STABILITY OF THE HALLEY COMETOSHEATH WITH RESISTIVITY AND PLASMA MOTION

The MHD stability of the cometary inner sheath determined by the balan ce between the inward Lorentz body force and the outward ion-neutral d rag force is investigated by numerically solving the wave equations wh ich include resistivity, plasma motion and plasma pressure with the he lp of two-point boundary value method. The eigenvalues and the eigenfu nctions are obtained numerically by treating the cometary inner sheath as a layer of finite thickness, bounded by the contact surface, that is, the diamagnetic cavity boundary. To gain an insight into the probl em, certain limiting cases of the wave equations are also discussed. T he diamagnetic cavity boundary and the adjacent layer of about 100-km thickness of comet Halley is found to be unstable. The effects of fini te plasma pressure, dissociative recombination, mass loading due to ph otoionization, resistivity, and plasma motion am found to be stablizin g but are unable to quench the instability completely. An estimate of tau(C)/tau(i) shows that it hes in the range of 10 to 20 or even highe r which appears to be sufficient for the effective penetration of the magnetic field perturbations into the cavity surface. Motion of the Ha lley ionopause has been confirmed by observations: according to Neubau er (1987), Halley ionopause seemed to have strong ripples with a wavel ength of several hundred kilometers.