Contextual Predictions for Parker Solar Probe I: Critical Surfaces and Regions

The solar corona and young solar wind may be characterized by critical surfaces -- the sonic, Alfv\'en, and first plasma-$\beta$ unity surfaces -- that demarcate regions where the solar wind flow undergoes certain crucial transformations. Global numerical simulations and remote sensing observations offer a natural mode for the study of these surfaces at large scales, thus providing valuable context for the high-resolution in-situ measurements expected from the soon-to-be-launched Parker Solar Probe (PSP). The present study utilizes global three-dimensional magnetohydrodynamic simulations of the solar wind to characterize the critical surfaces and investigate the flow in propinquitous regions. Effects of solar activity are incorporated by varying source magnetic dipole tilts and employing magnetogram-based boundary conditions. A magnetohydrodynamic turbulence model is self-consistently coupled to the bulk flow equations, enabling investigation of turbulence properties of the flow in the vicinity of critical regions. The simulation results are compared with a variety of remote sensing observations. A simulated PSP trajectory is used to provide contextual predictions for the spacecraft in terms of the computed critical surfaces. Broad agreement is seen in the interpretation of the present results in comparison with existing remote sensing results, both from heliospheric imaging and from radio scintillation studies. The trajectory analyses show that the period of time that PSP is likely to spend inside the $\beta=1$, sonic and Alfv\'en surfaces depends sensitively on the degree of solar activity and the tilt of the solar dipole and location of the heliospheric current sheet.