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.
This tutorial summarizes our present, pre-Rosetta understanding of the basic physical processes operating in cometary environments with particular attention to cometary magnetotails. Ionization of cometary gases is responsible for most plasma processes in the magnetospheres of active comets. The interaction between the superalfvenic, magnetized solar wind and cometary ions forms a number of plasma boundaries starting with the weak bow shock and continuing with the contact surface (diamagnetic cavity boundary), the recombination layer and the inner shock. The plasma tail is sensitive to changes in the solar wind and to varying cometary activity. These processes will be explored in detail.
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.