Shock Waves and Nonlinear Plasma Waves Mediated by Pickup Ions and Energetic Particles

Abstract:

The observations made by many spacecraft demonstrate that the distant solar wind in the outer heliosphere is indeed mediated by nonthermal energetic particles such as pickup ions (PUIs). They dominate internal pressure in the outer heliosphere and play a fundamental role in the dissipative process at shock waves and in determining their structure. In this talk, I give a brief summary of the structure of shocks in the outer heliosphere, the heliospheric termination shock (HTS), shocks in the inner heliosheath (IHS), and the very local interstellar medium (VLISM) shocks. We use a two-fluid (thermal gas and non-thermal PUIs) model to study the structure of shock waves and then we analyze the observed shocks in these regions. We show that a very small percentage of the solar wind flow energy at the upstream of the HTS is converted to downstream thermal heating, as it was observed by Voyager 2 (Richardson et al. 2008) and PUIs provide almost all the dissipative heating of the bulk flow energy at the HTS. Voyager 2 observations showed that the IHS is dominated by large energetic particle pressure (Decker et al. 2015). We show that the mediation of the IHS temperature due to the presence of many shocks result in the more effective production of energetic neutral atoms (ENAs) due to charge exchange between interstellar neutral gas and IHS PUIs. Our results show that the predicted ENA flux matches the observed IBEX ENA flux more closely when shock waves are present in the IHS. The first in situ VLISM shock observed by Voyager 1 was remarkably broad and had properties very different than shocks inside the heliosphere (Burlaga et al. 2013). We use our model and show that the VLISM is a collisional medium with respect to the thermal gas (unlike the collisionless heliosphere) and the weak VLISM shock observed by Voyager 1 is controlled by particle collisions and not mediated by PUIs. Thus, their broad thickness correspond to the collisionality.