Macrostructure of collisionless bow shocks: 1. Scale lengths

Abstract

The global structure of collisionless bow shocks is investigated using a 2.5‐dimensional electromagnetic hybrid code. This allows us to study the macrostructure of the shock while accounting for microphysical processes at the shock. The study entails the interaction of solar wind with magnetic dipoles of varying strength. For very weak dipoles the interaction does not lead to formation of a shock since the obstacle is not strong enough for the flow to become subsonic. For lager dipole strengths, a bow shock/wave is formed due to the presence of a plasma stagnation region in front of the dipole. It is found that the quasi‐perpendicular part of this boundary corresponds to a true shock wave, whereas the quasi‐parallel side consists of a magnetosonic wave followed by a rotational discontinuity. The backstreaming ions in the foreshock of this interaction lead to the generation of parallel propagating sinusoidal waves. These waves result in beam scattering, however, do not affect the solar wind. The formation of quasi‐parallel shock is tightly connected to the generation of oblique compressional waves. These waves are generated by backstreaming ions having a beam‐ring distribution function and are an inherent part of the shock dissipation processes. The results demonstrate that the two classes of 30 s ULF waves observed in the ion foreshock are unrelated. The results also demonstrate that at least in 2.5‐dimensional, plasma scales determine the nature of the bow shock to a large extent although system size can influence both particle acceleration and evolution of ULF waves.