Search for the magnetic neutral line in the near-Earth plasma sheet 2. Systematic study of Imp 6 magnetic field observations

Abstract

An extensive and comprehensive survey of substorm variations of the magnetic field within the plasma sheet is conducted on the basis of simultaneous magnetic field and energetic particle observations from the Imp 6 satellite. Two important results are as follows. (1) It is found that thinning of the plasma sheet during substorms is generally associated with only a slight southward dipping of the magnetic field vector, and such a dipping is observed even along the horn of the plasma sheet, as close to the earth as X ≃ −2 RE (or (X² + Y²)½ ≃ 6 RE), confirming and strengthening the results presented in the first paper of this series. In particular, it indicates that slight southward field dippings should not be used as supporting evidence for a neutral line formed earthward of the observing site. (2) Further, it is shown that the sign of the Bz component at the crossings of the midplane (or the so-called neutral sheet) at X < −20 RE, tailward of the expected location of the hypothetical neutral line, is predominantly positive during plasma sheet thinning at the substorm expansion phase. This set of observations indicates that the formation of a neutral line in the near-earth plasma sheet is far from conclusive. On the basis of our analysis and others we have constructed a plausible sequence of magnetospheric configuration changes during thinning of the plasma sheet which is associated with the substorm expansion phase.