Abstract
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.
Highlights
Energies from a few keV to tens of keVs within the discrete aurora indicate energization of the magnetospheric particles before they precipitate into the ionosphere
At the bottom of the potential drop a field-aligned electric field accelerates electrons towards the ionosphere and ions out of the ionosphere resulting in an upward field-aligned current (FAC)
The relation between field-aligned current j|| and the potential drop V can be described by the equation j||=K(V +TE/e) (e.g. Janhunen, 1999), where K is an experimental parameter, often referred to as the field-aligned conductance, TE is the electron temperature in the source region, and e is the electron charge
Summary
Energies from a few keV to tens of keVs within the discrete aurora indicate energization of the magnetospheric particles before they precipitate into the ionosphere. Based on a few events they concluded that inverted-V’s are substructures occurring during moderate magnetic activity with the Kp index ranging from 3 to 4+ They found out that the maximum accelerated energy of these structures is of the order of a keV, and their typical duration in the energytime spectrogram is 30–50 s, which corresponds to a distance of about 150–250 km. Bosqued et al (1985) reported the first statistical study of about 250 inverted-V events as detected by the AUREOL-3 satellite According to their results, the typical width of the V structures is 50–100 km and their peak energies are 1–5 keV.
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