Interpenetrating solar wind streams

Observations of solar wind interpenetrating ion streams are reviewed. They occur preferentially during periods of low but increasing plasma number density and most likely result from spatial and/or temporal inhomogeneities in the solar wind bulk velocity. The exact nature of these inhomogeneities is not known, but three conceptual models that account for the observations are summarized. Double ion streams are shown to occur during extended time periods during which the proton anisotropy and dimensionless heat flux undergo large coherent variations. This fact provides strong evidence that a major source for solar wind internal ion energy is its bulk convection energy. The observed continuum of proton velocity configurations further suggests that the ion internal energy state at 1 AU is often perturbed by bulk flow spatial and/or temporal inhomogeneities. Self‐consistent interactions with the interplanetary wave mix that results from these inhomogeneities as well as other aspects of the solar wind expansion establish preferred proton velocity configurations. These configurations can be summarized by simple statistical relations that couple the two independent third velocity moments to the second moments.