Lower Hybrid Waves Upstream of Comets and Their Implications for the Comet Halley Bow Wave

Observed and theoretical features concerning the nature of so-called cometary "bow shocks" or "bow waves" are discussed. Collective plasma effects associated with the presence of pickup ring ions (protons and water ions) in the vicinity of the supermagnetosonic to submagnetosonic transition region in the quasi-perpendicular limit are considered; the linear and nonlinear evolution of instabilities around the lower hybrid frequency is emphasized. It is shown that lower hybrid waves can lead to heating and produce distributions with magnitudes in reasonable agreement with Giotto data. The implications to the existence and structure of cometary bow shocks are discussed.