A 3-D global MHD model for the effect of neutral jets during the Deep Space 1 Comet 19P/Borrelly flyby

The Deep Space 1 (DS1) spacecraft passed the sunward side of Comet 19P/Borrelly in 2001. Along its relatively north-south orbit, a set of plasma density and velocity measurement revealed a northward shift of the plasma boundaries and the mass loading peak. Both onboard and ground based telescopes found evidence for asymmetric distribution of the dust and neutrals. In this paper, five mass-loading patterns are studied to present the first study of the effect of non-spherical neutral distribution profiles on the solar wind-cometary plasma interaction environment. Using magnetohydrodynamic simulations, it is found that a combination of Gaussian and cosine neutral jet distribution, with cosine being the major part, can fit the DS1 general plasma measurement well, with a total gas production rate of around 5×¹⁰²⁸ s⁻¹. These model-data comparisons indicate that the general plasma distribution around Comet Borrelly can be explained with its aspherical neutral jet distribution. However, such neutral jets by themselves are insufficient to produce the density offset in the central peak. Kinetic effects, such as finite gyroradius may be required to create the offset plasma peak.