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Altitude Distribution of the Auroral Acceleration Potential Determined from Cluster Satellite Data at Different Heights
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.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-1270-1616
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-5617-9765
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2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, no 5, 055002- p.Article in journal (Refereed) Published
Abstract [en]

Aurora, commonly seen in the polar sky, is a ubiquitous phenomenon occurring on Earth and other solar system planets. The colorful emissions are caused by electron beams hitting the upper atmosphere, after being accelerated by quasistatic electric fields at 1-2 RE altitudes, or by wave electric fields. Although aurora was studied by many past satellite missions, Cluster is the first to explore the auroral acceleration region with multiprobes. Here, Cluster data are used to determine the acceleration potential above the aurora and to address its stability in space and time. The derived potential comprises two upper, broad U-shaped potentials and a narrower S-shaped potential below, and is stable on a 5 min time scale. The scale size of the electric field relative to that of the current is shown to depend strongly on altitude within the acceleration region. To reveal these features was possible only by combining data from the two satellites.

Place, publisher, year, edition, pages
2011. Vol. 106, no 5, 055002- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-30986DOI: 10.1103/PhysRevLett.106.055002ISI: 000286878500008Scopus ID: 2-s2.0-79551585415OAI: oai:DiVA.org:kth-30986DiVA: diva2:404399
Note
QC 20110317Available from: 2011-03-17 Created: 2011-03-07 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Cluster multi-point observations of the auroral acceleration region
Open this publication in new window or tab >>Cluster multi-point observations of the auroral acceleration region
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. ix, 45 p.
Series
Trita-EE, ISSN 1653-5146 ; 2011:041
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-35500 (URN)978-91-7415-999-8 (ISBN)
Presentation
2011-05-31, AL seminarierum, Teknikringen 31, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20110701Available from: 2011-07-01 Created: 2011-07-01 Last updated: 2012-01-17Bibliographically approved
2. Multi-Spacecraft Observations of the Auroral Acceleration Region
Open this publication in new window or tab >>Multi-Spacecraft Observations of the Auroral Acceleration Region
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The two major agents for producing aurora are generally believed to be the quasi-static parallel electric fields, accelerating electrons in the auroral acceleration region (AAR), and Alfvén waves. The Cluster spacecraft quartet has made multi-spacecraft measurements in the AAR possible for the first time. Four event studies are included and discussed in this thesis, using Cluster data inside and at the top of the AAR, to address various open issues regarding the nature of the quasistatic electric potential structures, such as their altitude distribution, temporal and spatial variability, as well as their interactions with regions of Alfvén wave activity.

In Paper 1, Cluster data from the upper and lower parts of the AAR were used to determine the altitude and latitude distribution of the acceleration potential above the aurora and to address its stability in space and time. The acceleration potential pattern derived consisted of two equally broad and intense U-shaped electric potential structures and a narrower S-shaped potential structure located below. The acceleration potential distribution was found to be stable, both in intensity and width, over five minutes. Furthermore, the perpendicular (to the magnetic field) spatial scale of the electric field was found to be much smaller than that of the current in the lower AAR, but roughly equal in the upper AAR. Revealing of these features was possible only by combining data from the two Cluster spacecraft.

In Paper 2, the spatial and temporal characteristics and development of two AAR structures were studied, benefiting from a magnetic conjunction between two of the Cluster spacecraft and a short time difference (~1 min) between the spacecraft crossings. The configuration allowed for estimating the characteristic times of development for the two structures and the parallel electric field and potential drop for the more stable one. Potential structure 1, having a perpendicular width of ~80 km was short-lived, developing in less than 40 seconds and decaying in one minute. The parallel potential drop between 1.13 and 1.3 RE altitudes increased, whereas that above 1.3 RE remained almost unchanged during this time. For potential structure 2, having a width of ~50 km, the parallel potential drop increased by a factor of 3 below 1.3 RE during ~40 seconds, after which it remained stable for at least a minute. Also here, the parallel potential drop above 1.3 RE remained roughly unchanged. An average parallel electric field was estimated to be 0.56 mV/m.

In the auroral zone, the quasi-static properties are dominant in the plasma sheet (PS) and especially in the central plasma sheet (CPS), while the Alfvénic properties are generally strongest at the plasma sheet boundary layer (PSBL). Therefore it is of special interest to study the PSBL/CPS boundary, regarding how the two processes interact. In Papers 3 and 4 we addressed this matter. In each event, data from the Cluster fleet making an equatorward crossing of the highaltitude AAR were used together with a DMSP UV image of the oval. The particle and field data were used to infer the acceleration potentials of the observed arcs and their distribution in altitude and latitude. In Paper 3, observations demonstrate a quasi-static potential structure extending into the Alfvénic region of the polar cap boundary (PCB). The associated density cavity did not extend into the Alfvénic region, suggesting that the Alfvénic activity observed within the PCB region prevents the cavity formation. The results show that Alfvénic and quasi-static acceleration operate jointly in the PCB region. In Paper 4, Cluster passed over a system of East-West aligned auroral arcs. The upper extent of the AAR was found to be at least 0.5 RE higher in altitude than generally expected. Overlapping Alfvénic and quasi-static regions were found within the PSBL and inside Region 2 of downward currents. Growth of small-scale potential structures was observed in the PSBL and in the middle of Region 2, during the ~1.5 minutes between the Cluster 4 and Cluster 3 passages. During this period, the Alfvénic regions retreated both at the poleward and equatorward oval boundaries, while the quasi-static potentials intensified.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. x, 59 p.
Series
Trita-EE, ISSN 1653-5146 ; 2012:048
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-104472 (URN)978-91-7501-526-2 (ISBN)
Public defence
2012-11-21, Sal H1, Teknikringen 33, KTH, Stockholm, 13:00 (English)
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Supervisors
Note

QC 20121105

Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2012-12-04Bibliographically approved

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Marklund, Göran T.Karlsson, TomasLindqvist, Per-Arne

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