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The role of heavy ions in the collisionless parallel shock of Venus and Mars is investigated where the solar wind interacts directly with the planetary ionosphere. We consider O+ ions in the planetary plasma and He++ ions (α particles) in the solar wind as the heavy-ion species. By solving a linear dispersion relation of the ion cyclotron beam instability, which provides a dissipation in the parallel shock, we find that the dominant frequency and wavenumber are not much modified when the heavy ions are included in a realistic amount. Particle simulations are performed in order to examine the effects and dynamical behavior of the heavy ions in the parallel shock. The re-formation of the parallel shock is observed, which confirms that the electromagnetic waves that make up the parallel shock are not affected by the heavy ions. Notable pickup of O+ ions by electromagnetic waves is not detected. Instead, strong heating of the solar wind He++ ions is observed in the parallel shock region.

An implicit-particle simulation of the collisionless parallel shock created at the interface between an injected beam and a stationary plasma is performed in one-dimensional geometry. The solar wind plasma, which consists of ions and electrons, is injected into a stationary dense plasma that corresponds to the planetary ionosphere. Electromagnetic waves with right-hand circular polarization that propagate upstream (R− waves) are generated at the interface of the two plasmas, which decelerate the solar wind to form a shock. The shock transition region is not monotonic but consists of two distinct regions, a pedestal and a shock ramp. The transition region, which contains the ionopause, is a few thousand electron skin depths long. The parallel shock varies in time and periodically collapses and re-forms. The right-hand circularly polarized electromagnetic waves that propagate downstream (R+ waves) are excited at the shock ramp. Nonlinear wave-particle interaction between the solar wind and the R+ waves causes wave condensation and density modulation. These R+ waves may be sweeping away the downstream plasma to suppress its thermal diffusion across the shock. The electrons at the shock ramp exhibit a flat-topped velocity distribution along the magnetic field owing to the ion acoustic-like electrostatic waves.

Heating at collisionless shocks due to the kinetic cross-field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi-linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are described.