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Statistical altitude distribution of the auroral density cavity
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.
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
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.

Place, publisher, year, edition, pages
2015. Vol. 120, no 2, 996-1006 p.
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-155704DOI: 10.1002/2014JA020691ISI: 000351360800011Scopus ID: 2-s2.0-84924811630OAI: oai:DiVA.org:kth-155704DiVA: diva2:761995
Note

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

Available from: 2014-11-10 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically 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)
Opponent
Supervisors
Note

QC 20141110

Available from: 2014-11-10 Created: 2014-11-07 Last updated: 2014-11-10Bibliographically approved
2. Cluster investigations of the extent and altitude distribution of the auroral density cavity
Open this publication in new window or tab >>Cluster investigations of the extent and altitude distribution of the auroral density cavity
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The auroral density cavity constitutes the boundary between the cold, dense ionospheric plasma and the hot, tenuous plasma sheet plasma. The auroral density cavity is characterized by low electron density and particle populations modified by parallel electric fields. Inside the cavity the electron densities can be as much as a factor 100-1000 lower than same altitude outside the cavity.The Cluster mission's wide range of instruments, long lifetime and ability to make multi-spacecraft observations has been very successful. Over its 15 year lifespan, the Cluster satellites have gathered data on auroral density cavities over a large altitude range and throughout an entire solar cycle, providing a vast data material.The extent of the density cavity and acceleration region is large compared to the typical altitude coverage of a satellite crossing the cavity. This makes it difficult to produce a comprehensive altitude/density profile from a single crossing. In order to facilitate comparisons between data from different events, we introduce a new reference frame, pseudo altitude. Pseudo altitude describes the satellites' position relative to the acceleration region, as opposed to relative to the Earth. This pseudo altitude is constructed by dividing the parallel potential drop below the satellite with the total parallel potential drop. A pseudo altitude of 0 corresponds to the bottom of the acceleration region and a pseudo altitude of 1 to the top of the acceleration region. As expected, the pseudo altitude increases with altitude. The electron density exhibits an anti-correlation with the pseudo altitude, the density becomes lower close to the upper edge of the acceleration region. The upper edge of the acceleration region is located between a geocentric altitude of 4.375 and 5.625 RE. Above the upper edge of the acceleration region, the electron density continues to decrease for the entire range of the study, 3.0-6.5 RE. This is much further than the geocentric altitude range of 2-3 RE which is suggested by previous models. We can conclude that the auroral density cavity is not confined by the auroral acceleration region, as suggested by previous models, and may extend all the way to the plasma sheet.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xi, 58 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:100
Keyword
Auroral density cavity, auroral accelaration, Cluster, in situ observation, electron density, pseudo altitude
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-176285 (URN)978-91-7595-729-6 (ISBN)
Public defence
2015-11-20, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20151102

Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2015-11-02Bibliographically approved

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Alm, LoveMarklund, GöranKarlsson, Tomas

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