Interaction of the Solar Wind with Comet Hale‐Bopp: Global Interaction and Microphysics

Abstract

Comet Hale-Bopp, discovered at a heliocentric distance of ~7.2 AU, promises to become a striking spectacle as it approaches perihelion (~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 trajectory 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 encounter distance of ~0.9 AU. It is shown that while Hale-Bopp's H2O production rate may increase by 3.5-6 orders of magnitude in this range of heliocentric distances, depending on the assumed model of nuclear rotation, the dimensions of the region over which the solar wind is strongly perturbed as measured by the cometocentric distance of the cometary bow shock changes by only 1 order of magnitude. This effect primarily arises from the increasingly strong depletion effect of neutral particles ("neutrals") as the comet approaches the Sun. Because of this effect, the size of the shock at 1 AU is less than its size at 2 AU, despite the larger production rate of H2O at the smaller distance. Also, although this shock is always produced by mass loading of the inflowing solar wind by heavy cometary ions, its nature changes from "soft" or typically "comet-like" at large heliocentric distances to "hard" or "planet-like" near perihelion. This neutral depletion effect has important consequences for the microphysics as well. Strong Alfvén wave turbulence, which served as a distant precursor of the bow shock of Halley's comet, will not be generated by Hale-Bopp near perihelion. Also, downstream of the shock, the excitation of MHD waves, which led to the isotropization of "pick-up" cometary ions in the solar-wind frame near Halley's comet, will be suppressed, leading to a highly anisotropic (ring-type) distribution near Hale-Bopp. It is also shown that electron energization resulting from lower hybrid turbulence leads to observable X-ray emission, which will become progressively harder as the comet approaches the Sun. Finally, it is argued that the Townsend criterion for a discharge produced by energetic electrons is likely to be more easily fulfilled by comet Hale-Bopp at heliocentric distances of ≤3 AU than by Halley's comet at encounter, thereby leading to anomalous ionization of the neutral gas outflow by the so-called Alfvén or critical velocity ionization mechanism.