the density gradients (all in the range 4.2-.9 km) but not to the median proton gyroradius or the proton inertial scale length at these times and locations (22-30km). (The scales are mapped to the ionospheric altitude for reference.)

    The electric field scale sizes during summer months and high geomagnetic activity (K-p>3) are typically 2-3 km, smaller than the typical 4-5 km scale sizes during winter months and low geomagnetic activity (K-p <= 3), indicating a dependence on ionospheric conductivity.

  • 25.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    Auroral Electric Fields From Satellite Observations and Numerical Modelling2001Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is about electrodynamics of thehigh-latitude/auroral region of near-Earth space. The work ismainly based on electric field measurements made by the doubleprobe instrument on the Freja satellite at altitudes ofapproximately 800-1700 km, together with measurements fromother instruments on the same satellite, and ground-basedinstrumentation. A useful tool for interpreting observationaldata is also numerical modelling, and this is the subject ofpart of the work. The electric field measurements address threesubjects. The first one is that of very intense, divergingelectric fields at Freja altitude. Statistics and case studiesshow that these structures have scale-sizes of the order of 1km, and are associated with regions devoid of electronprecipitation (which at times can be identified with theoptical phenomenon of black aurora), with downward currents,ion heating, density depletions, and upward acceleratedelectron beams. A numerical model is used to study theionospheric response to intense small-scale current systems. Itis shown that on time scales of the order of 1 minute, deepdensity cavities and enhancements of the electric fieldresults. The second subject is the electrodynamics of thewestward traveling surge (WTS) and other large-scale auroralspirals. Freja measurements show that the electric field isdirected towards the surge/spiral center, and that the surgehead is associated with extremely intense, small-scale,converging electric fields, and field-aligned currents. Datasuggest that a significant part of the upward current of thesurge head is closed by localized, downward-directed currents,whereas no clear signature of a Cowling channel feeding thecurrent to the surge head from distributed downward currents inthe wake of the surge is observed. The third type ofmeasurements concerns the phenomenon of subauroral ion drifts(SAID), or equivalently subauroral electric fields (SAEF). Acomprehensive statistics shows the distribution in local timeand latitude of the SAID, and their correlation withgeomagnetic activity. It is concluded that the SAID areassociated with the low-conductivity region of the mid-latitudetrough, and that they may be a consequence of closure ofsubstorm-related currents through this region, which may becomeeven more low-conducting by the ionospheric response to theapplied current and electric field. Finally a new formulationof a classification scheme of auroral arc-associated electricfields is given. It is shown that this generalization enables aflexible way of modelling and predicting the arc-associatedfield in various situations.

    Keywords: Satellite measurements, electric fields,ionosphere, magnetosphere, diverging electric fields, downwardcurrent region, black aurora, westward traveling surge, auroralbulge, subauroral ion drifts (SAID), subauroral electric fields(SAEF), mid-latitude trough, numerical modelling, auroral arcclassification, auroral arc-associated electric field.

  • 26.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    On electric field patterns associated with night-side discrete auroral arcs: A generalization of an auroral arc classification scheme2001Report (Other academic)
  • 27.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    The Acceleration Region of Stable Auroral Arcs2012In: Auroral Phenomenology And Magnetospheric Processes: Earth And Other Planets, American Geophysical Union (AGU), 2012, p. 227-239Conference paper (Refereed)
    Abstract [en]

    The acceleration region above stable, discrete auroral arcs is reviewed. Substantial observational evidence shows that the acceleration of auroral electrons associated with these arcs is achieved by an electric potential structure above the aurora at altitudes of around 0.5-2 R-E. The morphology, internal structure, and lifetime of the predominantly U-shaped potential structure are discussed, based on observations by a number of spacecrafts. The parallel (to the geomagnetic field) electric field component of the potential structure, which accelerates the auroral electrons, is discussed in terms of relatively recent direct observations. The most important theories for how the parallel electric field is sustained are also described. The altitude distribution predicted from the various theories is compared to observations, and briefly discussed, as well as their relation to each other.

  • 28.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    Trotignon, J-G
    Valliéres, X.
    Facsko, G.
    Localized density enhancements in the magnetosheath: Three-dimensional morphology and possible importance for impulsive penetration2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, p. A03227-Article in journal (Refereed)
    Abstract [en]

    We use Cluster multipoint density measurements, using the spacecraft potential, to identify localized density enhancements (>50%) in the magnetosheath, and estimate their three-dimensional morphology and orientation. Typically one dimension of the density enhancements is shorter than others, is directed perpendicular to the background magnetic field, and varies from similar to 0.1 R-E to 10 R-E, with the other two dimensions a factor 3-10 greater. The density structures are oriented with the longest sides in the general direction of the bow shock and magnetopause. Examples of density structures both convecting with the same velocity as the background magnetosheath flow ("embedded plasmoids"), and convecting with an excess x(GSE) velocity component ("fast plasmoids") are found. Possible importance for the impulsive penetration mechanism for plasma entry in the magnetosphere is analyzed by comparing the results to laboratory results, via a parameter scaling. The estimation of the three-dimensional topology of the density enhancements will enable a comparison with localized magnetosheath populations inside the magnetosphere, observed earlier, to determine if these originate from penetrated magnetosheath density enhancements.

  • 29.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Eriksson, A. I.
    Odelstad, E.
    Andre, M.
    Dickeli, G.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    Richter, I.
    Rosetta measurements of lower hybrid frequency range electric field oscillations in the plasma environment of comet 67P2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 4, p. 1641-1651Article in journal (Refereed)
    Abstract [en]

    Electric field measurements from cometary environments are very rare but can provide important information on how plasma waves help fashion the plasma environment. The long dwelling time of the Rosetta spacecraft close to comet 67P/Churyumov-Gerasimenko promises to improve this state. We here present the first electric field measurements from 67P, performed by the Rosetta dual Langmuir probe instrument LAP. Measurements of the electric field from cometocentric distances of 149 and 348 km are presented together with estimates of plasma density changes. Persistent wave activity around the local H2O+ lower hybrid frequency is observed, with the largest amplitudes observed at sharp plasma gradients. We demonstrate that the necessary requirements for the lower hybrid drift instability to be operating are fulfilled. We suggest that lower hybrid waves are responsible for the creation of a warm electron population, the origins of which have been unknown so far, by heating ambient electrons in the magnetic field-parallel direction.

  • 30.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Hamrin, M.
    Nilsson, H.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Pitkänen, T.
    Magnetic forces associated with bursty bulk flows in Earth's magnetotail2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 9, p. 3122-3128Article in journal (Refereed)
    Abstract [en]

    We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 RE geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 RE, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near-Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere. Key Points Direct measurements of magnetic force acting on BBF plasma BBF plasma is accelerated toward Earth until it reaches 15 RE, then braked Knowledge of the forces on BBFs provides clue to their fate close to Earth.

  • 31.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    Gunell, H.
    Hamrin, M.
    On the origin of magnetosheath plasmoids and their relation to magnetosheath jets2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 9, p. 7390-7403Article in journal (Refereed)
    Abstract [en]

    We investigate localized magnetosheath and solar wind density enhancements, associated with clear magnetic field changes, and therefore referred to as magnetosheath/solar wind plasmoids, respectively. Using Cluster data, we show that there are two distinct populations of magnetosheath plasmoids, one associated with a decrease of magnetic field strength (diamagnetic plasmoids), and one with an increased magnetic field strength (paramagnetic plasmoids). The diamagnetic magnetosheath plasmoids have scale sizes of the order of 1-10 R-E, while the paramagnetic ones are an order of magnitude smaller. The diamagnetic plasmoids are not associated with any change in the magnetosheath plasma flow velocity, and they are classified as embedded plasmoids in the terminology of Karlsson et al. (2012). The paramagnetic plasmoids may either be embedded or associated with increases in flow velocity (fast plasmoids). A search for plasmoids in the pristine solar wind resulted in identification of 62 diamagnetic plasmoids with very similar properties to the magnetosheath diamagnetic plasmoids, making it probable that the solar wind is the source of these structures. No paramagnetic plasmoids are found in the pristine solar wind, indicating that these are instead created at the bow shock or in the magnetosheath. We discuss the relation of the plasmoids to the phenomenon of magnetosheath jets, with which they have many properties in common, and suggest that the paramagnetic plasmoids can be regarded as a subset of these or a closely related phenomenon. We also discuss how the results from this study relate to theories addressing the formation of magnetosheath jets.

  • 32.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Dawn-dusk asymmetries in auroral morphology and processes2017In: Dawn-Dusk Asymmetries in Planetary Plasma Environments, Wiley Blackwell , 2017, p. 295-305Chapter in book (Other academic)
    Abstract [en]

    We address the dawn-dusk asymmetries in auroral emissions in the main auroral oval, and discuss their origins in terms of the underlying asymmetries of the precipitating particles. These, in turn, are associated with asymmetries in the mechanisms responsible for the transport and acceleration of the precipitating particles. We briefly discuss the reasons for the asymmetries of these processes, which include dawn-dusk asymmetries in particle drifts and in the ionospheric conductivity, the direction of the interplanetary magnetic field, and substorm-related asymmetries in field-aligned currents and flows. Finally, we briefly discuss dawn-dusk asymmetries associated with auroral emissions in the polar cap. 

  • 33.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Raines, Jim M.
    Slavin, James A.
    Sundberg, Torbjorn
    Isolated magnetic field structures in Mercury's magnetosheath as possible analogues for terrestrial magnetosheath plasmoids and jets2016In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 129, p. 61-73Article in journal (Refereed)
    Abstract [en]

    We have investigated MESSENGER magnetic field data from the Mercury magnetosheath and near solar wind, to identify isolated magnetic field structures (defined as clear, isolated changes in the field magnitude). Their properties are studied in order to determine if they may be considered as analogues to plasmoids and jets known to exist in Earth's magnetosheath. Both isolated decreases of the magnetic field absolute value ('negative magnetic field structures') and increases ('positive structures') are found in the magnetosheath, whereas only negative structures are found in the solar wind. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggests that they are analogous to diamagnetic plasmoids found in Earth's magnetosheath and near solar wind. The latter have earlier been identified with solar wind magnetic holes. Positive magnetic field structures are only found in the magnetosheath, concentrated to a region relatively close to the magnetopause. Their proximity to the magnetopause, their scale sizes, and the association of a majority of the structures with bipolar magnetic field signatures identify them as flux transfer events (which generally are associated with a decrease of plasma density in the magnetosheath). The positive magnetic field structures are therefore not likely to be analogous to terrestrial paramagnetic plasmoids but possibly to a sub-population of magnetosheath jets. At Earth, a majority of magnetosheath jets are associated with the quasi-parallel bow shock. We discuss some consequences of the findings of the present investigation pertaining to the different nature of the quasi-parallel bow shock at Mercury and Earth.

  • 34.
    Karlsson, Tomas
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory.
    A statistical study of intense low-altitude electric fields observed by Freja1996In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 23, p. 1005-1008Article in journal (Refereed)
    Abstract [en]

    Extremely intense (up to 2 V/m) and fine-structured (1–5 km) electric fields have been observed by the Freja satellite at altitudes of 1400–1770 km. To study the occurrence and characteristics of these intense electric field events, a database was set up by searching 7 months' worth of Freja data for events with peak values greater than 200 mV/m. The intense electric field events are distributed over all local times, but they are mostly concentrated to the midnight and early morning sector of the auroral oval. The events are seen to be associated with low ambient ionospheric conductivities, but are not activity dependent. The most intense events (located in the midnight sector) are associated with the smallest scale sizes, whereas the less intense events on the day side have larger scale sizes.

    The findings are consistent with the interpretation that a majority of the intense electric field events observed by Freja are associated with the small-scale, east-west-aligned, low-conductivity bands devoid of auroral emissions, known as black auroral bands, or with the curls that may develop when these bands go unstable. Several observations of diverging electric fields of around 1 V/m at 800 km altitude, within the southern auroral oval, may serve as support for the prediction that intense electric fields should exist also at ionospheric altitudes.

  • 35.
    Karlsson, Tomas
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Results from the DC Electric Field Experiment on the Freja Satellite1999In: Advances in Space Research, Vol. 23, p. 1657-1665Article in journal (Refereed)
  • 36.
    Karlsson, Tomas
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory.
    Simulations of effects of small-scale auroral current closure in the return current region1998In: Physics of space plasmas, Vol. 15, p. 401-Article in journal (Other academic)
  • 37.
    Karlsson, Tomas
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Blomberg, Lars
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Malkki, A
    Subauroral electric fields observed by the Freja satellite: A statistical study1998In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, ISSN 0148-0233, Vol. 103, no A3, p. 4327-4341Article in journal (Refereed)
    Abstract [en]

    Over 12 months of Freja electric field data have been scanned for subauroral electric fields (SAEF) to enable a comprehensive study of the ionospheric signatures of such electric fields. SAEF are encountered from 1800 to 0200 MLT in agreement with an earlier study. However, a large majority of the SAEF are encountered at a time slightly premidnight (2200 - 2300 MLT), with rather few occurrences before 2000 MLT and after 2400 MLT. Furthermore, the strength of the subauroral electric field is generally much larger for events close to 2200 MLT than for other events. The data confirm that SAEF occur during the substorm recovery phase but also show that SAEF occur earlier during recovery when located close to 2200 MLT than at other local times. The dependence on season and geomagnetic activity is studied, and it is found that the SAEF are more commonly observed at times of high activity when the subauroral electric fields are also generally stronger, except close to winter solstice, when strong electric fields are observed during low activity. The potentials associated with the SAEF and the relation to interplanetary magnetic field B-y are also studied. The observations are discussed in context with substorm-related field-aligned currents and the midlatitude trough, and we present evidence that support and refine one of the proposed production mechanisms.

  • 38.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran T.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Axnäs, Ingvar
    On enhanced aurora and low-altitude parallel electric fields2005In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 72, no 5, p. 419-422Article in journal (Refereed)
    Abstract [en]

    Enhanced auroras are bands of enhanced intensity, sometimes observed along the bottom edges of auroral arcs. It is here proposed that they sometimes are due to local electron energization by low-altitude. downward-directed DC electric fields. which arise as a consequence of the ionospheric response to small-scale auroral current systems. Simulations of such structures are presented. and the electron energization mechanism is discussed. The enhanced auroras would, in this model, appear parallel and close to (perhaps only a few kin front) an ordinary auroral arc. Detailcd 3-D observations of auroral arc structures could resolve if enhanced auroras sometimes take this form.

  • 39.
    Karlsson, Tomas
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Marklund, Göran T.
    KTH, Superseded Departments, Alfvén Laboratory.
    Figueiredo, Sonia
    KTH, Superseded Departments, Alfvén Laboratory.
    Johansson, Tommy
    KTH, Superseded Departments, Alfvén Laboratory.
    Buchert, S.
    Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 7, p. 2463-2472Article in journal (Refereed)
    Abstract [en]

    Cluster multipoint measurements of the electric and magnetic fields from a crossing of auroral field lines at an attitude of 4 R-E are used to show that it is possible to resolve the ambiguity of temporal versus spatial variations in the fields. We show that the largest electric fields (of the order of 300 mV/m when mapped down to the ionosphere) are of a quasi-static nature, unipolar, associated with upward electron beams, stable on a time scale of at least half a minute, and located in two regions of downward current. We conclude that they are the high-altitude analogues of the intense return current/black auroral electric field structures observed at lower altitudes by Freja and FAST. In between these structures there are temporal fluctuations, which are shown to likely be downward travelling Alfven waves. The periods of these waves are 20-40 s, which is not consistent with periods associated with either the Alfvenic ionospheric resonator, typical field line resonances or substorm onset related Pi2 oscillations. The multipoint measurements enable us to estimate a lower limit to the perpendicular wavelength of the Alfven waves to be of the order of 120 kin, which suggests that the perpendicular wavelength is similar to the dimension of the region between the two quasi-static structures. This might indicate that the Alfven waves are ducted within a wave guide, where the quasi-static structures are associated with the gradients making up this waveguide.

  • 40.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sjokvist, M
    Wallin, S
    Sjolander, K
    MAGIC service system2005In: 17th ESA Symposium on European Rocket and Balloon Programmes and Related Research, 2005, Vol. 590, p. 269-273Conference paper (Refereed)
    Abstract [en]

    The "MAGIC" (Mesospheric Aerosol - Genesis, Interaction and Composition) payload was launched from Esrange, 10 January 2005, with a single stage improved Orion 14 inch motor to an altitude of 95.1km. SSC (Swedish Space Corporation) in cooperation with MISU (Meteorological Institute of Stockholm), LASP (The Laboratory for Atmospheric and Space Physics, at the University of Colorado) and NRL (Naval Research Laboratory, Washington DC) conducted the project. The SNSB (Swedish National Space Board) funded the MAGIC project. The main scientific object was to collect cosmic dust particles and bring the samples back to the lab for analysis. The vital parts of the service system consist of a MIDAS gyro platform, an SSC telemetry system with wraparound antenna, an Ashtech G12 GPS receiver with wraparound antenna, a power system and an SSC recovery module with a motor separation system.

  • 41. Keiling, A.
    et al.
    Donovan, E.Bagenal, F.Karlsson, TomasKTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Auroral Phenomenology and Magnetospheric Processes: Earth And Other Planets2013Collection (editor) (Other academic)
  • 42. Keiling, A.
    et al.
    Marghitu, O.
    Vogt, J.
    Amm, O.
    Bunescu, C.
    Constantinescu, V.
    Frey, H.
    Hamrin, M.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nakamura, R.
    Nilsson, H.
    Semeter, J.
    Sorbalo, E.
    Magnetosphere-ionosphere coupling of global Pi2 pulsations2014In: Journal of Geophysical Research A: Space Physics, ISSN 2169-9380, Vol. 119, no 4, p. 2717-2739Article in journal (Refereed)
    Abstract [en]

    Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This super global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfven and fast modes, field line resonance, and possibly a forced cavity-type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.

  • 43. Kullen, A.
    et al.
    Buchert, S.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Johansson, T.
    Lileo, S.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Nilsson, H.
    Marchaudon, A.
    Fazakerley, A. N.
    Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 11, p. 3279-3293Article in journal (Refereed)
    Abstract [en]

    The role of intense high-altitude electric field (E-field) peaks for large-scale plasma convection is investigated with the help of Cluster E-field, B-field and density data. The study covers 32 E-field events between 4 and 7 R-E geocentric distance, with E-field magnitudes in the range 500 1000 mV/m when mapped to ionospheric altitude. We focus on E-field structures above the ionosphere that are typically coupled to discrete auroral arcs and their return current region. Connected to such E-field peaks are rapid plasma flows directed along the discrete arcs in opposite directions on each side of the arc. Nearly all the E-field events occur during active times. A strong dependence on different substorm phases is found: a majority of intense E-field events appearing during substorm expansion or maximum phase are located on the night-side oval, while most recovery events occur on the dusk-to-dayside part of the oval. For most expansion and maximum phase cases, the average background plasma flow is in the sunward direction. For a majority of recovery events, the flow is in the anti-sunward direction. The net plasma flux connected to a strong E-field peak is in two thirds of the cases in the same direction as the background plasma flow. However, in only one third of the cases the strong flux caused by an E-field peak makes an important contribution to the plasma transport within the boundary plasma sheet. For a majority of events, the area covered by rapid plasma flows above discrete arcs is too small to have an effect on the global convection. This questions the role of discrete auroral arcs as major driver of plasma convection.

  • 44. Kullen, A.
    et al.
    Cumnock, Judy A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Seasonal dependence and solar wind control of transpolar arc luminosity2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A8Article in journal (Refereed)
    Abstract [en]

    The influence of the solar wind and the interplanetary magnetic field (IMF) on the luminosity of transpolar arcs (TPAs) is examined by taking into account seasonal effects. The study focuses on those transpolar arcs that appear after an IMF By sign change during steady northward IMF. It includes 21 northern hemisphere events identified in a previous study from global UV images taken by the Polar spacecraft between 1996 and 2000. Sorting the TPA events by sign of the Earth dipole tilt we find that the TPAs which appear in the dark hemisphere are on average much weaker than TPAs in the sunlit hemisphere. For the dark hemisphere events, no clear correlation between solar wind parameters and TPA luminosity is found. However, in the sunlit hemisphere, a clear dependence on solar wind and IMF conditions is seen. The TPA brightness is strongly influenced by IMF magnitude, northward IMF Bz and solar wind speed. A weak, negative correlation with the ion density is found. The TPA luminosity in the sunlit hemisphere is much more strongly controlled by the magnetic energy flux than by the kinetic energy flux of the solar wind. This explains the absence of transpolar arcs for the two By sign change cases for positive dipole tilts with lowest magnetic energy flux values. The strong influence of the Earth dipole tilt on the transpolar arc luminosity appears due to the dependence of the ionospheric conductivity on solar EUV emissions.

  • 45. Kullen, A.
    et al.
    Ohtani, S.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Geomagnetic signatures of auroral substorms preceded by pseudobreakups2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114Article in journal (Refereed)
    Abstract [en]

    The evolution of ten growth-phase pseudobreakups and subsequent substorms, identified in Northern Hemisphere Polar UV images during winter 1998-1999, are compared to the AE index, the unified PC indices, and GOES B field data. Comparing substorm onset (auroral breakup) with GOES data and AE and PC indices, it is found that an exact onset determination from these parameters is in most cases not possible. The three weakest substorms leave no clear signatures in the auxiliary parameters. For the other events, the AE increase appears with a time delay of 5-15 min after onset. The PC indices increase, as expected, before the AE index. The time span between PC increase and onset varies widely (-26 to 5 min). A tail dipolarization is seen in GOES data with a time delay of 2-31 min after onset. The dipolarization delay at geosynchronous orbit appears because of the GOES displacement from the tail onset region. Using the mapped GOES distance from the auroral breakup region as an estimate of GOES displacement from the breakup source region, we find that the tail dipolarization region expands in average with an azimuthal speed of 0.22 MLT min(-1) and an equatorward speed of 0.09 degrees min(-1). Pseudobreakups leave hardly any signature in AE or PC index data except in the four strongest substorm cases. In these cases, a bump appears in the PC indices during the pseudobreakup. A bump in geosynchronous B field data is found only in those two cases where GOES is located very close to the pseudobreakup tail source region.

  • 46.
    Kullen, Anita
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fear, R. C.
    Milan, S. E.
    Carter, J. A.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    The statistical difference between bending arcs and regular polar arcs2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 12, p. 10443-10465Article in journal (Refereed)
    Abstract [en]

    In this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary magnetic field (IMF) B-y direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arc's nightside end remains attached to the oval, eventually becoming hook-shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during B-y-dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90 degrees about 20min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF B-y and initial arc location. In our data set, the highest correlations are found with IMF B-y appearing at least 1-2 h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection.

  • 47.
    Kullen, Anita
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Karlsson, Tomas
    KTH, Superseded Departments, Alfvén Laboratory.
    On the relation between solar wind, pseudobreakups, and substorms2004In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 109, no A12Article in journal (Refereed)
    Abstract [en]

    A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3-month period in winter 1998-1999. Data from the ACE solar wind monitor are examined in order to determine the influence of solar wind parameters on the occurrence of different substorm and pseudobreakup types. The results confirm that the IMF clock angle and the amount of solar wind energy flux control the strength of a substorm. The majority of large substorms appear when the IMF is strongly southward and the solar wind energy flux is high. Most small substorms occur during weakly positive or zero IMF B-z and low solar wind energy flux values. Pseudobreakups are associated with even lower energy fluxes than small substorms and appear typically for weakly positive IMF B-z. These results are in agreement with the scenario that pseudobreakups essentially are very weak substorms. Pseudobreakups appear during quiet times and during the growth phase or the recovery phase of weak or medium strong substorms. Time periods of enhanced geomagnetic activity with recurrent substorms are devoid of pseudobreakups. A detailed analysis of the different pseudobreakup types reveals that quiet time pseudobreakups appear predominantly during northward IMF. At least 20 percent of these appear at the poleward oval boundary. Optically, they do not differ much from very weak substorms. Growth phase pseudobreakups develop typically at the end of a 1 to 2 hour long excursion from northward to weakly southward IMF and are followed by quite weak substorms. A large majority of recovery phase pseudobreakups occur at a strongly polewardly displaced oval boundary at the end of a very active recovery phase. A considerable decrease of the polar cap size during the preceding substorm is connected to a northward turning of the IMF.

  • 48.
    Kullen, Anita
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Cumnock, Judy A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sundberg, Torbjörn
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Occurrence and properties of substorms associated with pseudobreakups2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, p. A12310-Article in journal (Refereed)
    Abstract [en]

    We investigate how substorms with and without growth-phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth-phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field E-m is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating E-m over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much E-m reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth-phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with E-m, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by E-m and AE, respectively. Comparing growth-phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth-phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth-phase pseudobreakups or substorms.

  • 49.
    Li, Bin
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Alm, Love
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Masson, Arnaud
    Statistical altitude distribution of Cluster auroral electric fields, indicating mainly quasi-static acceleration below 2.8 R-E and Alfvenic above2014In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 11, p. 8984-8991Article in journal (Refereed)
    Abstract [en]

    Results are presented from a statistical study of high-altitude electric fields and plasma densities using Cluster satellite data collected during 9.5years between 2 and 4 R-E. The average electric fields are most intense on the nightside and associated with an extensive plasma density cavity, with densities of 1cm(-3) or less. The intense electric fields are concentrated in two regions, separated by an altitude gap at about 2.8 R-E. Below this, the average electric field magnitudes reach about 50mV/m (mapped to the ionosphere) between 22 and 01 magnetic local time (MLT). Above 3 R-E, the fields are about twice as high and spread over a broader MLT range. These fields occur in a region where the (E/B)/V-A ratio is close to unity, which suggests an Alfvenic origin. The intense low-altitude electric fields are interpreted to be quasi-static, associated with the auroral acceleration region. This is supported by their location in MLT and altitude, and by a (E/B)/V-A ratio much below unity. The local electric field minimum between the two regions indicates a partial closure of the electrostatic potentials in the lower region. These results show similarities with model results of reflected Alfven waves by Lysak and Dum (1983), and with the O-shaped potential model, with associated wave-particle interaction at its top, proposed by Janhunen et al. (2000).

  • 50.
    Li, Bin
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Marklund, Göran
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sadeghi, Soheil
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Vaivads, Andris
    Fazakerley, Andrew
    Zhang, Yongliang
    Lucek, Elizabeth
    Sergienko, Tima
    Nilsson, Hans
    Masson, Arnaud
    Inverted-V and low-energy broadband electron acceleration features of multiple auroras within a large-scale surge2013In: Journal of Geophysical Research: Space Physics, ISSN 2169-9380, Vol. 118, no 9, p. 5543-5552Article in journal (Refereed)
    Abstract [en]

    Results are presented from a Cluster C2 satellite crossing through the acceleration region of multiple auroral structures within a large-scale surge, simultaneously monitored by DMSP F17 imager data. The magnetic and electric field data are consistent with the auroral distribution at large and medium scales. We identify the quasi-static acceleration above and below C2 orbit by downgoing inverted-V electrons and parallel electric potential drops, respectively. In the poleward surge region, within or adjacent to the inverted-V arcs, intense low-energy (broadband) electron fluxes are seen as well as a rough equality between E/B and the Alfven velocity, suggesting that these are of Alfvenic origin. The most poleward and equatorward auroral structure is found to be Alfvenic and quasi-static, respectively. In between, the structures are of mixed origin. We estimate the relative role of the acceleration processes by the contributions to the downward electron energy flux by electrons above and below 1.62keV. Although these are local estimates, they should be representative also below Cluster altitude, except for two regions of intense downward Poynting flux, the power of which will be dissipated at lower altitudes and increasing the Alfvenic contribution. This is also supported by intense fluxes of low-energy, broadband, upgoing electrons observed within these regions. Otherwise, the inverted-V contribution dominates for most of the auroral structures observed by Cluster. The Alfvenic contribution to the mixed arc emissions is to extend these to higher altitudes, as shown by numerical simulation results.

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