Evaluating the Parker Model With the Trans-Heliospheric Survey

The solar wind's expansion through the heliosphere is often studied by “radial trends”—the average trends in fluid parameters as functions of radial distance from the Sun. This study utilizes the latest version of the Trans-Heliospheric Survey, a compilation of in-situ magnetic field and proton measurements from 13 different spacecraft spanning over 60 years and three orders of magnitude in radial distance to explore global changes throughout the heliosphere. These data are used to evaluate theoretical models, in particular the Parker model of the solar wind's bulk speed and magnetic field. The Parker model matches the speed data well in the inner heliosphere, but at roughly 0.5 au, the speed data plateaus while the model predicts an ongoing speed increase. In contrast, the model fits the magnetic field data remarkably well across all observed solar distances (0.06 to $79au$).