Dynamic MHD modeling of solar wind corotating stream interaction regions observed by Pioneer 10 and 11

Abstract

An opportunity to test an MHD one‐dimensional time‐dependent model of corotating solar wind streams presented itself during the period from September 30 to November 25, 1973, when Pioneer 11 and Pioneer 10 were near radial alignment with the sun. This observational period was characterized by five or six corotating interaction regions which streamed past the two spacecraft. Conditions were well suited for a comparison with the one‐dimensional MHD model because of the presence of two prerequisites: (1) multiple‐spacecraft radial alignment and (2) temporally varying conditions at the solar wind source. Plasma and magnetic field observations at Pioneer 11, located at approximately 2.8 AU, are used as the input (or forcing functions) for the model of Steinolfson et al. (1975a, b). The outputs, consisting of the solar wind velocity, density, proton temperature, azimuthal magnetic field, and the various energy and mass fluxes, are directly compared with the observations by Pioneer 10, which was located at approximately 4.9 AU. The test of the theoretical model was successful in the sense that both the phasing of the streams and the various solar wind properties were satisfactorily reproduced. The amplitudes of the radial velocity, density, and azimuthal magnetic field were also reasonably well predicted; the proton temperature, however, generally peaked at values about twice those observed. A limitation of the model, the neglect of thermal energy exchange, was thereby revealed. One of the streams was also analyzed in greater detail. The MHD simulation satisfactorily predicted features such as the development of the forward and reverse shocks as well as the interface between them, the increasing displacement between these shocks with heliocentric distance, and the local magnetic field maxima between the shocks. Generally, our results lead us to suggest that future study should be directed to the roles of collective proton‐electron interactions and thermal conduction within one‐ and two‐dimensional time‐dependent MHD flows.