Steepening of kinetic magnetosonic waves into shocklets: Simulations and consequences for planetary shocks and comets

Abstract

Numerical simulations are used to investigate the nonlinear evolution of oblique low‐frequency electromagnetic (kinetic magnetosonic) waves, which have been observed upstream of planetary bow shocks and at comet Giacobini‐Zinner. The observations show that the waves are elliptically polarized and have a sinusoidal form when their amplitude is small, but they become steepened and linearly polarized as they grow in amplitude. These waves have been referred to as shocklets. A high‐frequency whistler wave packet is commonly seen at the steepened edge of the shocklets. To investigate the generation and the nonlinear evolution of kinetic magnetosonic waves in a self‐consistent manner, an electromagnetic hybrid code (particle ions, fluid electrons) is used in two stages. In the first stage, a large nonperiodic box is utilized to generate a series of small‐amplitude waves, one of which is then isolated for the second stage of the simulations, in which much higher resolution is employed to investigate further growth and the nonlinear evolution of the isolated wave. The results show that the original small‐amplitude elliptically polarized wave grows and steepens, such that its polarization changes and becomes somewhat linear. The steepening process is associated with the coherent generation of a broad spectrum of waves on the magnetosonic whistler branch, which propagate at various phase and group velocities. As a result, the waveform spreads in space, causing the change in polarization. Because the phase and group velocities of the whistler waves increase with frequency, the high end of the spectrum generated during the steepening propagates ahead of the steepened front and forms a high‐frequency wave packet, whose amplitude decreases with distance from the steepened edge of the magnetosonic wave. This drop in amplitude is due to the transitory nature of the wave packet. By investigating the behavior of the solar wind upstream and within the steepened wave it is shown that these waves are very similar to subcritical dispersive shock waves and can result in local heating and deceleration of the solar wind. This interpretation of the steepened waves has a number of implications for planetary bow shocks and the nature of the transition region at comet Giacobini‐Zinner. The presence of shocklets upstream of a planetary bow shock can modify its local structure by changing the solar wind Mach number and temperature, or by colliding with the shock. Further, it is suggested that the transition region at comet Giacobini‐Zinner consists of solar wind plasma which has been heated and decelerated by a series of steepened waves, until it becomes subsonic in the comet's rest frame.