An acceleration mechanism for cometary plasma tails

Abstract

Cometary plasma tail formation by the interaction between the solar wind plasma flow and the plasma at the head of the coma is discussed using unipolar electric generation theory. The cometary "plasma tail" here is defined as the part of the traditional type 1 ion tail. The plasma in the "plasma tail" is almost directly accelerated from the cometary ionopause along the sun‐nucleus line where the tail current flows. For steady state solar wind conditions, the cometary "plasma tail" velocity distribution is obtained self‐consistently. The solution of a kinetic equation gives the velocity of the cometary plasma as a function of the cometary tail position, x, as v=vo [1‐ exp(‐x/xo)], where xo, and vo are the characteristic length for the interaction and the solar wind velocity, respectively. The characteristic length, xo is 3×106 km when the plasma density near the nucleus of the comet, Nc, is 106 cm−3 and the component of the interplanetary magnetic field, IMF, perpendicular to the solar wind flow is 3 nT. The tailward cometary plasma is finally accelerated to the speed of the solar wind. The effect of a nonuniformly distributed magnetic field on the cometary tail is also treated. The theory is compared with the observational plasma velocities in the tails of comet Bennett (1970II) and comet Halley (1985)