Compressive character of low frequency waves driven by newborn ions at comet Grigg-Skjellerup

Abstract

The investigation by Giotto of the environment of comet P/Grigg-Skjellerup has provided a large set of observations on cometary low-frequency waves for a weakly active comet, after the observations at comets P/Giacobini-Zinner and P/Halley. A large amount of waves are observed at frequencies close to the local water group ion cyclotron frequencies on the magnetic field and the plasma density in a wide region around the comet. Cross-analysis of the electron parameters with the magnetic field reveals the existence of a compressive wave component even at large distances from the nucleus. The inferred characteristics of the observed wave modes are quantitatively reproduced from a theoretical work studying the electromagnetic instabilities driven by the distribution of the newborn ions.