A REEXAMINATION OF THE LOCAL TIME ASYMMETRY OF LOBE ENCOUNTERS AT GEOSYNCHRONOUS ORBIT - CRRES, ATS-5, AND LANL OBSERVATIONS

The local time of occurrence of a geosynchronous plasma nux dropout or lobe encounter is dependent on the longitude of the observing spacecr aft. Satellites located on or west of the International Geomagnetic Re ference Field (IGRF) model magnetic dip prime meridian (magnetic dip m eridian) observe almost exclusively (77%) dawnside lobe encounters, wh ile satellites located east of the magnetic dip meridian observe mostl y duskside lobe encounters (67%). However, the transition from observi ng dawnside or duskside lobe encounters is not symmetric about the mag netic dip meridian. Three satellites (ATS 5, 1991-080, and 1990-095) l ocated essentially at the magnetic dip meridian observed a majority (7 3%) of dawnside encounters. This study therefore concludes that there is a dawn-dusk asymmetry of the synchronous orbit altitude magnetic eq uatorial plane during active geomagnetic periods with the dawnside mag netosphere 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 mod el dip prime meridian better organizes the local time occurrence frequ ency than does the magnetic dipole magnetic meridian, which has been s hown to organize the seasonal dependence of geosynchronous lobe encoun ters in previous studies (i.e., Thomsen et al., 1994). Hence this stud y emphasizes the importance of considering the relative position of sa tellites with respect to both the magnetic dipole equator and the geom agnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere.