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Cluster in situ studies of the auroral acceleration region
KTH, School of Electrical Engineering (EES), Space and Plasma Physics. (Physical Electrotechnology)
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 [en]
Cluster, Aurora acceleration region
National Category
Geophysics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-155668ISBN: 978-91-7595-334-2 (print)OAI: oai:DiVA.org:kth-155668DiVA: diva2:761703
Public defence
2014-11-25, Sal F3, Lindstedtsvägen 26, kTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20141110

Available from: 2014-11-10 Created: 2014-11-07 Last updated: 2014-11-10Bibliographically approved
List of papers
1. Cluster multipoint study of the acceleration potential pattern and electrodynamics of an auroral surge and its associated horn arc
Open this publication in new window or tab >>Cluster multipoint study of the acceleration potential pattern and electrodynamics of an auroral surge and its associated horn arc
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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.

Keyword
Parallel Electric-Fields, Particle-Acceleration, Nighttime Sector, Current Region, Double-Layers, Black Aurora, Substorms, Magnetosphere, Ionosphere, Plasma
National Category
Geophysics
Identifiers
urn:nbn:se:kth:diva-105654 (URN)10.1029/2012JA018046 (DOI)000310084600001 ()2-s2.0-84868024343 (Scopus ID)
Note

QC 20121126

Available from: 2012-11-26 Created: 2012-11-23 Last updated: 2017-12-07Bibliographically approved
2. Inverted-V and low-energy broadband electron acceleration features of multiple auroras within a large-scale surge
Open this publication in new window or tab >>Inverted-V and low-energy broadband electron acceleration features of multiple auroras within a large-scale surge
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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.

Keyword
auroral acceleration region, quasi-static acceleration, Alfvenic acceleration, aurora surge
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-133652 (URN)10.1002/jgra.50517 (DOI)000325483800014 ()2-s2.0-84889031018 (Scopus ID)
Note

QC 20131111

Available from: 2013-11-11 Created: 2013-11-08 Last updated: 2014-11-10Bibliographically approved
3. Statistical altitude distribution of Cluster auroral electric fields, indicating mainly quasi-static acceleration below 2.8 R-E and Alfvenic above
Open this publication in new window or tab >>Statistical altitude distribution of Cluster auroral electric fields, indicating mainly quasi-static acceleration below 2.8 R-E and Alfvenic above
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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).

Keyword
quasi-static acceleration, Alfvénic acceleration, Statistical study, auroral electric fields
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-155703 (URN)10.1002/2014JA020225 (DOI)000346792100019 ()2-s2.0-84995695217 (Scopus ID)
Funder
Swedish National Space Board
Note

QC 20150130

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved
4. Statistical altitude distribution of the auroral density cavity
Open this publication in new window or tab >>Statistical altitude distribution of the auroral density cavity
2015 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 2, 996-1006 p.Article in journal (Refereed) Published
Abstract [en]

The statistical altitude distribution of auroral density cavities located between 3.0 and 6.5 R-E is investigated using in situ observations from flux tubes exhibiting auroral acceleration. The locations of the observations are described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellite. The upper edge of the auroral acceleration region is observed between 4.375 and 5.625 R-E. Above 6.125 R-E, none of the events exhibit precipitating inverted V electrons, though the upward ion beam can be observed. This indicates that the satellites are located inside the same flux tube as, but above, the auroral acceleration region. The electron density decreases as we move higher into the acceleration region. The spacecraft potential continues to decrease once above the acceleration region, indicating that the density cavity extends above the acceleration region. From 3.0 to 4.375 R-E the pseudo altitude increases by 0.20 per R-E, consistent with a distributed parallel electric field. Between 4.375 and 5.625 R-E the pseudo altitude increases weakly, by 0.01 per R-E, due to an increasing number of events per altitude bin, which are occurring above the acceleration region. Above 5.625 R-E the pseudo altitude increases by 0.28 per R-E, due to a rapid increase in the number of events per altitude bin occurring above the acceleration region, indicating that the remaining parallel potential drop is concentrated in a narrow region at the upper edge of the acceleration region, rather than in a distributed parallel electric field.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-155704 (URN)10.1002/2014JA020691 (DOI)000351360800011 ()2-s2.0-84924811630 (Scopus ID)
Note

Updated from "Manuscript" to "Article". QC 20150420

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved

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