Hybrid code simulations of the solar wind interaction with Comet 19P/Borrelly

Detailed observations of the plasma environment of Comet 19P/Borrelly were made by the Plasma Experiment for Planetary Exploration (PEPE) during the Deep Space 1 (DS1) flyby on 22 September 2001. DS1 flew from north to south relative to the plane of the ecliptic on the sunward side of the comet and all plasma boundaries (i.e., first and last pickup ions, bow shock, decelerated solar wind) were observed to be shifted northward. This surprising result was initially attributed to large well‐collimated dayside jets directed ∼10° northward from the nucleus as observed by the DS1 MICAS camera. However, ground‐based observations of Schleicher et al. (2003) indicate a symmetric gas cloud with respect to the comet‐Sun line. In an attempt to resolve this conundrum, we have conducted a series of hybrid plasma simulations to investigate the effects of the large pickup ion gyroradius (i.e., comparable to the length scale of the cometary obstacle) at 1.36 AU. We find that that a maximum asymmetry, consistent with the observations, is generated when the pickup ion gyroradius is comparable to or less than the standoff distance of the bow shock ( B IMF ∼ 4 nT). The degree of asymmetry is reduced for larger and smaller magnetic field strengths.