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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, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.ORCID-id: 0000-0002-1594-1861
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.ORCID-id: 0000-0001-6997-7037
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.ORCID-id: 0000-0003-1270-1616
Visa övriga samt affilieringar
2014 (Engelska)Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 11, s. 8984-8991Artikel i tidskrift (Refereegranskat) 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).

Ort, förlag, år, upplaga, sidor
2014. Vol. 119, nr 11, s. 8984-8991
Nyckelord [en]
quasi-static acceleration, Alfvénic acceleration, Statistical study, auroral electric fields
Nationell ämneskategori
Annan fysik
Identifikatorer
URN: urn:nbn:se:kth:diva-155703DOI: 10.1002/2014JA020225ISI: 000346792100019Scopus ID: 2-s2.0-84995695217OAI: oai:DiVA.org:kth-155703DiVA, id: diva2:761994
Forskningsfinansiär
Rymdstyrelsen
Anmärkning

QC 20150130

Tillgänglig från: 2014-11-10 Skapad: 2014-11-10 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
Ingår i avhandling
1. Cluster in situ studies of the auroral acceleration region
Öppna denna publikation i ny flik eller fönster >>Cluster in situ studies of the auroral acceleration region
2014 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2014. s. x, 41
Serie
TRITA-EE, ISSN 1653-5146 ; 2014:058
Nyckelord
Cluster, Aurora acceleration region
Nationell ämneskategori
Geofysik
Forskningsämne
Fysik
Identifikatorer
urn:nbn:se:kth:diva-155668 (URN)978-91-7595-334-2 (ISBN)
Disputation
2014-11-25, Sal F3, Lindstedtsvägen 26, kTH, Stockholm, 14:00 (Engelska)
Opponent
Handledare
Anmärkning

QC 20141110

Tillgänglig från: 2014-11-10 Skapad: 2014-11-07 Senast uppdaterad: 2014-11-10Bibliografiskt granskad

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Marklund, GöranAlm, LoveKarlsson, TomasLindqvist, Per-Arne

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