Linear Polarization Observations of Coronal Pseudostreamers

Pseudostreamers are common coronal structures that appear at the solar limb as streamers bordering regions of the same magnetic polarity. They are locations prone to sympathetic eruptions and sources of an hybrid kind of solar wind with properties intermediate between slow and moderately fast wind. The pseudostreamer magnetic topological skeleton, and in particular the magnetic X-point at their cusp, imprints a very clear signature in linear polarization that distinguish pseudostreamers from the larger helmet streamers or two nearby streamers. We use polarimetric data taken with the Coronal Multichannel Polarimeter (CoMP) at the Mauna Loa Solar Observatory from 2013 to 2017 to analyze pseudostreamers and compare them with PFSS models. The solarsoft software package FORWARD is used to derive from the models a synthetic linear polarization image that can be directly compared with the observations and to compute the magnetic field expansion factor from both observations and models. We find differences between observations and PFSS extrapolations. In particular, the height of the magnetic X-point is always higher in the observations, indicating that PFSS models systematically underestimate the height of the cusp null point and confirming the previous result of Gibson et al. in 2017 based on a single pseudostreamer on April 18 2015. Gibson et al. also noted that the expansion factor for the case they studied was significantly larger in CoMP observations than expected from a potential field extrapolation with possible implications for solar wind speed. Our statistical analysis shows this is not always the case. We find no clear trend in how the expansion factor derived from observations compares to models and, at least at the heights covered by the CoMP field-of-view (up to 1.35 solar radii), the observed expansion factor cannot be used as meaningful diagnostic for solar wind speed.