A Comparison of Parker Spiral and WSA Footpoint Longitudes for Solar Energetic Particle Events.

Determination of solar coronal magnetic footpoints is an essential part of solar energetic particle (SEP) investigations. This is usually done by combining an assumption of a Parker spiral field geometry with the spacecraft observed solar wind (SW) speed. An alternative method is to use a SW model based on synoptic solar magnetic field maps and a potential field source surface (PFSS) model, such as the Wang-Sheeley-Arge (WSA) model. For 24 recent SEP event onsets we compare the ecliptic Carrington longitude footpoints obtained from the Parker spiral method with those from the WSA plus Schatten current sheet (SCS) model. For each SEP event we use synoptic maps from the day of and two days before the SEP event and do 12 realizations for each day. This enables us to determine the median WSA longitudes and their standard deviations as well as their differences from the assumed Parker spiral longitudes. As a further validation of each method we compare the 1 AU observed SW magnetic polarities with the WSA and the Parker spiral source polarities. The comparison of these two methods of determining solar source longitudes will provide estimates of the uncertainties of SEP source longitudes.