Shock Heating of the Solar Wind Plasma

Observational data from IMP, Voyagers and Pioneers show that (a) the solar rotational average of the solar wind entropy increases with the heliocentric distance; (b) in 1973-1982 the shock strength increases with the heliocentric distance outside 1 AU, reach a maximum near 5 AU, and then decrease with the distance; (c) the average entropy increases across shocks also reaches a maximum near 5 AU. When a shock propagates through the solar wind, the shock heating increases the entropy of the solar wind plasma by approximately 0.8 x 10-23 J/K/proton. We use a simulation study to calculate the changes in entropy associated with the evolution of a solar wind structure between 1 and 15 AU. Shock heating is the only heating mechanism included in the model. The calculated entropy increase agrees very well with that calculated from observational data. This result infers that shocks are chiefly responsible for the heating of the solar wind plasma between 1 and 15 AU.