A kinetic study of solar wind mass loading and cometary bow shocks

Abstract

A detailed numerical study is conducted to understand the kinetic processes associated with solar wind mass loading due to pickup of cometary ions and the formation of cometary bow shocks. Because in this study we are most interested in phenomena which take place over time and spatial scales associated with ion dynamics, electrons are treated as a massless fluid. On the other hand, solar wind protons and heavy cometary ions are treated kinetically. It is found that solar wind deceleration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind, as well as the microscopic fields associated with low‐frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. These electromagnetic waves can grow to very large amplitudes and their nonlinear evolution is controlled by their wave normal angle. At parallel propagation where the waves are noncompressional, parametric instabilities seem to be operative, while the oblique, compressional waves are found to steepen and form shocklets. As for the nature of cometary bow shocks, three different regimes are found, based on the shock normal angle. For shock normal angles at or near 90° (quasi‐perpendicular), a typically thin transition region (shock) is formed through steepening of fast magnetosonic pulses. This shock is a pure proton shock in that no sharp change in density or velocity of the cometary ions is observed across the shock. At intermediate shock normal angles (oblique), a much wider transition region is seen where the solar wind is gradually heated and decelerated. This region, whose width is much larger than the gyroradius of cometary ions, is associated with a high level of turbulence due to ion pickup instabilities and is unlike a typical fast magnetosonic shock. Finally, at small shock normal angles (quasi‐parallel), a narrow transition region is found which is similar to a parallel terrestrial shock. The formation of this shock is not due to wave steepening but rather is the result of scattering and heating of the solar wind protons by the large‐amplitude electromagnetic waves that are generated through the relative drift between the protons and the cometary ions. These results are compared and shown to be in qualitative agreement with the recent observations at comets.