Shock Wave Structure Mediated by Energetic Particles

Energetic particles such as cosmic rays, Pick Up Ions (PUIs), and solar energetic particles can affect all facets of plasma physics and astrophysical plasma. Energetic particles play an especially significant role in the dissipative process at shocks and in determining their structure. The very interesting recent observations of shocks in the inner heliosphere found that many shocks appear to be significantly mediated by solar energetic particles which have a pressure that exceeds considerably both the thermal gas pressure and the magnetic field pressure. Energetic particles contribute an isotropic scalar pressure to the plasma system at the leading order, as well as introducing dissipation via a collisionless heat flux (diffusion) at the next order and a collisionless stress tensor (viscosity) at the second order. Cosmic-ray modified shocks were discussed by Axford et al. (1982), Drury (1983), and Webb (1983). Zank et al. (2014) investigated the incorporation of PUIs in the supersonic solar wind beyond 10AU, in the inner Heliosheath and in the Very Local Interstellar Medium. PUIs do not equilibrate collisionally with the background plasma in these regimes. In the absence of equilibration between plasma components, a separate coupled plasma description for the energetic particles is necessary. This model is used to investigate the structure of shock waves assuming that we can neglect the magnetic field. Specifically, we consider the dissipative role that both the energetic particle collisionless heat flux and viscosity play in determining the structure of collisionless shock waves. We show that the incorporation of both energetic particle collisionless heat flux and viscosity is sufficient to completely determine the structure of a shock. Moreover, shocks with three sub-shocks converge to the weak sub-shocks. This work differs from the investigation of Jokipii and Williams (1992) who restricted their attention to a cold thermal gas. For a cold thermal non-magnetized gas, all shocks are smoothed by cosmic ray diffusion and therefore viscosity is not an important process.