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| Titel |
The formation of transpolar arcs |
| VerfasserIn |
Robert Fear, Steve Milan |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052071
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| Zusammenfassung |
| Transpolar arcs are auroral features which form on the night side of the auroral
oval, and extend into the polar cap. They occur predominantly during intervals
of northward IMF (Troshichev et al., 1988, Valladares et al., 1994), and there is
some evidence for IMF BY control of the local time at which the arc initially forms
(Gussenhoven, 1982; Elphinstone et al., 1990; Makita et al., 1991; Kullen et al., 2002).
Milan et al. (2005) proposed that transpolar arcs might be related to magnetotail
reconnection following lengthy periods of dayside reconnection with a significant IMF BY
component. Such intervals of dayside reconnection lead to the presence of a cross-tail
component of the magnetic field in the magnetotail (Cowley, 1981), and subsequent
magnetotail reconnection results in fast eastward or westward ionospheric flows which are
asymmetric across midnight MLT (Grocott et al., 2003, 2004). Milan et al. argued
that transpolar arcs might be formed by the build-up of closed magnetic flux near
midnight MLT, where closed magnetic field lines have one foot in the pre-midnight
hemisphere, and the other post-midnight, and therefore their flow back toward the dayside
magnetosphere is frustrated. If the Milan et al. (2005) hypothesis is correct, then an
anticorrelation should be observed between IMF BY and the local time at which transpolar
arcs form, and fast ionospheric flows should be observed directed away from the
arc and across the midnight meridian. In this presentation, we show the results
of a statistical study of 209 transpolar arcs observed by the FUV cameras on the
IMAGE satellite between June 2000 and September 2005. Most of the events occur
when the IMF has a northward component. There is an anticorrelation between
the magnetic local time at which the arcs form and the IMF BY component in the
time leading up to the start of the arc; the anticorrelation is strongest when the
IMF is averaged over the 4 hours prior to the start of the event. Examination of
SuperDARN data also reveals that most of the events for which there is good ionospheric
scatter are also associated with fast eastward or westward ionospheric flows. These
observations are consistent with the mechanism proposed by Milan et al. (2005). |
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