Plasma Flow and Critical Velocity Ionization in Cometary Comae

Abstract

The plasma flow in the cometary coma is described by a simple relation which results from the balance of magnetic stresses and drag forces, the latter being mainly due to ion mass loading. Inertial and pressure effects are not explicitly considered, but lumped into a correcting factor of order unity. The relation is used to infer the ionization rate due to the critical velocity effect. The limits, rcrit and rlim, between which the effect dominates over photoionization are evaluated. Critical velocity ionization stabilizes the plasma flow speed at η−1/2 vcrit, where η is the overall efficiency of energy transfer to the ionizing electrons. This speed is estimated to be close to 20 km/s. At the outer limit, rlim, there is a strong rise of plasma density as seen from outside. Inside rcrit, recombination and ion-neutral friction come into play. The contact surface is found to be largely determined by ion-neutral collisions. A second neutral component of the order of 10² cm−3 is predicted to originate from recombination inside rlim. It is characterized by an antisunward flow with ∼ 20 km/s. Numerical values for the radial profile of ni and of the critical boundaries (except the bow shock) are given. The high plasma density found by ICE at the closest approach to comet Giacobini-Zinner is interpreted as a result of the critical velocity effect.