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Electron density and parallel electric field 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.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, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 120, no 11, 9428-9441 p.Article in journal (Refereed) Published
Abstract [en]

We present an event study in which Cluster satellites C1 and C3 encounters the flux tube of a stable auroral arc in the pre-midnight sector. C1 observes the mid cavity, while C3 enters the flux tube of the auroral arc at an altitude which is below the acceleration region, before crossing into the top half of the acceleration region. This allows us to study the boundary between the ionosphere and the density cavity, as well as large portion of the upper density cavity. The position of the two satellites, in relation to the acceleration region, is described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellites.The electron density exhibits an anti-correlation with the pseudo altitude, indicating that the lowest electron densities are found near the top of the density cavity. Over the entire pseudo altitude range, the electron density distribution is similar to a planar sheath, formed out of a plasma sheet dominated electron distribution, in response to the parallel electric field of the acceleration region. This indicates that the parallel electric fields on the ionosphere-cavity boundary, as well as the mid cavity parallel electric fields, are part of one unified structure rather than two discrete entities.The results highlight the strong connection between the auroral density cavity and auroral acceleration as well as the necessity of studying them in a unified fashion.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2015. Vol. 120, no 11, 9428-9441 p.
Keyword [en]
Auroral density cavity
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-176281DOI: 10.1002/2015JA021593ISI: 000368252100014Scopus ID: 2-s2.0-84954399402OAI: oai:DiVA.org:kth-176281DiVA: diva2:866362
Note

QC 20160216

Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2017-12-01Bibliographically 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, GöranKarlsson, Tomas

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