Flux dropouts of plasma and energetic particles at geosynchronous orbit during large geomagnetic storms: Entry into the lobes

Abstract

Geosynchronous orbit spacecraft occasionally observe the magnetospheric tail lobe during disturbed geomagnetic conditions. These lobe intervals are characterized by an absence of detectable ion flux above 1 eV and an absence of electron flux above several hundred eV. This study is an extension of Thomsen et al. (1994) and examines geosynchronous orbit plasma measurements for lobe encounters during a 4‐year period involving three spacecraft, for a total of 85 spacecraft‐months of observations. During the interval surveyed, 160 lobe encounters or close encounters were observed. These events are divided into the following two classes: events observed near midnight and those observed along the flanks. Both classes of events were strongly associated with very active geomagnetic conditions as indicated by Kp (mean value of 5), Dst (mean value of −104 nT), and the Auroral Boundary Index, particularly the flank events. Fifty percent of the lobe encounters occurred within ±24 hours of a geosynchronous magnetopause crossing, which indicates that many of the lobe encounters occurred when there was severe compression and/or erosion of the magnetosphere. Most of the magnetopause‐crossing‐associated events were flank events. The flank‐lobe events were also generally associated with a large interplanetary magnetic field magnitude, with the By or Bz component being dominant. Dropouts in the energetic electron and ion fluxes were observed concurrently with the thermal ion plasma dropouts, though they often extended over a longer time interval. For most of the events a geosynchronous injection of energetic electrons and/or protons was observed following the beginning of the thermal ion dropout, often at a different satellite than the one observing the lobe. For a limited subset of the lobe events (17 events) in which a GOES satellite was nearby, the magnetic field data indicate that the field is often highly stretched or radial near the time of the lobe events. We conclude that the near‐midnight and flank‐lobe events are due to two different mechanisms as follows: the near‐midnight events are consistent with the growth phase picture of large geomagnetic substorms, while the flank events are due to a major reconfiguration of the magnetosphere during large geomagnetic storms which are triggered by the passage of a large solar wind disturbance such as a fast coronal mass ejection. These observations show that the magnetosphere can be highly distorted at synchronous orbit, even far from midnight.