A statistical evaluation of ballistic backmapping for the slow solar wind: the interplay of solar wind acceleration and corotation

Mapping solar wind plasma back to its source is often achieved using the 'two-step ballistic backmapping' method. Solar wind observations are mapped through the heliosphere to the edge of a PFSS model, by assuming a constant speed, radial, plasma flow. Tracing field lines through the model gives the source location at 1R(circle dot) The heliospheric mapping component hinges upon the argument that two known sources of error, stemming from solar wind acceleration and non-radial flow, effectively cancel. This assumption has not been tested statistically. In this study, we evaluate the heliospheric portion of two-step backmapping, in addition to mapping using models with explicit radial acceleration, and azimuthal velocity, v(phi), derived from angular momentum conservation. We estimate longitudinal mapping offsets, Delta phi, between 326 Earth-observed crossings of the heliospheric current sheet (HCS), and corresponding crossings at 2.5R(circle dot) from PFSS models. While the detailed solar wind models can be optimized to produce Delta phi in good average agreement with HCS crossing data, the ballistic mapping performs almost as well, although all residuals have a sizeable standard deviation sigma similar to 16 degrees. We conclude that the proposed error cancellation likely contributes to the good performance of ballistic mapping. However, interplanetary acceleration and the height of effective solar wind corotation are both smaller than previously assumed. Our results further suggest that early Parker Solar Probe observations of large v(phi) around 36R(circle dot) do not represent the overall solar wind, due to the requirement for it to be balanced by increased acceleration.