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Cluster multipoint study of the acceleration potential pattern and electrodynamics of an auroral surge and its associated horn arc
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.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
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2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, no 10, A10223- p.Article in journal (Refereed) Published
Abstract [en]

Cluster results are presented from the acceleration region of an auroral surge and connected horn arc, observed during an extended time period of substorm activity. The Cluster spacecraft crossed different magnetic local time (MLT) sectors of the surge and horn, with lag times of 2-10 min. Acceleration potential patterns are derived for the horn arc and for the double arc (surge and horn) at the surge front and deeper into the surge. The parallel potential drop of the horn arc ranged between 4 and 7 kV. At the surge front, two weakly coupled U-potentials with parallel potential drops of 8 (7) kV and 7 (5) kV were derived for the surge and horn, respectively, from the C3 (C4) data. A similar, more coupled pattern was derived for the region deeper into the surge. We also address how the field-aligned currents of the surge and horn system close in the ionosphere. The Cluster data allow almost simultaneous estimates of the latitudinal current closure at various MLT sectors. Significant net upward currents are derived for the horn and surge, whereas the currents at the surge front were found to be balanced. The net upward horn current is proposed to be fed by the zonal divergence of the westward Pedersen current in the horn, consistent with the acceleration potential decrease in the westward horn direction. The net upward surge current is proposed to be fed by the divergence of a westward electrojet and by localized downward currents adjacent to the surge.

Place, publisher, year, edition, pages
2012. Vol. 117, no 10, A10223- p.
Keyword [en]
Parallel Electric-Fields, Particle-Acceleration, Nighttime Sector, Current Region, Double-Layers, Black Aurora, Substorms, Magnetosphere, Ionosphere, Plasma
National Category
Geophysics
Identifiers
URN: urn:nbn:se:kth:diva-105654DOI: 10.1029/2012JA018046ISI: 000310084600001Scopus ID: 2-s2.0-84868024343OAI: oai:DiVA.org:kth-105654DiVA: diva2:571874
Note

QC 20121126

Available from: 2012-11-26 Created: 2012-11-23 Last updated: 2017-12-07Bibliographically 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.
Series
TRITA-EE, ISSN 1653-5146 ; 2014:058
Keyword
Cluster, Aurora acceleration region
National Category
Geophysics
Research subject
Physics
Identifiers
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)
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Supervisors
Note

QC 20141110

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

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