INTERACTION OF THE SOLAR-WIND WITH COMET HALE-BOPP - GLOBAL INTERACTION AND MICROPHYSICS

Comet Hale-Bopp, discovered at a heliocentric distance of similar to 7 .2 AU, promises to become a striking spectacle as it approaches perihe lion (similar to 0.914 AU) on 1997 April 1. In this paper we describe the changes of the global interaction of the comet with the solar wind and some of the associated microphysics as the comet follows its traj ectory from 4 AU to 1 AU from the Sun. Hale-Bopp's production rate of H2O at 4 AU is comparable to that of Halley's comet at the latter's en counter distance of similar to 0.9 AU. It is shown that while Hale-Bop p's H2O production rate may increase by 3.5-6 orders of magnitude in t his range of heliocentric distances, depending on the assumed model of nuclear rotation, the dimensions of the region over which the solar w ind is strongly perturbed as measured by the cometocentric distance of the cometary bow shock changes by only 1 order of magnitude. This eff ect primarily arises from the increasingly strong depletion effect of neutral particles (''neutrals'') as the comet approaches the Sun. Beca use of this effect, the size of the shock at 1 AU is less than its siz e at 2 AU, despite the larger production rate of H2O at the smaller di stance. Also, although this shock is always produced by mass loading o f the inflowing solar wind by heavy cometary ions, its nature changes from ''soft'' or typically ''comet-like'' at large heliocentric distan ces to ''hard'' or ''planet-like'' near perihelion. This neutral deple tion effect has important consequences for the microphysics as well. S trong Alfven wave turbulence, which served as a distant precursor of t he bow shock of Halley's comet, will not be generated by Hale-Bopp nea r perihelion. Also, downstream of the shock, the excitation of MHD wav es, which led to the isotropization of ''pick-up'' cometary ions in th e solar-wind frame near Halley's comet, will be suppressed, leading to a highly anisotropic (ring-type) distribution near Hale-Bopp. It is a lso shown that electron energization resulting from lower hybrid turbu lence leads to observable X-ray emission, which will become progressiv ely harder as the comet approaches the Sun. Finally, it is argued that the Townsend criterion for a discharge produced by energetic electron s is likely to be more easily fulfilled by comet Hale-Bopp at heliocen tric distances of less than or equal to 3 AU than by Halley's comet at encounter, thereby leading to anomalous ionization of the neutral gas outflow by the so-called Alfven or critical velocity ionization mecha nism.