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  • 51.
    Graham, Daniel B.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Electrostatic solitary waves with distinct speeds associated with asymmetric reconnection2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 2, p. 215-224Article in journal (Refereed)
    Abstract [en]

    Electrostatic solitary waves (ESWs) are observed at the magnetopause with distinct time scales. These ESWs are associated with asymmetric reconnection of the cold dense magnetosheath plasma with the hot tenuous magnetospheric plasma. The distinct time scales are shown to be due to ESWs moving at distinct speeds and having distinct length scales. The length scales are of order 5-50 Debye lengths, and the speeds range from approximate to 50 km s(-1) to approximate to 1000 km s(-1). The ESWs are observed near the reconnection separatrices. The observation of ESWs with distinct speeds suggests that multiple instabilities are occurring. The implications for reconnection at the magnetopause are discussed.

  • 52.
    Graham, Daniel B.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Fazakerley, A. N.
    Electron Dynamics in the Diffusion Region of an Asymmetric Magnetic Reconnection2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 21, p. 215004-Article in journal (Refereed)
    Abstract [en]

    During a magnetopause crossing near the subsolar point Cluster observes the ion diffusion region of antiparallel magnetic reconnection. The reconnecting plasmas are asymmetric, differing in magnetic field strength, density, and temperature. Spatial changes in the electron distributions in the diffusion region are resolved and investigated in detail. Heating of magnetosheath electrons parallel to the magnetic field is observed. This heating is shown to be consistent with trapping of magnetosheath electrons by parallel electric fields.

  • 53.
    Graham, Daniel. B.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri. V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Andre, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Whistler emission in the separatrix regions of asymmetric magnetic reconnection2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 3, p. 1934-1954Article in journal (Refereed)
    Abstract [en]

    At Earth's dayside magnetopause asymmetric magnetic reconnection occurs between the cold dense magnetosheath plasma and the hot tenuous magnetospheric plasma, which differs significantly from symmetric reconnection. During magnetic reconnection the separatrix regions are potentially unstable to a variety of instabilities. In this paper observations of the separatrix regions of asymmetric reconnection are reported as Cluster crossed the magnetopause near the subsolar point. The small relative motion between the spacecraft and plasma allows spatial changes of electron distributions within the separatrix regions to be resolved over multiple spacecraft spins. The electron distributions are shown to be unstable to the electromagnetic whistler mode and the electrostatic beam mode. Large-amplitude whistler waves are observed in the magnetospheric and magnetosheath separatrix regions, and outflow region. In the magnetospheric separatrix regions the observed whistler waves propagate toward the X line, which are shown to be driven by the loss in magnetospheric electrons propagating away from the X line and are enhanced by the presence of magnetosheath electrons. The beam mode waves are predicted to be produced by beams of magnetosheath electrons propagating away from the X line and potentially account for some of the electrostatic fluctuations observed in the magnetospheric separatrix regions.

  • 54. 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.

  • 55.
    Hamrin, M.
    et al.
    Umea Univ, Dept Phys, Umea, Sweden..
    Andersson, L.
    LASP, Boulder, CO USA..
    Vaivads, Andris
    Uppsala universitet, Institutionen för fysik och astronomi.
    Pitkanen, T.
    Umea Univ, Dept Phys, Umea, Sweden..
    Gunell, H.
    Belgian Inst Space Aeron, Brussels, Belgium..
    The use of the power density for identifying reconnection regions2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 10, p. 8644-8662Article in journal (Refereed)
    Abstract [en]

    In the vicinity of magnetic reconnection, magnetic energy is transferred into kinetic energy. A reconnection region hence corresponds to a load, and it should manifest itself as large and positive values of the power density, E<bold>J </bold>>> 0, where E is the electric field and J the current density. In this article we analyze Cluster plasma sheet data from 2001-2004 to investigate the use of the power density for identifying possible magnetic reconnection events from large sets of observed data. From theoretical arguments we show that an event with E<bold>J</bold>20pW/m(3) in the Earth's magnetotail observed by the Cluster instruments (X <- 10R(E) and |Y|less than or similar to 10RE) is likely to be associated with reconnection. The power density can be used as a primary indicator of potential reconnection regions, but selected events must be reviewed separately to confirm any possible reconnection signatures by looking for other signatures such as Hall electric and magnetic fields and reconnection jets. The power density can be computed from multispacecraft data, and we argue that the power density can be used as a tool for identifying possible reconnection events from large sets of data, e.g., from the Cluster and the Magnetospheric Multiscale missions.

  • 56. Hamrin, M.
    et al.
    Norqvist, P.
    Marghitu, O.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    Scale size and life time of energy conversion regions observed by Cluster in the plasma sheet2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 11, p. 4147-4155Article in journal (Refereed)
    Abstract [en]

    In this article, and in a companion paper by Hamrin et al. (2009) [Occurrence and location of concentrated load and generator regions observed by Cluster in the plasma sheet], we investigate localized energy conversion regions (ECRs) in Earth's plasma sheet. From more than 80 Cluster plasma sheet crossings (660 h data) at the altitude of about 15-20 R-E in the summer and fall of 2001, we have identified 116 Concentrated Load Regions (CLRs) and 35 Concentrated Generator Regions (CGRs). By examining variations in the power density, E.J, where E is the electric field and J is the current density obtained by Cluster, we have estimated typical values of the scale size and life time of the CLRs and the CGRs. We find that a majority of the observed ECRs are rather stationary in space, but varying in time. Assuming that the ECRs are cylindrically shaped and equal in size, we conclude that the typical scale size of the ECRs is 2 R-E less than or similar to Delta 1 S-ECR less than or similar to 5 R-E. The ECRs hence occupy a significant portion of the mid altitude plasma sheet. Moreover, the CLRs appear to be somewhat larger than the CGRs. The life time of the ECRs are of the order of 1-10 min, consistent with the large scale magnetotail MHD simulations of Birn and Hesse (2005). The life time of the CGRs is somewhat shorter than for the CLRs. On time scales of 1-10 min, we believe that ECRs rise and vanish in significant regions of the plasma sheet, possibly oscillating between load and generator character. It is probable that at least some of the observed ECRs oscillate energy back and forth in the plasma sheet instead of channeling it to the ionosphere.

  • 57. 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.

  • 58. Hamrin, M.
    et al.
    Rönnmark, K.
    Borlin, N.
    Vedin, J.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    GALS - Gradient Analysis by Least Squares2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 11, p. 3491-3499Article in journal (Refereed)
    Abstract [en]

    We present a method, GALS (Gradient Analysis by Least Squares) for estimating the gradient of a physical field from multi-spacecraft observations. To obtain the best possible spatial resolution, the gradient is estimated in the frame of reference where structures in the field are essentially locally stationary. The estimates are refined iteratively by a least squares method. We show that GALS is not very sensitive to the spacecraft configuration and resolves structures much smaller than the characteristic size of the spacecraft distribution. Furthermore, GALS requires little user input. GALS has been tested on synthetic magnetic field data and data from the Cluster FGM instrument. GALS will also be useful for other types of data. The results indicate that GALS is robust and superior to the curlometer method for estimating the current from magnetic field measurements.

  • 59.
    Hamrin, Maria
    et al.
    Umeå universitet, Institutionen för fysik.
    Marghitu, Octav
    Rönnmark, Kjell
    Umeå universitet, Institutionen för fysik.
    Klecker, Berndt
    André, Mats
    Buchert, S
    Kistler, L
    McFadden, J
    Rème, H
    Vaivads, Andris
    Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, p. 637-49Article in journal (Refereed)
    Abstract [en]

    Here and in the companion paper, Marghitu et al. (2006), we investigate plausible auroral generator regions in the nightside auroral magnetosphere. In this article we use magnetically conjugate data from the Cluster and the FAST satellites during a 3.5-h long event from 19-20 September 2001. Cluster is in the Southern Hemisphere close to apogee, where it probes the plasma sheet and lobe at an altitude of about 18 RE. FAST is below the acceleration region at approximately 0.6 RE. Searching for clear signatures of negative power densities, E(.)J < O, in the Cluster data we can identify three concentrated generator regions (CGRs) during our event. From the magnetically conjugate FAST data we see that the observed generator regions in the Cluster data correlate with auroral precipitation. The downward Poynting flux observed by Cluster, as well as the scale size of the CGRs, are consistent with the electron energy flux and the size of the inverted-V regions observed by FAST. To our knowledge, these are the first in-situ observations of the crossing of an auroral generator region. The main contribution to E(.)J < O comes from the GSE E(y)J(y). The electric field E-y is weakly negative during most of our entire event and we conclude that the CGRs occur when the duskward current J(y) grows large and positive. We find that our observations are consistent with a local southward expansion of the plasma sheet and/or rather complicated, 3-D wavy structures propagating over the Cluster satellites. We find that the plasma is working against the magnetic field, and that kinetic energy is being converted into electromagnetic energy. Some of the energy is transported away as Poynting flux.

  • 60.
    Hamrin, Maria
    et al.
    Umeå universitet, Institutionen för fysik.
    Rönnmark, Kjell
    Umeå universitet, Institutionen för fysik.
    Börlin, Niclas
    Umeå universitet, Institutionen för datavetenskap.
    Vaivads, Andris
    GALS: gradient analysis by least squares2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 11, p. 3491-3499Article in journal (Refereed)
    Abstract [en]

    We present a method, GALS (Gradient Analysis by Least Squares) for estimating the gradient of a physical field from multi-spacecraft observations. To obtain the best possible spatial resolution, the gradient is estimated in the frame of reference where structures in the field are essentially locally stationary. The estimates are refined iteratively by a least squares method. We show that GALS is not very sensitive to the spacecraft configuration and resolves structures much smaller than the characteristic size of the spacecraft distribution. Furthermore, GALS requires little user input. GALS has been tested on synthetic magnetic field data and data from the Cluster FGM instrument. GALS will also be useful for other types of data. The results indicate that GALS is robust and superior to the curlometer method for estimating the current from magnetic field measurements.

  • 61. Hasegawa, H.
    et al.
    Retino, A.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Nakamura, T. K. M.
    Teh, W. -L
    Sonnerup, B. U. Oe.
    Schwartz, S. J.
    Seki, Y.
    Fujimoto, M.
    Saito, Y.
    Reme, H.
    Canu, P.
    Kelvin-Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, no 12, p. A12207-Article in journal (Refereed)
    Abstract [en]

    We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at similar to 1500 magnetic local time (MLT) by the Geotail spacecraft and at similar to 1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 x 10(4) km) is considerably longer than predicted by the linear theory from the thickness (similar to 1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length similar to 100 km and of magnetic shear similar to 60 degrees existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvenic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.

  • 62. Hasegawa, H.
    et al.
    Retino, A.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Nakamura, R.
    Takada, T.
    Miyashita, Y.
    Reme, H.
    Lucek, E. A.
    Retreat and reformation of X-line during quasi-continuous tailward-of-the-cusp reconnection under northward IMF2008In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, no 15, p. L15104-Article in journal (Refereed)
    Abstract [en]

    We present observations on 19-20 November 2006 by the Cluster spacecraft that were skimming the high-latitude dusk-flank magnetopause, which are consistent with more than one reconnection X-line present on the tailward side of the cusp under northward IMF. Evidence of quasi-continuous reconnection over 16 hours exists in the form of Alfvenic acceleration of magnetosheath ions found almost always when either of the satellites traversed the boundary. The data indicate that a dominant X-line was sunward of Cluster for most of the time, but ion velocity distributions consisting of two magnetosheath populations demonstrate that for part of the time, more than one X-line existed. Further, the motion of reconnected field lines shows that some X-line(s) retreated tailward. It is inferred that following the X-line retreat, another X-line reformed sunward of Cluster, leading tomultiple X-lines.

  • 63. Hietala, H.
    et al.
    Laitinen, Tiera V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Andreeova, K.
    Vainio, R.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Palmroth, M.
    Pulkkinen, T. I.
    Koskinen, H. E. J.
    Lucek, E. A.
    Reme, H.
    Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 24, p. 245001-Article in journal (Refereed)
    Abstract [en]

    The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.

  • 64. Hietala, H.
    et al.
    Partamies, N.
    Laitinen, T. V.
    Clausen, L. B. N.
    Facsko, G.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Koskinen, H. E. J.
    Dandouras, I.
    Reme, H.
    Lucek, E. A.
    Supermagnetosonic subsolar magnetosheath jets and their effects: from the solar wind to the ionospheric convection2012In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 30, no 1, p. 33-48Article in journal (Refereed)
    Abstract [en]

    It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than similar to 6 R-E transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.

  • 65. Horbury, T.
    et al.
    Louarn, P.
    Fujimoto, M.
    Baumjohann, W.
    Blomberg, Lars
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Barabash, S.
    Canu, P.
    Glassmeier, K. -H
    Koskinen, H.
    Nakamura, R.
    Owen, C.
    Pulkkinen, T.
    Roux, A.
    Sauvaud, J. -A
    Schwartz, S. J.
    Svenes, K.
    Vaivads, A.
    Cross-scale: A multi-spacecraft mission to study cross-scale coupling in space plasmas2006In: European Space Agency, (Special Publication) ESA SP, 2006, no 598, p. 561-568Conference paper (Refereed)
    Abstract [en]

    Collisionless astrophysical plasmas exhibit complexity on many scales: if we are to understand their properties and effects, we must measure this complexity. We can identify a small number of processes and phenomena, one of which is dominant in almost every space plasma region of interest: shocks, reconnection and turbulence. These processes act to transfer energy between locations, scales and modes. However, this transfer is characterised by variability and 3D structure on at least three scales: electron kinetic, ion kinetic and fluid. It is the nonlinear interaction between physical processes at these scales that is the key to understanding these phenomena and predicting their effects. However, current and planned multi-spacecraft missions such as Cluster and MMS only study variations on one scale in 3D at any given time - we must measure the three scales simultaneously fully to understand the energy transfer processes. We propose a mission, called Cross-Scale, to study these processes. Cross-Scale would comprise three nested groups, each consisting of up to four spacecraft. Each group would have a different spacecraft separation, at approximately the electron and ion gyroradii, and a larger MHD scale. We would therefore be able to measure variations on all three important physical scales, simultaneously, for the first time. The spacecraft would fly in formation through key regions of near-Earth space: The solar wind, bowshock, magnetosheath, magnetopause and magnetotail.

  • 66. Huang, S. Y.
    et al.
    Fu, H. S.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Yuan, Z. G.
    Pang, Y.
    Zhou, M.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Deng, X. H.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Zhang, L.
    Fu, S.
    Li, H. M.
    Wang, D. D.
    Dawn-dusk scale of dipolarization front in the Earth's magnetotail: multi-cases study2015In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 357, no 1, article id 22Article in journal (Refereed)
    Abstract [en]

    We analyze three dipolarization front (DF) events to investigate their dawn-dusk scales in the Earth's magnetotail using the Cluster measurements in year 2007, when the spacecraft separation is about 1.8 Re (Re is the Earth's radius) and is appropriate for investigating the DF scale. Based on the Minimum Variance Analysis (MVA) and the general shape of the DF, we found that Cluster detected the center and the flank (or just beyond the flank) of DF in the same event. This means that the scale of DF is about 3.6 Re in the dawn-dusk direction, larger than that reported in previous studies. Using the semicircle function to fit the observations, we got the dawn-dusk scale of similar to 3.2-3.6 Re, consistent with the rough estimation. Considering large separation among the spacecraft, the timing analysis cannot be used to obtain the normal of DF and the propagation velocity along the normal. One should be careful when performing timing analysis of DF using the Cluster data, and have to carry on MVA analysis to check the normal of DF before do timing analysis.

  • 67.
    Huang, S. Y.
    et al.
    Wuhan Univ, Sch Elect Informat, Wuhan, Peoples R China.;UPMC, Lab Phys Plasmas, CNRS, Ecole Polytech, Palaiseau, France..
    Fu, H. S.
    Beihang Univ, Sch Space & Environm, Beijing, Peoples R China..
    Yuan, Z. G.
    Wuhan Univ, Sch Elect Informat, Wuhan, Peoples R China..
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Retino, A.
    UPMC, Lab Phys Plasmas, CNRS, Ecole Polytech, Palaiseau, France..
    Zhou, M.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Graham, Daniel B.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Fujimoto, K.
    Natl Astron Observ Japan, Div Theoret Astron, Mitaka, Tokyo, Japan..
    Sahraoui, F.
    UPMC, Lab Phys Plasmas, CNRS, Ecole Polytech, Palaiseau, France..
    Deng, X. H.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Ni, B.
    Wuhan Univ, Sch Elect Informat, Wuhan, Peoples R China..
    Pang, Y.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Fu, S.
    Wuhan Univ, Sch Elect Informat, Wuhan, Peoples R China..
    Wang, D. D.
    Wuhan Univ, Sch Elect Informat, Wuhan, Peoples R China..
    Zhou, X.
    Liaoning Univ, Sch Phys, Shenyang, Peoples R China..
    Two types of whistler waves in the hall reconnection region2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 7, p. 6639-6646Article in journal (Refereed)
    Abstract [en]

    Whistler waves are believed to play an important role during magnetic reconnection. Here we report the near-simultaneous occurrence of two types of the whistler-mode waves in the magnetotail Hall reconnection region. The first type is observed in the magnetic pileup region of downstream and propagates away to downstream along the field lines and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type, propagating toward the X line, is found around the separatrix region and probably is generated by the electron beam-driven whistler instability or erenkov emission from electron phase-space holes. These observations of two different types of whistler waves are consistent with recent kinetic simulations and suggest that the observed whistler waves are a consequence of magnetic reconnection.

  • 68.
    Huang, S. Y.
    et al.
    Wuhan Univ, Sch Elect Informat, Wuhan 430072, Peoples R China.;UPMC, Ecole Polytech, CNRS, Lab Phys Plasmas, Palaiseau, France..
    Retino, A.
    UPMC, Ecole Polytech, CNRS, Lab Phys Plasmas, Palaiseau, France..
    Phan, T. D.
    Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA..
    Daughton, W.
    Los Alamos Natl Lab, Los Alamos, NM USA..
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Karimabadi, H.
    SciberQuest Inc, Del Mar, CA USA..
    Zhou, M.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Sahraoui, F.
    UPMC, Ecole Polytech, CNRS, Lab Phys Plasmas, Palaiseau, France..
    Li, G. L.
    Univ New Hampshire, Ctr Space Sci, Durham, NH 03824 USA..
    Yuan, Z. G.
    Wuhan Univ, Sch Elect Informat, Wuhan 430072, Peoples R China..
    Deng, X. H.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Fu, H. S.
    Beihang Univ, Sch Astronaut, Space Sci Inst, Beijing 100191, Peoples R China..
    Fu, S.
    Wuhan Univ, Sch Elect Informat, Wuhan 430072, Peoples R China..
    Pang, Y.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Peoples R China..
    Wang, D. D.
    Wuhan Univ, Sch Elect Informat, Wuhan 430072, Peoples R China..
    In situ observations of flux rope at the separatrix region of magnetic reconnection2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 1, p. 205-213Article in journal (Refereed)
    Abstract [en]

    We present the first in situ observations of a small-scale flux rope locally formed at the separatrix region of magnetic reconnection without large guide field. Bidirectional electron beams (cold and hot beams) and density cavity accompanied by intense wave activity substantiate the crossing of the separatrix region. Density compression and one parallel electron beam are detected inside the flux rope. We suggest that this flux rope is locally generated at the separatrix region due to the tearing instability within the separatrix current layer. This observation sheds new light on the 3-D picture of magnetic reconnection in space plasma.

  • 69.
    Huang, S. Y.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Zhou, M.
    Fu, Huishan
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Retino, A.
    Deng, X. H.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Cully, Christopher M.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    He, J. S.
    Sahraoui, F.
    Yuan, Z. G.
    Pang, Y.
    Electron acceleration in the reconnection diffusion region: Cluster observations2012In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, no L11103Article in journal (Refereed)
    Abstract [en]

    We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.

  • 70.
    Huang, S. Y.
    et al.
    Wuhan Univ, Sch Elect Informat, Wuhan, Hubei, Peoples R China.
    Yuan, Z. G.
    Wuhan Univ, Sch Elect Informat, Wuhan, Hubei, Peoples R China.
    Fu, H. S.
    Beihang Univ, Sch Astronaut, Space Sci Inst, Beijing, Peoples R China.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Sahraoui, F.
    UPMC, EcolePolytech, CNRS, Lab Phys Plasmas, Palaiseau, France.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Retino, A.
    UPMC, EcolePolytech, CNRS, Lab Phys Plasmas, Palaiseau, France.
    Zhou, M.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.
    Graham, Daniel B.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Fujimoto, K.
    Natl Astron Observ Japan, Div Theoret Astron, Mitaka, Tokyo, Japan.
    Deng, X. H.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.
    Ni, B. B.
    Wuhan Univ, Sch Elect Informat, Wuhan, Hubei, Peoples R China.
    Pang, Y.
    Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.
    Fu, S.
    Wuhan Univ, Sch Elect Informat, Wuhan, Hubei, Peoples R China.
    Wang, D. D.
    Wuhan Univ, Sch Elect Informat, Wuhan, Hubei, Peoples R China.
    Observations of Whistler Waves in the Magnetic Reconnection Diffusion Region2018In: 2ND URSI ATLANTIC RADIO SCIENCE MEETING (AT-RASC), IEEE , 2018Conference paper (Refereed)
    Abstract [en]

    Whistler waves are believed to play an important role during magnetic reconnection. In this paper, we report the simultaneous occurrence of two types of the whistler waves in the magnetotail reconnection diffusion region. The first type is observed in the pileup region of downstream and propagates away along the field lines to downstream, and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type is found around the separatrix region and propagates towards the X-line, and is possibly aenerated by the electron beam-driven whistler instability or Cerenkov emission from electron phase-space holes. Our observations of two different types of whistler waves are well consistent with recent kinetic simulations, and suggest that the observed whistler waves are the consequences of magnetic reconnection.Moreover, we statistically investigate the whistler waves in the magnetotail reconnection region, and construct the global distribution and occurrence rate of the whistler waves based on the two-dimensional reconnection model. It is found that the occurrence rate of the whistler waves is large in the separatrix region (113,1B0j>0.4) and pileup region ([B,./Bol<0.2, 161>45'), but very small in the X-line region. The statistical results are well consistent with the case study.

  • 71. Huang, S. Y.
    et al.
    Zhou, M.
    Sahraoui, F.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Deng, X. H.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    He, J. S.
    Fu, Huishan
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Li, H. M.
    Yuan, Z. G.
    Wang, D. D.
    Observations of turbulence within reconnection jet in the presence of guide field2012In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, no L11104Article in journal (Refereed)
    Abstract [en]

    We present the first comprehensive observations of turbulence properties within high speed reconnection jet in the plasma sheet with moderate guide field. The power spectral density index is about -1.73 in the inertial range, and follows the value of -2.86 in the ion dissipation range. The turbulence is strongly anisotropic in the wave-vector space with the major power having its wave-vector highly oblique to the ambient magnetic field, suggesting that the turbulence is quasi-2D. The measured "dispersion relations" obtained using the k-filtering technique are compared with theory and are found to be consistent with the Alfven-Whistler mode. In addition, both Probability Distribution Functions and flatness results show that the turbulence in the reconnection jet is intermittent (multifractal) at scales less than the proton gyroradius/inertial lengths. The estimated electric field provided by anomalous resistivity caused by turbulence is about 3 mV/m, which is close to the typical reconnection electric field in the magnetotail.

  • 72. Johlander, A.
    et al.
    Schwartz, S. J.
    Vaivads, Andris
    Khotyaintsev, Yu. V.
    Gingell, I.
    Peng, Bo
    KTH, School of Computer Science and Communication (CSC), Computational Science and Technology (CST).
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), Computational Science and Technology (CST).
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ergun, R. E.
    Marklund, G. T.
    Plaschke, F.
    Magnes, W.
    Strangeway, R. J.
    Russell, C. T.
    Wei, H.
    Torbert, R. B.
    Paterson, W. R.
    Gershman, D. J.
    Dorelli, J. C.
    Avanov, L. A.
    Lavraud, B.
    Saito, Y.
    Giles, B. L.
    Pollock, C. J.
    Burch, J. L.
    Rippled Quasiperpendicular Shock Observed by the Magnetospheric Multiscale Spacecraft2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 16, article id 165101Article in journal (Refereed)
    Abstract [en]

    Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.

  • 73.
    Johlander, Andreas
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Gingell, Imogen
    Schwartz, Steven J
    Giles, Barbara L
    Torbert, Roy B
    Russell, Christopher T
    Shock ripples observed by the MMS spacecraft: ion reflection and dispersive properties2018In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 60, article id 125006Article in journal (Refereed)
    Abstract [en]

    Shock ripples are ion-inertial-scale waves propagating within the front region of magnetized quasi-perpendicular collisionless shocks. The ripples are thought to influence particle dynamics and acceleration at shocks. With the four magnetospheric multiscale (MMS) spacecraft, it is for the first time possible to fully resolve the small scale ripples in space. We use observations of one slow crossing of the Earth’s non-stationary bow shock by MMS. From multi-spacecraft measurements we show that the non-stationarity is due to ripples propagating along the shock surface. We find that the ripples are near linearly polarized waves propagating in the coplanarity plane with a phase speed equal to the local Alfvén speed and have a wavelength close to 5 times the upstream ion inertial length. The dispersive properties of the ripples resemble those of Alfvén ion cyclotron waves in linear theory. Taking advantage of the slow crossing by the four MMS spacecraft, we map the shock-reflected ions as a function of ripple phase and distance from the shock. We find that ions are preferentially reflected in regions of the wave with magnetic field stronger than the average overshoot field, while in the regions of lower magnetic field, ions penetrate the shock to the downstream region.

  • 74.
    Johlander, Andreas
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Retinò, A.
    Dandouras, I.
    Univ Toulouse 3, F-31062 Toulouse, France.;CNRS, IRAP, Toulouse, France..
    Ion Injection At Quasi-Parallel Shocks Seen By The Cluster Spacecraft2016In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 817, no 1, article id L4Article in journal (Refereed)
    Abstract [en]

    Collisionless shocks in space plasma are known to be capable of accelerating ions to very high energies through diffusive shock acceleration (DSA). This process requires an injection of suprathermal ions, but the mechanisms producing such a suprathermal ion seed population are still not fully understood. We study acceleration of solar wind ions resulting from reflection off short large-amplitude magnetic structures (SLAMSs) in the quasi-parallel bow shock of Earth using in situ data from the four Cluster spacecraft. Nearly specularly reflected solar wind ions are observed just upstream of a SLAMS. The reflected ions are undergoing shock drift acceleration (SDA) and obtain energies higher than the solar wind energy upstream of the SLAMS. Our test particle simulations show that solar wind ions with lower energy are more likely to be reflected off the SLAMS, while high-energy ions pass through the SLAMS, which is consistent with the observations. The process of SDA at SLAMSs can provide an effective way of accelerating solar wind ions to suprathermal energies. Therefore, this could be a mechanism of ion injection into DSA in astrophysical plasmas.

  • 75. Kempf, Yann
    et al.
    Pokhotelov, Dimitry
    von Alfthan, Sebastian
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Palmroth, Minna
    Koskinen, Hannu E. J.
    Wave dispersion in the hybrid-Vlasov model: Verification of Vlasiator2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 11Article in journal (Refereed)
    Abstract [en]

    Vlasiator is a new hybrid-Vlasov plasma simulation code aimed at simulating the entire magnetosphere of the Earth. The code treats ions (protons) kinetically through Vlasov's equation in the six-dimensional phase space while electrons are a massless charge-neutralizing fluid [M. Palmroth et al., J. Atmos. Sol.-Terr. Phys. 99, 41 (2013); A. Sandroos et al., Parallel Comput. 39, 306 (2013)]. For first global simulations of the magnetosphere, it is critical to verify and validate the model by established methods. Here, as part of the verification of Vlasiator, we characterize the low-beta plasma wave modes described by this model and compare with the solution computed by the Waves in Homogeneous, Anisotropic Multicomponent Plasmas (WHAMP) code [K. Ronnmark, Kiruna Geophysical Institute Reports No. 179, 1982], using dispersion curves and surfaces produced with both programs. The match between the two fundamentally different approaches is excellent in the low-frequency, long wavelength range which is of interest in global magnetospheric simulations. The left-hand and right-hand polarized wave modes as well as the Bernstein modes in the Vlasiator simulations agree well with the WHAMP solutions. Vlasiator allows a direct investigation of the importance of the Hall term by including it in or excluding it from Ohm's law in simulations. This is illustrated showing examples of waves obtained using the ideal Ohm's law and Ohm's law including the Hall term. Our analysis emphasizes the role of the Hall term in Ohm's law in obtaining wave modes departing from ideal magnetohydrodynamics in the hybrid-Vlasov model. 

  • 76.
    Khotyaintsev, Yuri
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Cully, Christopher
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Owen, C. J.
    Plasma Jet Braking: Energy Dissipation and Nonadiabatic Electrons2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, no 16Article in journal (Refereed)
    Abstract [en]

    We report in situ observations by the Cluster spacecraft of wave-particle interactions in a magnetic flux pileup region created by a magnetic reconnection outflow jet in Earth's magnetotail. Two distinct regions of wave activity are identified: lower-hybrid drift waves at the front edge and whistler-mode waves inside the pileup region. The whistler-mode waves are locally generated by the electron temperature anisotropy, and provide evidence for ongoing betatron energization caused by magnetic flux pileup. The whistler-mode waves cause fast pitch-angle scattering of electrons and isotropization of the electron distribution, thus making the flow braking process nonadiabatic. The waves strongly affect the electron dynamics and thus play an important role in the energy conversion chain during plasma jet braking.

  • 77.
    Khotyaintsev, Yuri V.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Fujimoto, M.
    Retino, A.
    Owen, C. J.
    Observations of Slow Electron Holes at a Magnetic Reconnection Site2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 16Article in journal (Refereed)
    Abstract [en]

    We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to f(pe) are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.

  • 78. Khotyaintsev, Yuri
    et al.
    Vaivads, Andris
    Ogawa, Yoshimoto
    Popielawska, Barbara
    André, Mats
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Buchert, Stephan
    Décréau, P.
    Lavraud, B.
    Réme, H.
    Cluster observations of high-frequency waves in the exterior cusp2004In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, p. 2403-2411Article in journal (Refereed)
  • 79.
    Laitinen, T. V.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Reme, H.
    Local influence of magnetosheath plasma beta fluctuations on magnetopause reconnection2010In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, no 5, p. 1053-1063Article in journal (Refereed)
    Abstract [en]

    We present observations from two subsolar Cluster magnetopause crossings under southward interplanetary magnetic field and strong mirror mode fluctuations in the magnetosheath. In both events the reconnection outflow jets show strong variations on the timescale of one minute. We show that at least some of the recorded variations are truly temporal, not spatial. On the same timescale, mirror mode fluctuations appear as strong magnetic fluctuations in the magnetosheath next to the magnetopause. This suggests that mirror modes can cause the variations either through modulation of continuous reconnection or through triggering of bursty reconnection. Using a theoretical scaling law for asymmetric reconnection we show that modulation of reconnection at a single x-line can explain the observations of the first event. The second event cannot be explained by a single modulated x-line: there the evidence points to patchy and bursty reconnection.

  • 80.
    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.

  • 81.
    Li, Wenya
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Andre, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Graham, Daniel B.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Toledo-Redondo, S.
    European Space Agcy, Sci Directorate, ESAC, Madrid, Spain..
    Norgren, Cecilia
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Henri, P.
    CNRS, LPC2E, Orleans, France..
    Wang, C.
    Natl Space Sci Ctr, Beijing, Peoples R China..
    Tang, B. B.
    Natl Space Sci Ctr, Beijing, Peoples R China..
    Lavraud, B.
    Univ Toulouse UPS, Inst Rech Astrophys & Planetol, Toulouse, France.;CNRS, UMR 5277, Toulouse, France..
    Vernisse, Y.
    Univ Toulouse UPS, Inst Rech Astrophys & Planetol, Toulouse, France..
    Turner, D. L.
    Aerosp Corp, Dept Space Sci, El Segundo, CA 90245 USA..
    Burch, J.
    Southwest Res Inst, San Antonio, TX USA..
    Torbert, R.
    Univ New Hampshire, Ctr Space Sci, Durham, NH 03824 USA..
    Magnes, W.
    Austrian Acad Sci, Space Res Inst, Graz, Austria..
    Russell, C. T.
    Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90024 USA..
    Blake, J. B.
    Aerosp Corp, Dept Space Sci, El Segundo, CA 90245 USA..
    Mauk, B.
    Johns Hopkins Univ, Appl Phys Lab, Laurel, MD USA..
    Giles, B.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Pollock, C.
    Denali Sci, Healy, AL USA..
    Fennell, J.
    Aerosp Corp, Dept Space Sci, El Segundo, CA 90245 USA..
    Jaynes, A.
    Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80309 USA..
    Avanov, L. A.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Dorelli, J. C.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Gershman, D. J.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.;Univ Maryland, Dept Astron, College Pk, MD 20742 USA..
    Paterson, W. R.
    NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Saito, Y.
    Japan Aerosp Explorat Agcy, Tokyo, Japan..
    Strangeway, R. J.
    Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90024 USA..
    Kinetic evidence of magnetic reconnection due to Kelvin-Helmholtz waves2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 11, p. 5635-5643Article in journal (Refereed)
    Abstract [en]

    The Kelvin-Helmholtz (KH) instability at the Earth's magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multiscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvenic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low-energy (below 100eV) magnetosheath electrons enter the magnetosphere and are heated in the field-aligned direction at the high-density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves.

  • 82.
    Lindstedt, T.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Fear, R. C.
    Lavraud, B.
    Haaland, S.
    Owen, C. J.
    Separatrix regions of magnetic reconnection at the magnetopause2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 10, p. 4039-4056Article in journal (Refereed)
    Abstract [en]

    Using data from the four Cluster spacecraft we study the separatrix regions of magnetic reconnection sites at the dayside magnetopause under conditions when reconnection is occurring in the magnetopause current layer which separates magnetosheath plasma from the hot magnetospheric plasma sheet. We define the separatrix region as the region between the separatrix - the first field line opened by reconnection - and the reconnection jet (outflow region). We analyze eight separatrix region crossings on the magnetospheric side of the magnetopause and present detailed data for two of the events. We show that characteristic widths of the separatrix regions are of the order of ten ion inertial lengths at the magnetopause. Narrow separatrix regions with widths comparable to a few ion inertial lengths are rare. We show that inside the separatrix region there is a density cavity which sometimes has complex internal structure with multiple density dips. Strong electric fields exist inside the separatrix regions and the electric potential drop across the regions can be up to several kV. On the magnetosheath side of the region there is a density gradient with strong field aligned currents. The observed strong electric fields and currents inside the separatrix region can be important for a local energization of ions and electrons, particularly of ionospheric origin, as well as for magnetosphere-ionosphere coupling.

  • 83.
    Lindstedt, T.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Nilsson, H.
    Swedish Institute of Space Physics, Kiruna.
    Waara, M.
    Swedish Institute of Space Physics, Kiruna.
    Oxygen energization by localized perpendicular electric fields at the cusp boundary2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, no 9, article id L09103Article in journal (Refereed)
    Abstract [en]

    We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency.

  • 84. Lindstedt, T
    et al.
    Khotyaintsev, YV
    Vaivads, Andris
    André, M
    Nilsson, H
    Waara, Martin
    Umeå universitet, Institutionen för fysik.
    Oxygen energization by localized perpendicular electric fields at the cusp boundary2010In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, no 9, article id L09103Article in journal (Refereed)
    Abstract [en]

    We report Cluster observations of oxygen energization by several keV at the boundary between the high latitude cusp and lobe. A localized electric field at the cusp/lobe boundary is responsible for a significant part of the observed energization. Such electric fields can be related to the separatrix region of reconnection at the magnetopause. Ions are accelerated as they move non-adiabatically in the spatially inhomogeneous electric field. Additional heating may be provided by low frequency waves at the oxygen gyrofrequency. Citation: Lindstedt, T., Y. V. Khotyaintsev, A. Vaivads, M. Andre, H. Nilsson, and M. Waara (2010), Oxygen energization by localized perpendicular electric fields at the cusp boundary.

  • 85. Lu, Quanming
    et al.
    Huang, Can
    Xie, Jinlin
    Wang, Rongsheng
    Wu, Mingyu
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Wang, Shui
    Features of separatrix regions in magnetic reconnection: Comparison of 2-D particle-in-cell simulations and Cluster observations2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, no 11, article id A11208Article in journal (Refereed)
    Abstract [en]

    In collisionless magnetic reconnection, the in-plane Hall currents are carried mainly by the magnetized electrons. The in-plane Hall currents are directed toward the X line along the magnetic field lines just inside the separatrices and away from the X line along the separatrices. Such a current system leads to the quadrupole out-of-plane magnetic field with the peaks between the regions carrying the in-plane currents. Simultaneously, the electron flow toward the X line along the separatrices causes electron density depletions along the separatrices. In this paper, the features of separatrix regions in magnetic reconnection and the relations between the electron density depletions and the out-of-plane magnetic field are investigated with both two-dimensional particle-in-cell simulations and Cluster observations. We conclude that the electron density depletions are formed because of the magnetic mirror, and they are outside the peaks of the out-of-plane magnetic field. Such a theoretical prediction is confirmed by both simulations and observations.

  • 86. Marghitu, Octav
    et al.
    Hamrin, Maria
    Umeå universitet, Institutionen för fysik.
    Klecker, Berndt
    Vaivads, Andris
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    McFadden, Jim
    Buchert, Stephan
    Kistler, Lynn M
    Dandouras, Iannis
    André, Mats
    Rème, Henri
    Experimental investigation of auroral generator regions with conjugate Cluster and FAST data2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, p. 619-635Article in journal (Refereed)
  • 87. Meng, Zhou
    et al.
    XiaoHua, Deng
    Song, Fu
    RongXin, Tang
    YunHui, Hu
    ShiYou, Li
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Xi, Lin
    MingHui, Lin
    XiaoMin, Zhou
    Observation of the lower hybrid waves near the three-dimensional null pair2009In: Science in China Series G: Physics Mechanics and Astronomy, ISSN 1672-1799, E-ISSN 1862-2844, Vol. 52, no 4, p. 626-630Article in journal (Refereed)
    Abstract [en]

    Magnetic reconnection is a fundamental process in plasma, which is thought to play important roles both in laboratory and natural plasmas through affecting magnetic topology, heating and accelerating particles. During an event on Oct. 1st, 2001, the Cluster tetrahedron circled around the magnetic reconnection region several times, and Xiao et al. first identified the null pair and found that the spectrum of the null-point oscillation shows the maximum power near the lower-hybrid frequency. In this paper we report the observation of electromagnetic and electrostatic wave enhancements near lower hybrid frequency associated with the reconnection process near the null pair. The lower hybrid waves (LHWs) with quasi-perpendicular propagation were identified and also confirmed by the power law of the spectrum of electric and magnetic fields.

  • 88.
    Nakamura, R
    et al.
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Haerendel, G
    Baumjohann, W
    Vaivads, Andris
    Kucharek, H
    Klecker, B
    Georgescu, E
    Birn, J
    Kistler, LM
    Mukai, T
    Kokubun, S
    Eglitis, P
    Frank, LA
    Sigwarth, JB
    Substorm observations in the early morning sector with Equator-S and Geotail1999In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 17, no 12, p. 1602-1610Article in journal (Refereed)
    Abstract [en]

    Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03-04 MLT) at a radial distance o

  • 89.
    Nakamura, R
    et al.
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Haerendel, G
    Baumjohann, W
    Vaivads, Andris
    Kucharek, H
    Klecker, B
    Georgescu, E
    Birn, J
    Kistler, LM
    Mukai, T
    Kokubun, S
    Eglitis, P
    Frank, LA
    Sigwarth, JB
    Substorm observations in the early morning sector with Equator-S and Geotail1999In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 17, no 12, p. 1602-1610Article in journal (Refereed)
    Abstract [en]

    Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03-04 MLT) at a radial distance o

  • 90.
    Norgren, Cecilia
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Graham, Daniel. B.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri. V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Slow electron holes in multicomponent plasmas2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 18, p. 7264-7272Article in journal (Refereed)
    Abstract [en]

    Electrostatic solitary waves (ESWs), often interpreted as electron phase space holes, are commonly observed in plasmas and are manifestations of strongly nonlinear processes. Often slow ESWs are observed, suggesting generation by the Buneman instability. The instability criteria, however, are generally not satisfied. We show how slow electron holes can be generated by a modified Buneman instability in a plasma that includes a slow electron beam on top of a warm thermal electron background. This lowers the required current for marginal instability and allows for generation of slow electron holes for a wide range of beam parameters that covers expected plasma distributions in space, for example, in magnetic reconnection regions. At higher beam speeds, the electron-electron beam instability becomes dominant instead, producing faster electron holes. The range of phase speeds for this model is consistent with a statistical set of observations at the magnetopause made by Cluster.

  • 91.
    Norgren, Cecilia
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Slow electron phase space holes: Magnetotail observations2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 6, p. 1654-1661Article in journal (Refereed)
    Abstract [en]

    We report multispacecraft observations of slow electrostatic solitary waves in the plasma sheet boundary layer. The electrostatic solitary waves are embedded in a region with field-aligned electron flows and are interpreted as electron phase space holes. We make unambiguous velocity and length estimates of the electron holes, v(EH)approximate to 500 km/s and l(||)approximate to 2-4(De), where l(||) is the parallel half width. We do not detect any magnetic signature of the holes. The electrostatic potentials of the holes are of the order e/k(B)T(e)approximate to 10%, indicating that they can affect electron motion and further couple the electron and ion dynamics.

  • 92.
    Norgren, Cecilia
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Lower Hybrid Drift Waves: Space Observations2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 5, article id 055001Article in journal (Refereed)
    Abstract [en]

    Lower hybrid drift waves (LHDWs) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of the Cluster satellites (C3 and C4) located in Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDWs and to determine the phase velocity and wavelength of the LHDWs. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDWs is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation integral delta Edt . v = phi(delta B parallel to). The electrostatic potential fluctuations correspond to similar to 10% of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.

  • 93. Pedersen, A.
    et al.
    Decreau, P.
    Escoubet, C. P.
    Gustafsson, G.
    Laakso, H.
    Lindqvist, Per-Arne
    KTH, Superseded Departments (pre-2005), Alfvén Laboratory.
    Lybekk, B.
    Masson, A.
    Mozer, F.
    Vaivads, Andris
    Four-point high time resolution information on electron densities by the electric field experiments (EFW) on cluster2001In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 19, no 12-okt, p. 1483-1489Article in journal (Refereed)
    Abstract [en]

    For accurate measurements of electric fields, spherical double probes are electronically controlled to be at a positive potential of approximately 1 V relative to the ambient magnetospheric plasma. The spacecraft will acquire a potential which balances the photoelectrons escaping to the plasma and the electron flux collected from the plasma. The probe-to-plasma potential difference can be measured with a time resolution of a fraction of a second, and provides information on the electron density over a wide range of electron densities from the lobes (similar to0.01 cm(-3)) to the magnetosheath (> 10 cm(-3)) and the plasmasphere (> 100 cm(-3)). This technique has been perfected and calibrated against other density measurements on GEOS, ISEE-1, CRRES, GEOTAIL and POLAR. The Cluster spacecraft potential measurements opens the way for new approaches, particularly near boundaries and gradients where four-point measurements will provide information never obtained before. Another interesting point is that onboard data storage of this simple parameter can be done for complete orbits and thereby will provide background information for the shorter full data collection periods on Cluster. Preliminary calibrations against other density measurements on Cluster will be reported.

  • 94.
    Perri, Silvia
    et al.
    Univ Calabria, Dipartimento Fis, Via P Bucci, I-87036 Arcavacata Di Rende, Italy..
    Servidio, Sergio
    Univ Calabria, Dipartimento Fis, Via P Bucci, I-87036 Arcavacata Di Rende, Italy..
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Valentini, Francesco
    Univ Calabria, Dipartimento Fis, Via P Bucci, I-87036 Arcavacata Di Rende, Italy..
    Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas2017In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 231, no 1, article id 4Article in journal (Refereed)
    Abstract [en]

    In situ heliospheric measurements allow us to resolve fluctuations as a function of frequency. A crucial point is to describe the power spectral density as a function of the wavenumber, in order to understand the energy cascade through the scales in terms of plasma turbulence theories. The most favorable situation occurs when the average wind speed is much higher than the phase speed of the plasma modes, equivalent to the fact that the fluctuations' dynamical times are much longer than their typical crossing period through the spacecraft (frozen-in Taylor approximation). Using driven compressible Hall-magneothydrodynamics simulations, in which an "imaginary" spacecraft flies across a time-evolving turbulence, here we explore the limitations of the frozen-in assumption. We find that the Taylor hypothesis is robust down to sub-proton scales, especially for flows with mean velocities typical of the fast solar wind. For slow mean flows (i.e., speeds of the order of the Alfven speed) power spectra are subject to an amplitude shift throughout the scales. At small scales, when dispersive decorrelation mechanisms become significant, the frozen-in assumption is generally violated, in particular for k-vectors almost parallel to the average magnetic field. A discussion in terms of the spacetime autocorrelation function is proposed. These results might be relevant for the interpretation of the observations, in particular for existing and future space missions devoted to very high-resolution measurements.

  • 95. Retino, A.
    et al.
    Nakamura, R.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Hayakawa, T.
    Tanaka, K.
    Kasahara, S.
    Fujimoto, M.
    Shinohara, I.
    Eastwood, J. P.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Baumjohann, W.
    Daly, P. W.
    Kronberg, E. A.
    Cornilleau-Wehrlin, N.
    Cluster observations of energetic electrons and electromagnetic fields within a reconnecting thin current sheet in the Earth's magnetotail2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A12, article id A12215Article in journal (Refereed)
    Abstract [en]

    We study the acceleration of energetic electrons during magnetotail reconnection by using Cluster simultaneous measurements of three-dimensional electron distribution functions, electric and magnetic fields, and waves in a thin current sheet. We present observations of two consecutive current sheet crossings where the flux of electrons 35 127 keV peaks within an interval of tailward flows. The first crossing shows the signatures of a tailward moving flux rope. The observed magnetic field and density indicate that the flux rope was very dynamic, and a comparison with numerical simulation suggests a crossing right after coalescence of smaller flux ropes. The second crossing occurs within the ion diffusion region. The flux of electrons is largest within the flux rope where they are mainly directed perpendicular to the magnetic field. At the magnetic separatrices, the fluxes are smaller, but the energy spectra are harder and electrons are mainly field aligned. Reconnection electric fields E-Y similar to 7 mV/m are observed within the diffusion region, whereas in the flux rope, EY are much smaller. Waves around lower hybrid frequency do not show a clear correlation with energetic electrons. We interpret the field-aligned electrons at the separatrices as directly accelerated by the reconnection electric field in the diffusion region, whereas we interpret the perpendicular electrons as trapped within the flux rope and accelerated by a combination of betatron acceleration with nonadiabatic pitch-angle scattering. Our observations indicate that thin current sheets during dynamic reconnection are important for in situ production of energetic electrons and that simultaneous measurements of electrons and electromagnetic fields within thin sheets are crucial to understand the acceleration mechanisms.

  • 96.
    Retino, Alessandro
    et al.
    Univ Paris 06, Observ Paris, Ecole Polytech, CNRS,LPP UMR 7648, Route Saclay, F-91128 Palaiseau, France..
    Spallicci, Alessandro D. A. M.
    Univ Orleans, Observ Sci Univers Reg Ctr, UMS 3116, CNRS,UMR LPC2E 7328, 3A Ave Rech Sci, F-45071 Orleans, France..
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Solar wind test of the de Broglie-Proca massive photon with Cluster multi-spacecraft data2016In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 82, p. 49-55Article in journal (Refereed)
    Abstract [en]

    Our understanding of the universe at large and small scales relies largely on electromagnetic observations. As photons are the messengers, fundamental physics has a concern in testing their properties, including the absence of mass. We use Cluster four spacecraft data in the solar wind at 1 AU to estimate the mass upper limit for the photon. We look for deviations from Ampere's law, through the curlometer technique for the computation of the magnetic field, and through the measurements of ion and electron velocities for the computation of the current. We show that the upper bound for m(gamma) lies between 1.4 x 10(-49) and 3.4 x 10(-51) kg, and thereby discuss the currently accepted lower limits in the solar wind.

  • 97.
    Retinò, Alessandro
    et al.
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Sundkvist, David
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Mozer, F.
    André, Mats
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Owen, C.J.
    In situ evidence of magnetic reconnection in turbulent plasma2007In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 3, no 4, p. 235-238Article in journal (Refereed)
    Abstract [en]

    Magnetic reconnection is a universal process leading to energy conversion in plasmas. It occurs in the Solar System, in laboratory plasmas and is important in astrophysics . Reconnection has been observed so far only at large-scale boundaries between different plasma environments . It is not known whether reconnection occurs and is important in turbulent plasmas where many small-scale boundaries can form. Solar and laboratory measurements as well as numerical simulations indicate such possibility. Here we report, for the first time, in situ evidence of reconnection in a turbulent plasma. The turbulent environment is the solar wind downstream of the Earths bow shock. We show that reconnection is fast and electromagnetic energy is converted into heating and acceleration of particles. This has significant implications for laboratory and astrophysical plasmas where both turbulence and reconnection should be common.

  • 98.
    Retinò, Alessandro
    et al.
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Vaivads, Andris
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    André, Mats
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Khotyaintsev, Yuri
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Cluster multispacecraft observations at the high latitude duskside magnetopause: implications for continuous and component magnetic reconnection2005In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 2, p. 461-473Article in journal (Refereed)
    Abstract [en]

    We report multispacecraft Cluster observations of magnetic reconnection at the high-latitude magnetopause/magnetospheric boundary layer (MP/BL) under mainly northward interplanetary magnetic field (IMF) conditions. The event we study is on 3 December 2001 when the Cluster spacecraft were skimming the high-latitude duskside MP/BL during a period of about four hours. The orbit and configuration of the spacecraft were such that at least one satellite was present in the MP/BL during most of that period. We present the evidence of reconnection in the form of tangential stress balance between the magnetosheath and the MP/BL (Walen test) and in several cases in the form of transmitted magnetosheath ions in the MP/BL and incident/reflected magnetosheath ions in the magnetosheath boundary layer (MSBL). The observations are consistent with magnetic reconnection occurring tailward of the cusp and going on continuously for a period of about four hours. The observed directions of the reconnection flows are consistent with the IMF orientation, thus indicating that reconnection is globally controlled by the IMF. Observations of a few flow reversals suggest passages of the spacecraft close to the X-line. The observation of low magnetic shear across the magnetopause during a flow reversal is consistent with component merging at least in one case. The observation of reconnection flows on the duskside magnetopause irrespective of the change in the sign of the IMF By also suggests a better agreement with the component merging model, though antiparallel merging cannot be excluded because the distance from the X-line is not known.

  • 99.
    Retinò, Alessandro
    et al.
    Uppsala universitet, Institutionen för astronomi och rymdfysik.
    Vaivads, Andris
    Saharoui, F.
    Khotyaintsev, Y.
    Pickett, J.S.
    Bavassano Cattaneo, M. B.
    Marcucci, M. F.
    Morooka, M.
    Owen, C.J.
    Buchert, S. C.
    Cornilleau-Wehrlin, N.
    Structure of the separatrix region close to a magnetic reconnection X-line: Cluster observations2006In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 33Article in journal (Refereed)
  • 100.
    Rosenqvist, Lisa
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Buchert, Stephan
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Opgenoorth, Hermann
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Lu, Gang
    Magnetospheric energy budget during huge geomagnetic activity using Cluster and ground-based data2006In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A10, article id A10211Article in journal (Refereed)
    Abstract [en]

    The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the European Incoherent Scatter (EISCAT) radars and magnetometers observed the ionosphere during a sequence of intense substorm-like geomagnetic activity in October 2003. We attempt to estimate the local and global energy flow from the magnetosheath into the magnetotail and the ionosphere under these extreme conditions. We make for the first time direct observational estimates of the local solar wind power input using Cluster measurements. The global power input based on Cluster observations was found to be between 17 and 40 TW at the onset of the substorm intensification. However, spacecraft observations and global modelling of the magnetotail suggest that it is most probably closer to 17 TW. This is more than two times lower than the predicted epsilon parameter value ( 37 TW). Energy deposition in the ionosphere has been estimated locally with EISCAT and globally with the assimilated mapping of ionospheric electrodynamics (AMIE) technique. The amount of the global solar wind power input ( 17 TW) that is dissipated via Joule heating in the ionosphere is found to be 30%. The corresponding ratio based on empirical estimates is only 3%. However, empirical proxies seem to underestimate the magnitude of the Joule heating rate as compared to AMIE estimates (similar to a factor 4) and the epsilon parameter is more than twice as large as the Cluster estimate. In summary, the observational estimates provide a good balance between the energy input to the magnetosphere and deposition in the ionosphere. Empirical proxies seem to suffer from overestimations ( epsilon parameter) and underestimations ( Joule heating proxies) when pushed to the extreme circumstances during the early main phase of this storm period.

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