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Multi-Spacecraft Observations of the Auroral Acceleration Region
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
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: urn:nbn:se:kth:diva-104472ISBN: 978-91-7501-526-2 (print)OAI: oai:DiVA.org:kth-104472DiVA: diva2:564812
Public defence
2012-11-21, Sal H1, Teknikringen 33, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20121105

Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2012-12-04Bibliographically approved
List of papers
1. Altitude Distribution of the Auroral Acceleration Potential Determined from Cluster Satellite Data at Different Heights
Open this publication in new window or tab >>Altitude Distribution of the Auroral Acceleration Potential Determined from Cluster Satellite Data at Different Heights
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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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-30986 (URN)10.1103/PhysRevLett.106.055002 (DOI)000286878500008 ()2-s2.0-79551585415 (Scopus ID)
Note
QC 20110317Available from: 2011-03-17 Created: 2011-03-07 Last updated: 2017-12-11Bibliographically approved
2. Spatiotemporal features of the auroral acceleration region as observed by Cluster
Open this publication in new window or tab >>Spatiotemporal features of the auroral acceleration region as observed by Cluster
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2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, no 12, A00K19- p.Article in journal (Refereed) Published
Abstract [en]

A pair of negative electric potential structures associated with inverted-V aurora is investigated using electric and magnetic field, ion and electron data from the Cluster spacecraft, crossing the auroral acceleration region (AAR) at different altitudes above the Northern hemisphere midnight auroral oval. The spatial and temporal development of the acceleration structures is studied, given the magnetic conjunction opportunity and the one minute difference between the Cluster spacecraft crossings. The configuration allowed for estimation of characteristic times of development for the two structures and of the parallel electric field and potential drop for the more stable one. The first potential structure had a width of similar to 80 km (projected to the ionosphere) and was relatively short-lived, developing in less than 40 s and decaying in one minute. The parallel potential drop increased between altitudes of 1.13 R(E) and 1.3 R(E), whereas the acceleration potential above 1.3 R(E) remained almost unchanged during that time. This intensification occurred mainly after the time when the associated upward current had reached its maximum value. The second structure had a width of similar to 50 km and was subject to an increase by a factor of 3 of the parallel potential drop below 1.3 R(E), during about 40 s, after which it remained rather stable for one minute or more. Similarly here, the acceleration potential above 1.3 R(E) remained roughly unchanged. For the more stable second structure, an average parallel electric field between 1.13 and 1.3 R(E) could be estimated (similar to 0.56 mV/m). The conductance along the flux tube was also stable for one minute or more.

Keyword
Aurora, Quasi-static potential structures, Magnetosphere-Ionospher interaction, field-aligned currents, FAC, auroral acceleration region, AAR
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-35499 (URN)10.1029/2011JA016505 (DOI)000297979400001 ()2-s2.0-83455201438 (Scopus ID)
Note
Updated from submitted to published. Previous title: Spatio-temporal features of the auroral acceleration region as observed by Cluster. QC 20120117Available from: 2011-07-01 Created: 2011-07-01 Last updated: 2017-12-11Bibliographically approved
3. Evolution in space and time of the quasi-static acceleration potential of inverted-V aurora and its interaction with Alfvenic boundary processes
Open this publication in new window or tab >>Evolution in space and time of the quasi-static acceleration potential of inverted-V aurora and its interaction with Alfvenic boundary processes
Show others...
2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, A00K13- p.Article in journal (Refereed) Published
Abstract [en]

Results are presented from Cluster crossings of the acceleration region of two inverted-V auroras located in the poleward part of an extensive substorm bulge. The particle and field data are used to infer the acceleration potentials of the arcs and their distribution in altitude and latitude. The C1 data are consistent with a symmetric potential pattern, composed of two negative U potentials and one positive U potential in between, and the C3 and C4 data are consistent with an asymmetric pattern, where the dominating potential structure extends deep into the polar cap boundary (PCB) region. The two patterns may either correspond to different stages of evolution of the same double arc system or represent two longitudinally separated double arc systems. For all spacecraft, the potential well of the poleward arc extends into the PCB region, whereas the density cavity does not but remains confined to R1. This suggests that the Alfvenic activity observed within the PCB region prevents the cavity formation, consistent with the associated FACs being roughly balanced over this region. The results show that Alfvenic and quasi-static acceleration operates jointly in the PCB region, varying from being about equally important (on C1) to being predominantly quasi-static (on C3/C4). The presence (absence) of an upward electron beam, associated with a positive potential structure and a downward current, observed by C1 (C4/C3) is expected from its short life time, shorter than the time lag between the Cluster spacecraft. The evolution involves both a broadening and a density reduction of the associated downward current sheet to below the critical current density above which parallel electric fields will form. The deepest potential well of 13 kV observed by C4 was located in Region 1, adjacent to the PCB region and coinciding with the deepest density cavity, with a minimum density of 0.1 cm(-3). The interface between Region 1 and the PCB region, coinciding with the steep density gradient, appears to be the leading edge of the cavity.

Keyword
PARALLEL ELECTRIC-FIELDS, PARTICLE-ACCELERATION, CURRENT REGION, DOUBLE-LAYERS, BLACK AURORA, MAGNETOSPHERE, IONOSPHERE, PLASMA
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-41293 (URN)10.1029/2011JA016537 (DOI)000294622600001 ()2-s2.0-80052537367 (Scopus ID)
Note
QC 20110928Available from: 2011-09-28 Created: 2011-09-26 Last updated: 2017-12-08Bibliographically approved
4. Cluster Observations of Quasi-Static Potential Structures Overlapping with Alfvénic Regions
Open this publication in new window or tab >>Cluster Observations of Quasi-Static Potential Structures Overlapping with Alfvénic Regions
Show others...
2012 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202Article in journal (Other academic) Submitted
Abstract [en]

Results are presented from an equatorward crossing of the Cluster spacecraft through the high-altitude auroral acceleration region, over a system of East-West aligned auroral arcs in the Southern hemisphere auroral oval. The event occurred during quiet geomagnetic conditions during the expansion phase of a weak substorm. Acceleration potential profiles of the quasi-static structures are determined from the particle and field data. The observations reveal a dynamically developing system of small-scale and large-scale quasi-static structures overlapping with Alfvénic regions. Such overlaps are found within the PSBL and inside the Region 2 of downward currents. No density cavities are seen in the overlap regions. Growths of small-scale potential structures were 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, and the large-scale quasi-static potentials intensified.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-104471 (URN)
Note

QS 2012

Available from: 2012-11-05 Created: 2012-11-05 Last updated: 2017-12-07Bibliographically approved

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