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Investigations of auroral electric fields and currents
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The Cluster spacecraft have been used to investigate auroral electric fields and field-aligned currents (FACs) at geocentric distances between 4 and 7 Re. The electric fields have been measured by the EFW instrument, consisting of two pairs of spherical probes, and the FACs have been calculated from measurements of the magnetic field by the FGM fluxgate magnetometer. CIS ion and PEACE electron measurements have also been used. Event studies as well as statistical studies have been used to determine the characteristics of the auroral electric fields. In two events where regions of both spatial and temporal electric field variations could be identified, the quasi-static electric fields were, compared to the Alfvén waves, found to be more intense and contribute more to the downward Poynting flux. With the use of the four Cluster spacecraft, the quasi-static electric field structures were found to be relatively stable on the time scale of at least half a minute. Quasi-static electric fields were found throughout the altitude range covered by Cluster in the auroral region. The electric field structures were found both in the upward and downward current regions. Bipolar and monopolar electric fields, corresponding to U- and S-shaped potential structures, have been found at different plasma boundaries, consistent with the view that the plasma conditions and the geometry of the current system are related to the shape of the electric field. The type of the bipolar electric field structures (convergent or divergent) was further found to be consistent with the FAC direction. The typical scale sizes of the electric field structures have been determined to be between 4 and 5 km, when mapped to ionospheric altitude. The most intense FACs associated with intense electric fields were found for small FAC widths. The widths of upward and downward FACs were similar.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , vii, 48 p.
Series
Trita-EE, ISSN 1653-5146 ; 2007:056
Keyword [en]
auroral physics, auroral electric fields, auroral potential structures, auroral particle acceleration
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-4512ISBN: 978-91-7178-761-3 (print)OAI: oai:DiVA.org:kth-4512DiVA: diva2:12624
Public defence
2007-11-09, F3, Lindstedsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100730Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2010-07-30Bibliographically approved
List of papers
1. Intense high-altitude auroral electric fields: temporal and spatial characteristics
Open this publication in new window or tab >>Intense high-altitude auroral electric fields: temporal and spatial characteristics
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2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 7, 2485-2495 p.Article in journal (Refereed) Published
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.

Keyword
Magnetospheric physics (electric fields; auroral phenomena; magnetosphere-ionosphere interactions); electric field; magnetic field; magnetosphere
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7556 (URN)10.5194/angeo-22-2485-2004 (DOI)000223620300020 ()2-s2.0-4344694040 (Scopus ID)
Note
QC 20100730. Meeting on Spatio-Temporal Analysis and Multipoint Measurements in Space. Orleans, FRANCE. MAY 12-16, 2003 Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2017-12-14Bibliographically approved
2. A statistical study of intense electric fields at 4-7 R-E geocentric distance using Cluster
Open this publication in new window or tab >>A statistical study of intense electric fields at 4-7 R-E geocentric distance using Cluster
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2005 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 7, 2579-2588 p.Article in journal (Refereed) Published
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).

Keyword
Magnetospheric physics (Electric fields; Auroral phenomena; Magnetosphere-ionosphere interactions)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7557 (URN)10.5194/angeo-23-2579-2005 (DOI)000233568900028 ()2-s2.0-27844562666 (Scopus ID)
Note
QC 20100730Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2017-12-14Bibliographically approved
3. On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries
Open this publication in new window or tab >>On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries
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2006 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, no 6, 1713-1723 p.Article in journal (Refereed) Published
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.

Keyword
aurora; boundary layer; electric field; magnetosphere; plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7558 (URN)10.5194/angeo-24-1713-2006 (DOI)000239724500020 ()2-s2.0-33751071835 (Scopus ID)
Note
QC 20100730Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2017-12-14Bibliographically approved
4. Cluster observations of an auroral potential and associated field-aligned current reconfiguration during thinning of the plasma sheet boundary layer
Open this publication in new window or tab >>Cluster observations of an auroral potential and associated field-aligned current reconfiguration during thinning of the plasma sheet boundary layer
2007 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no 1, 10.1029/2006JA011804- p.Article in journal (Refereed) Published
Abstract [en]

Cluster observations are used to illustrate the reconfiguration of an auroral potential structure encountered at the poleward boundary of the central plasma sheet within the Southern Hemisphere premidnight auroral oval. The reconfiguration from a symmetric U shape to an asymmetric S shape takes place between two consecutive crossings by Cluster spacecraft 1 and 2, moving along roughly the same orbits and separated in time by 16 min. During this time the plasma conditions poleward of the boundary changed dramatically. The fluxes of energetic electrons decreased, as did the intensities of the associated small-scale field-aligned currents (FACs) and the ambient plasma density. These changes were particularly pronounced in a narrow region adjacent to the boundary. The reconfiguration of the potential structure, and of the associated FAC system consistent with this, are consistent with the predictions by Marklund et al. (2004).

Keyword
PARALLEL ELECTRIC-FIELDS; RETURN CURRENT REGION; PARTICLE-ACCELERATION; BLACK AURORA; MAGNETOSPHERE; EVOLUTION; ARCS
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7559 (URN)10.1029/2006JA011804 (DOI)000243532100001 ()2-s2.0-34547935961 (Scopus ID)
Note
QC 20100730Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2017-12-14Bibliographically approved
5. Scale sizes of intense auroral electric fields observed by Cluster
Open this publication in new window or tab >>Scale sizes of intense auroral electric fields observed by Cluster
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2007 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 25, no 11, 2413-2425 p.Article in journal (Refereed) Published
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.

Keyword
altitude; aurora; cluster analysis; electric field; electrical conductivity; geomagnetic field; ionosphere; scale effect; size; winter
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-7560 (URN)10.5194/angeo-25-2413-2007 (DOI)000251998900013 ()2-s2.0-36749036009 (Scopus ID)
Note
QC 20100730. Uppdaterad från Submitted till Published 20100730.Available from: 2007-10-22 Created: 2007-10-22 Last updated: 2017-12-14Bibliographically approved

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Citation style
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