Shock propagation in the outer heliosphere. 1. Pickup ions and gasdynamics

Pickup ions are expected to play a fundamental role in determining the global structure of the heliosphere since they dominate the internal energy of the outer solar wind beyond the ionization cavity. The dynamical effect of pickup ions on the propagation characteristics of interplanetary shocks is addressed here. It is found that (1) forward and reverse shocks in a heliosphere mediated by pickup ions have higher speeds than expected of shocks in an adiabatic solar wind without pickup ions; (2) identical initial conditions evolve differently in a pickup ion mediated heliosphere compared to an adiabatic heliosphere, producing more complicated velocity, density and temperature structure, and often with multiple forward-reverse shock pairs; (3) pickup ions act to weaken and dissipate shocks in the outer heliosphere although the velocity jumps can remain large; (4) density fluctuations in the outer pickup ion mediated heliosphere do not correlate with velocity fluctuations as well as they do in the inner heliosphere, and (5) the amplitudes of density fluctuations in a pickup ion mediated heliosphere are considerably smaller than those found in an adiabatic heliosphere without pickup ions.