A reexamination of the local time asymmetry of lobe encounters at geosynchronous orbit: CRRES, ATS 5, and LANL observations

Mark B. Moldwin, Mark I. Fernandez, Hamid K. Rassoul, Michelle F. Thomsen, Samuel J. Bame, David J. McComas, J. F. Fennell

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The local time of occurrence of a geosynchronous plasma flux dropout or lobe encounter is dependent on the longitude of the observing spacecraft. Satellites located on or west of the International Geomagnetic Reference Field (IGRF) model magnetic dip prime meridian (magnetic dip meridian) observe almost exclusively (77%) dawnside lobe encounters, while satellites located east of the magnetic dip meridian observe mostly duskside lobe encounters (67%). However, the transition from observing dawnside or duskside lobe encounters is not symmetric about the magnetic dip meridian. Three satellites (ATS 5, 1991-080, and 1990-095) located essentially at the magnetic dip meridian observed a majority (73%) of dawnside encounters. This study therefore concludes that there is a dawn-dusk asymmetry of the synchronous orbit altitude magnetic equatorial plane during active geomagnetic periods with the dawnside magnetosphere experiencing more radical distortions than the duskside. A case study of near-simultaneous lobe observations from CRRES and 1989-046 supports this conclusion. Note that it is found that the IGRF model dip prime meridian better organizes the local time occurrence frequency than does the magnetic dipole magnetic meridian, which has been shown to organize the seasonal dependence of geosynchronous lobe encounters in previous studies (i.e., Thomsen et al., 1994). Hence this study emphasizes the importance of considering the relative position of satellites with respect to both the magnetic dipole equator and the geomagnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere.

Original languageEnglish (US)
Article number97JA03728
Pages (from-to)9207-9216
Number of pages10
JournalJournal of Geophysical Research: Space Physics
Issue numberA5
StatePublished - 1998
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Space and Planetary Science


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