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Ann. Geophys., 25, 989-999, 2007
www.ann-geophys.net/25/989/2007/
© European Geosciences Union 2007
Interhemispheric comparison of average substorm onset locations: evidence for deviation from conjugacy
H. Wang1,*, H. Lühr2, S. Y. Ma1, and H. U. Frey3
1College of Electronic Informatics, Wuhan University, Wuhan 430079, P. R. China
2GeoForshungsZentrum Potsdam, 14473 Potsdam, Germany
3Space Sciences Laboratory, University of California, Berkeley, USA
*now at: Department of Atmospheric, Oceanic and Space science, University of Michigan, Ann Arbor, MI 48105, USA
Abstract. Based on 2760 well-defined substorm onsets in the Northern
Hemisphere and 1432 in the Southern Hemisphere observed by the FUV
Imager on board the IMAGE spacecraft, a detailed statistical study
is performed including both auroral regions. This study focuses on
the hemispheric comparisons. Southward pointing interplanetary
magnetic field (IMF) is favorable for substorm to occur, but still
30% of the events are preceded by northward IMF. The magnetic
latitude (MLat) of substorm onset depends mainly on the merging
electric field (Em) with a relationship of |dMLat|=
−5.2 Em0.5, where dMLat is the deviation from onset MLat. In
addition, seasonal effects on onset MLat are also detected, with
about 2 degrees higher latitudes during solstices than equinoxes.
Both IMF By and solar illumination have a significant influence
on the magnetic local time (MLT) of onsets. An average relation,
dMLT=0.25 By between IMF By and the deviation from onset
MLT, was found. The By dependence varies slightly with the
onset latitude. At lower latitudes (higher activity) it is
reduced. After removal of the relationship with IMF By a linear
relationships remains between the solar zenith angle and onset MLT
with dMLT=1 min/deg. Therefore, both solar illumination and IMF
By can contribute to hemispheric longitudinal displacements of
substorm onset locations from conjugacy. No indications for
systematic latitudinal displacements between the hemispheres have
been found.
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