On the Relationship between Magnetic Expansion Factor and Observed Speed of the Solar Wind from Coronal Pseudostreamers
Samantha Wallace, C. Nick Arge, Nicholeen Viall, Ylva Pihlström
Abstract
Abstract For the past 30+ yr, the magnetic expansion factor ( f s ) has been used in empirical relationships to predict solar wind speed ( v obs ) at 1 au based on an inverse relationship between these two quantities. Coronal unipolar streamers (i.e., pseudostreamers) undergo limited field line expansion, resulting in f s -dependent relationships to predict the fast wind associated with these structures. However, case studies have shown that the in situ observed pseudostreamer solar wind was much slower than that derived with f s . To investigate this further, we conduct a statistical analysis to determine if f s and v obs are inversely correlated for a large sample of periods when pseudostreamer wind was observed at multiple 1 au spacecraft (i.e., ACE, STEREO-A/B). We use the Wang–Sheeley–Arge model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) photospheric field maps to identify 38 periods when spacecraft observe pseudostreamer wind. We compare the expansion factor of the last open field lines on either side of a pseudostreamer cusp with the corresponding in situ measured solar wind speed. We find that only slow wind ( v obs < 500 km s −1 ) is associated with pseudostreamers and that there is not a significant correlation between f s and v obs for these field lines. This suggests that field lines near the open–closed boundary of pseudostreamers are not subject to the steady-state acceleration along continuously open flux tubes assumed in the f s – v obs relationship. In general, dynamics at the boundary between open and closed field lines such as interchange reconnection will invalidate the steady-state assumptions of this relationship.