Evaluation of a potential field source surface model with elliptical source surfaces via ballistic back mapping of in situ spacecraft data

Abstract

Context. The potential field source surface (PFSS) model is an important tool that helps link the solar coronal magnetic field to the solar wind. Due to its simplicity, it allows for predictions to be computed rapidly and requires little input data, though at the cost of reduced accuracy compared to more complex models. So far, PFSS models have almost exclusively been computed for a spherical outer boundary or "source surface". Changing this to an elliptical source surface holds promise to increase its prediction accuracy without the necessity of incorporating complex computations or additional model assumptions. Aims. The main goal of this work is to evaluate the merit of adding another parameter, namely the ellipticity of the source surface, to the PFSS model. In addition, the applicability of the PFSS model during different periods of the solar activity cycle as well as the impact of the source surface radius are analyzed. Methods. To evaluate the model, in situ spacecraft data are mapped back to the source surface via a ballistic approach. The in situ magnetic field polarity is compared to the magnetic field polarity predicted by the model at the source surface. This method is based on the assumption that better performing models provide better agreements between the prediction and the measured magnetic field polarity. We employ data from the Advanced Composition Explorer and the twin Solar Terrestrial Relations Observatory (STEREO) for this analysis. Results. We show that the PFSS model performs slightly better with oblate elliptical source surfaces elongated along the solar equatorial plane, although the best found ellipticity varies for different spacecraft and periods. In addition, it is demonstrated that the performance of the presented analysis degrades during the active times of the solar activity cycle.