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Inverted-V and low-energy broadband electron acceleration features of multiple auroras within a large-scale surge
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: 0000-0003-1270-1616
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
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2013 (English)In: Journal of Geophysical Research: Space Physics, ISSN 2169-9380, Vol. 118, no 9, 5543-5552 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2013. Vol. 118, no 9, 5543-5552 p.
Keyword [en]
auroral acceleration region, quasi-static acceleration, Alfvenic acceleration, aurora surge
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-133652DOI: 10.1002/jgra.50517ISI: 000325483800014ScopusID: 2-s2.0-84889031018OAI: diva2:663305

QC 20131111

Available from: 2013-11-11 Created: 2013-11-08 Last updated: 2014-11-10Bibliographically 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
National Category
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, GöranKarlsson, TomasSadeghi, SoheilLindqvist, Per-Arne
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