LOW-FREQUENCY ELECTROMAGNETIC PLASMA-WAVES AT COMET-P GRIGG-SKJELLERUP - ANALYSIS AND INTERPRETATION

The propagation and polarization characteristics of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted with the following new findings: (1) Broad pickup-generated wave fields exist in the magnetoplasma around comet P/GS. (2) Outside the bow wave/shock, wave fields can be pictured as plane waves with propagation angles of about 10-degrees with respect to +/-B0underbar, where B0underbar the background magnetic field. (3) An envelope of very regular wave fields with almost exclusively left-hand polarization in the spacecraft frame exists around the bow wave/shock. More complex polarization properties are found farther away. (4) Because of the unusual interplanetary magnetoplasma characteristics the ratio M(Ar) of the local ring beam velocity component parallel to the magnetic field in the plasma frame of reference and the Alfven velocity varies between zero and a little greater than 1. As a consequence, the waves excited in the plasma frame must be predominantly left-hand Alfven waves propagating away from the Sun in contrast to the right-hand waves propagating toward the Sun in the same frame of reference at comets P/Giacobini-Zinner and P/Halley. (5) it is argued that the regular character of the waves close to but upstream of the bow wave/shock is due to the effects of nongyrotropy and/or nonlinear dispersive effects. (6) Magnetosheath waves are characterized as compressive, nonplanar waves with mixed polarization.