Ultra low frequency waves at the magnetic pile-up boundary of comet P/Halley

Abstract

Around the magnetic pile-up boundary observed by Giotto at 135,000 km from the nucleus of comet Halley, plasma and magnetic features display evidence of ultra-low frequency modes. Outside the pile-up region, a large region is characterized by an anti-correlation between the electron plasma density and the magnetic field strength. The study of the electron pressure tensor and the anisotropy of the distribution function lead to the identification of these modes as mirror modes. Inside the boundary, quasi-monochromatic large amplitude waves are observed with a very close correlation between the magnetic field magnitude and the electron density and have been identified as fast mode waves. In this region, the electron population can be described by two polytropic laws, for the parallel and the perpendicular pressures. These electron properties as well as possible generation mechanisms of the ULF waves are discussed.