Coronal Pseudostreamers

Abstract

In a recent study of the 2006 solar eclipse, we noted that there are two kinds of coronal streamers: "helmet streamers," which separate coronal holes of opposite magnetic polarity, and "pseudostreamers," which overlie twin loop arcades and separate holes of the same polarity. It is well known that the heliospheric plasma and current sheets represent the outward extension of helmet streamers. Using white-light data from the Large Angle and Spectrometric Coronagraph (LASCO), we here show that pseudostreamers likewise have plasma sheet extensions, across which the polarity does not reverse; these multiple sheets contribute significantly to the brightness of the K corona, although their internal densities tend to be lower than those in the heliospheric plasma sheet. We use current-free extrapolations of photospheric field measurements to simulate the observed brightness patterns in the outer corona, including the contributions of both helmet streamer and pseudostreamer plasma sheets. Running-difference images show that pseudostreamers are relatively quiescent, resembling large-scale plumes; preliminary analysis suggests flow speeds as high as 200 km s-1 at heliocentric distances of only ~3 R☉, supporting the prediction (based on their low flux tube divergence rates) that pseudostreamers are sources of fast solar wind.