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Pseudo altitude: A new perspective on 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.ORCID iD: 0000-0002-1594-1861
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-1270-1616
2013 (English)In: Journal of Geophysical Research A: Space Physics, ISSN 2169-9380, Vol. 118, no 7, 4341-4351 p.Article in journal (Refereed) Published
Abstract [en]

Studying the density distribution inside the auroral density cavity is complicated by the difficulties in achieving simultaneous measurements within the same flux tube at different altitudes. Comparisons between different events are complicated by variations in both the location of the density cavity and the location of the related potential structure. Describing the spacecraft's location inside the density cavity relative to the potential structure instead of the Earth offers a more practical and consistent frame of reference, a pseudo altitude. The pseudo altitude is determined by comparing the potential drop above the spacecraft, as determined from the characteristic energy of the downward electrons, with the parallel potential drop below the spacecraft, determined from the characteristic energy of the upward ions. A pseudo altitude of 0 corresponds to the bottom of the potential structure and a pseudo altitude of 1 to the top of the structure. Seven events from 2008 were selected, each of which corresponds to a Cluster crossing of a mainly quasi-static potential structure. All of the events exhibit a consistent anticorrelation between the pseudo altitude and the electron density. No upper limit of the density cavity can be observed, while all cavities have a lower limit above a pseudo altitude of 0.33. These observations show that the auroral density cavity is predominately concentrated to the upper parts of the quasi-static potential structure.

Place, publisher, year, edition, pages
2013. Vol. 118, no 7, 4341-4351 p.
Keyword [en]
auroral density cavity, pseudo altitude, electron density, cluster satellites
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-133552DOI: 10.1002/jgra.50408ISI: 000325073600036Scopus ID: 2-s2.0-84882778991OAI: oai:DiVA.org:kth-133552DiVA: diva2:662447
Note

QC 20131107

Available from: 2013-11-07 Created: 2013-11-06 Last updated: 2015-11-02Bibliographically approved
In thesis
1. 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, Göran T.Karlsson, Tomas

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