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| Titel |
The occurrence frequency of auroral potential structures and electric fields as a function of altitude using Polar/EFI data |
| VerfasserIn |
P. Janhunen  , A. Olsson, H. Laakso |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 22, no. 4 ; Nr. 22, no. 4 (2004-04-02), S.1233-1250 |
| Datensatznummer |
250014836
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| Publikation (Nr.) |
 copernicus.org/angeo-22-1233-2004.pdf |
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| Zusammenfassung |
The aim of the paper is to study how auroral potential structures
close at high altitude. We analyse all electric field data
collected by Polar on auroral field lines in 1996–2001 by integrating
the electric field along the spacecraft orbit to obtain the plasma
potential, from which we identify potential minima by an automatic
method. From these we estimate the associated effective mapped-down
electric field Ei, defined as the depth of the potential minimum
divided by its half-width in the ionosphere. Notice that although we
use the ionosphere as a reference altitude, the field Ei does not
actually exist in the ionosphere but is just a convenient
computational quantity. We obtain the statistical distribution of
Ei as a function of altitude, magnetic local time (MLT), Kp index
and the footpoint solar illumination condition. Surprisingly, we find
two classes of electric field structures. The first class consists of
the low-altitude potential structures that are presumably associated with
inverted-V regions and discrete auroral arcs and their set of
associated phenomena. We show that the first class exists only below
~3RE radial distance, and it occurs in all nightside MLT
sectors (RE=Earth radius). The second class exists only above
radial distance R=4RE and almost only in the midnight MLT sector,
with a preference for high Kp values. Interestingly, in the middle
altitudes (R=3–4RE) the number of potential minima is small,
suggesting that the low and high altitude classes are not simple
field-aligned extensions of each other. This is also underlined by
the fact that statistically the high altitude structures seem to be
substorm-related, while the low altitude structures seem to correspond
to stable auroral arcs. The new finding of the existence of the two
classes is important for theories of auroral acceleration, since it
supports a closed potential structure model for stable arcs, while
during substorms, different superposed processes take place that
are associated with the disconnected high-altitude electric field structures.
Key words. Magnetospheric physics (electric fields; auroral
phenomena) – Space plasma physics (electrostatic structures) |
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