Magnetosphere boundary observations along the Imp 7 orbit, 1. Boundary locations and wave level variations


  • F. L. Scarf,

  • L. A. Frank,

  • R. P. Lepping


We discuss and analyze magnetosphere boundary phenomena observed by the Imp 7 magnetic field, plasma, and plasma wave instruments in 1972 and 1973. The spacecraft crosses the dawn and dusk boundaries near 25 RE downstream, and the physical processes at the Imp 7 magnetopause appear to be intermediate between those observed over the poles and those observed at the lunar orbit. The Imp 7 orbit also traverses a downstream region near where ‘fireball’ phenomena occur, and this indicates that it is especially important to investigate the range of magnetopause phenomena detected with the plasma and fields payload complement. This report contains a brief description of the relevant Imp 7 instrumentation, a survey of the boundary locations for a 15-month period, a discussion of the different types of crossings (essentially, sharp and diffuse), and an analysis of the electromagnetic wave modes being detected in the broad low-frequency channel of the wave instrument. The wave mode discussion, based on comparison of Imp 7 data with low-frequency plasma sheet and magnetosheath observations from other spacecraft, leads to the conclusion that the broad low-frequency channel is sensitive to oscillations in the lower hybrid resonance region of the spectrum. Details of the broad diffuse boundary crossings will be discussed in a separate report.