Nonlinear model of high-speed solar wind streams

Abstract

A hydrodynamic model describing the generation and propagation of high-speed plasma streams in the solar wind is presented. The model is based upon numerical integrations of the conservation equations for a time-dependent, spherically symmetric, radial flow of interplanetary plasma. The nearly radial nature of the solar wind flow justifies the use of the model to approximate ‘corotating streams,’ e.g., nonspherically symmetric flows that are steady in a frame of reference rotating with the sun. The predicted variations in solar wind properties are in good agreement with those observed at 1 AU for a reasonable choice of parameters characterizing a ‘coronal disturbance’ at the heliocentric distance of 28 Rs. This choice must include a perturbation of the coronal temperature but need not include perturbations of the coronal density or mass efflux. The streams produced by such a disturbance ‘steepen’ in transit to the orbit of the earth, with formation of a pair of shocks predicted at slightly larger heliocentric distances. The average dependencies of density and temperature upon the solar wind speed deduced from the model resemble those inferred from solar wind observations. This suggests that the major density changes associated with high-speed streams are the products of interplanetary compression and rarefaction within the evolving stream structure. The same processes explain the deviations of proton temperature from their average dependence on solar wind speed; however, that basic dependence appears to reflect the temperature changes imposed on the plasma in the corona.