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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 26, issue 6
Ann. Geophys., 26, 1439–1449, 2008
https://doi.org/10.5194/angeo-26-1439-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 26, 1439–1449, 2008
https://doi.org/10.5194/angeo-26-1439-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  11 Jun 2008

11 Jun 2008

Statistical study of inverted-V structures in FAST data

N. Partamies1, E. Donovan2, and D. Knudsen2 N. Partamies et al.
  • 1Finnish Meteorological Institute, Helsinki, Finland
  • 2Institute for Space Research, University of Calgary, Calgary, Canada

Abstract. Since the 1970s the inverted-V structures have been observed in the satellite-borne particle data. These Λ-shaped enhancements of the differential energy flux on the high energy channels on the top of low number flux on the lower energies are traces of auroral particle acceleration by U-shaped potential drops along the field lines within the auroral zones. In this study, we show the results of a statistical study of inverted-V events in five years (1997–2001) of the Fast Auroral SnapshoT (FAST) electron spectrograms. We found 588 well-defined events in total, but the statistical results are also shown for a subset of 97 very best events.

The statistical analysis shows that the inverted-V's mainly occur within the auroral oval latitudes (65°–75° invariant latitude), in the pre-midnight sector (21–23 magnetic local time) and during the Northern Hemisphere winter months (November to February). The typical maximum energies of the inverted-V's are 2–4 keV, when a threshold value of 0.3 keV is used as a selection criterion. This corresponds to an acceleration potential drop of a few kV. The typical peak energy tends to increase towards the magnetic midnight. Most of the inverted-V events occur during magnetically quiet or moderate activity, Kp 1–4. The usual width of an inverted-V varies from 20 to 40 km and is comparable to the peak value of the typical meso-scale auroral arc width of 10–20 km. Furthermore, the polar cap potential values estimated from the SuperDARN radar data are always at least three times larger than the inverted-V acceleration potential drop.

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