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Statistical altitude distribution of Cluster auroral electric fields, indicating mainly quasi-static acceleration below 2.8 R-E and Alfvenic above
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0002-1594-1861
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-6997-7037
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-1270-1616
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2014 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 11, 8984-8991 p.Article in journal (Refereed) Published
Abstract [en]

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

Place, publisher, year, edition, pages
2014. Vol. 119, no 11, 8984-8991 p.
Keyword [en]
quasi-static acceleration, Alfvénic acceleration, Statistical study, auroral electric fields
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Other Physics Topics
URN: urn:nbn:se:kth:diva-155703DOI: 10.1002/2014JA020225ISI: 000346792100019OAI: diva2:761994
Swedish National Space Board

QC 20150130

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2015-01-30Bibliographically approved
In thesis
1. Cluster in situ studies of the auroral acceleration region
Open this publication in new window or tab >>Cluster in situ studies of the auroral acceleration region
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis addresses a central topic in auroral physics, namely particle accelerationproducing intense aurora as well as energetic plasma outflow. Cluster satellitemeasurements of electric and magnetic fields, electrons and ions, collected across auroralfield lines, are used to study various aspects of the quasi-static auroral accelerationregion (AAR), its relation to the auroral density cavity, and the relative role of quasistaticand Alfvénic acceleration for producing aurora.The acceleration potential structures and electro-dynamical features of a large-scaleauroral surge is studied based on data from the Cluster satellites, crossing differentmagnetic local time (MLT) sectors of a surge-horn system. This allows snapshots of theacceleration potential structure and of the current systems to be provided, including thefield-aligned current closure for the different segments of the surge-horn aurora.The relative role of quasi-static and Alfvénic acceleration for producing auroral arcs isaddressed for the case of a large-scale substorm surge, crossed by the Cluster C2 satellite. Thetwo contributions to the downward electron energy flux is estimated for each of the smallerscalearc structures crossed by C2 within and adjacent to the large-scale surge. For these, thequasi-static acceleration typically dominates, except for the polar cap boundary arc, and in thesurge head, where the Alfvénic contribution is significant.The occurrence of intense electric fields and associated plasma densities versus altitude andMLT is the subject of a statistical study based on 9.5 years of Cluster data, collected ataltitudes between 2 and 4 RE. Intense arc-associated electric fields occur in two altituderegions, separated by a gap around 2.8 RE. The low-altitude fields are interpreted as mainlyquasi-static and the high-altitude fields as mainly Alfvénic. The results which are supportedby estimates of the (ΔE/ΔB)/VA ratio, indicate that, on the average, the quasi-static fieldsextend up to 2.6 RE, above which a transition to Alfvénic fields occur.The auroral density cavity, intimately associated with the auroral acceleration process, wasthe subject of a statistical study based on Cluster data, collected between 2002 and 2007, ataltitudes between 2.0 RE and 5.5 RE. Decreasing electron densities are observed between 2 and 3.3 RE, and between 4.6 and 5.5 RE, corresponding to climbing the parallel potential hillof the AAR. Furthermore, the density is found to decrease while ascending above the AAR,indicating that the cavities are not necessarily confined by it.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 41 p.
TRITA-EE, ISSN 1653-5146 ; 2014:058
Cluster, Aurora acceleration region
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Research subject
urn:nbn:se:kth:diva-155668 (URN)978-91-7595-334-2 (ISBN)
Public defence
2014-11-25, Sal F3, Lindstedtsvägen 26, kTH, Stockholm, 14:00 (English)

QC 20141110

Available from: 2014-11-10 Created: 2014-11-07 Last updated: 2014-11-10Bibliographically approved

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Li, BinMarklund, GöranAlm, LoveKarlsson, TomasLindqvist, Per-Arne
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