FOUNTAIN MODEL FOR PLASMA ENVELOPES OF COMETS

By regarding a comet as a source of CO molecules, we investigate the dynamics of the CO+ envelopes of major plasma comets. Electron-collisional ionization within the envelopes is the main mechanism for plasma production. By analyzing the flow within it, a thin envelope is shown to be dynamically unstable at enhanced solar wind velocities, as long as the mean expansion velocity of the CO molecules is greater than 3 km/sec. An unstable envelope always contracts in towards the head; it also acts as the source of a sunward fountain of plasma which initiates a new outermost envelope. The material of each envelope flows into a number of tail rays, and the phenomenon of 'closing-in' rays is related to the decreasing pressure within the aging envelopes. The existence of an innermost stationary envelope, from which the main tail rays arise, is predicted. The varied and rapidly changing tail structure is largely due to the inherent instabilities of the ionization and plasma interaction mechanisms. The model is also applicable to smaller comets which neither possess distinct plasma envelopes nor exhibit fountain behaviour. © 1968.