Statistical study of the 'instantaneous' nightside auroral oval: The equatorward boundary of electron precipitation as observed by the Isis 1 and 2 satellites
Electron spectrograms from 351 passes of the Isis 1 and 2 satellites were utilized to study statistically the effects of the interplanetary magnetic field (IMF), substorm activity, and the earth's dipole tilt angle on the latitude of the equatorward boundary of the nightside (2000--0400 magnetic local time) 'instantaneous' auroral oval. The boundary location (in invariant latitude) of the instantaneous oval at hourly local time intervals was identified in terms of the equatorward boundary of the diffuse >100-eV electron precipitation. The following characteristics were noted: (1) The north-south component (B/sub z/) of the IMF plays the dominant role in controlling the motion of this boundary. The invariant latitude of the boundary is shown to shift by approximately +- 4/sup 0/ depending on the direction of the IMF (northward and southward, respectively) relative to its position corresponding to B/sub z/=0. This indicates an inward motion of the associated boundary in the magnetotail by about 5 earth radii when the IMF changes its direction from northward to southward with large magnitude. There is a significant difference in the amount of the shift between the evening and morning sectors: i.e., for the same decrease in B/sub z/ value the boundary moves more equatorward in the morning sector than in the evening sector. When the obtained oval particle boundary was projected onto the equatorial plane of the magnetotail along magnetic field lines, good agreement was found between the projected boundary and the drift boundary (the Alfven layer) of low-energy electrons in the presence of the dawn-dusk electric field. (2) Substorm activity seems to have a separate role in determining the latitude of the equatorward boundary of the nightside auroral precipitation region.
- Research Organization:
- Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA, Boulder, Colorado 80302
- OSTI ID:
- 5330832
- Journal Information:
- J. Geophys. Res.; (United States), Vol. 82:35
- Country of Publication:
- United States
- Language:
- English
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