The Evolution of Interplanetary Shocks Propagating into the Very Local Interstellar Medium

Voyager 1 has made in situ measurements of the very local interstellar medium (VLISM) since August 2012 and its magnetometer and plasma wave instrument have detected several VLISM shock waves. Interplanetary shocks propagate through the supersonic solar wind and then through the inner heliosheath after colliding with the heliospheric termination shock (HTS). Interplanetary shock waves are transmitted partially across the heliopause (HP) into the VLISM and partially reflected back into the inner heliosheath. Previous studies showed that the in situ VLISM shocks observed by Voyager 1 were very weak and remarkably broad and had properties different than shocks inside the heliosphere [1, 2]. We model the first VLISM shock observed by Voyager 1 and compare with observations. We calculate the collisionality of the thermal particles and the dissipation terms such as heat conduction and viscosity that are associated with Coulomb collisions in the VLISM. The VLISM is collisional with respect to the thermal plasma and the VLISM shock structure is determined by thermal proton-proton collisions, which is the dominant thermal collisional term. The VLISM shock is controlled by particle collisions and not mediated by PUIs since they do not introduce significant dissipation through the shock transition. As a result, we find that the extremely broadness of the weak VLISM shock is due to the thermal collisionality.