Magnetic structure of the distant geotail from −60 to −220 Re: ISEE‐3

Abstract

Magnetic field observations during the first ISEE‐3 pass into a previously unexplored region of the geotail (−80 to −220 Re) are discussed. The tail structures customarily detected closer to Earth are also identifiable in the distant tail field data: two lobes separated by a plasma sheet and embedded neutral sheet. A plasma sheet boundary layer, characterized by small diamagnetic field decreases and the presence of large amplitude (5 nT p‐p) electromagnetic waves, is often, but not always present. It is found that the tail maintains a well‐ordered magnetic field structure out to −220 Re. Near‐earth (−40 to −80 Re), the magnetic field in the plasma sheet is directed southward only 5% of the time, but this percentage gradually increases with distance until it is southward 50% the time at X = −220 Re. The implications of the above observations for reconnection models are discussed. Significant By fields are detected within the neutral sheet and the field rotation often takes place primarily in the X‐Y plane. These transverse fields may be associated with tailward moving plasmoids or with a shearing of the lobes.