Measurements of Coronal Rotation and Small Coronal Ejections Using the STEREO COR1 Coronagraphs

Abstract

I present results of two studies conducted using the STEREO COR1 coronagraph: single- and dual-spacecraft measurements of white-light coronal rotation rates in 2007 and 2008 as a function of latitude and altitude, and trajectory characteristics of small, upward-moving coronal density enhancements. Single-spacecraft coronal rotation measurements indicate that the rotation rate of the corona in the COR1 field of view is independent of latitude and altitude, consistent with previous studies that showed rotation in the corona is very rigid compared with rotation in the photosphere. The equatorial rotation rate for this study was found to be 27.06 ± 0.08 days in 2007 and 26.97 ± 0.10 days in 2008. Using the measured rotation periods, I extract the average coronal intensity as a function of longitude at each latitude at 1.8 solar radii, and compare them to reconstructions of the coronal electron density. The longitudinal structure derived from the rotation measurements is very similar to the electron density measurements, indicating that the observed rigidity of the coronal rotation does not seem to be due to projection of low-latitude features onto higher latitudes as some authors have speculated. It has been suggested that the relative rigidity of the coronal rotation may be at least in part a measurement effect, due to the selective measurement of large and/or long-lived features in coronagraph rotation studies. Following the measurements of coronal rotation in the STEREO COR1 field of view using standard coronagraph rotation measurement techniques, I present for the first time short time lag coronagraph rotation measurements, using the unique capabilities of the STEREO dual-spacecraft mission. Finally, I present results of a systematic search of nineteen days' worth of COR1 data for small, faint, outward-moving density enhancements, sometimes referred to in the literature as plasma blobs. In the past these plasma blobs have been studied in the LASCO C2 and C3 fields of view, which extend only as low as 2 - 2.5 solar radii, and it was believed that their origin was in pointed cusps at the top of the streamer belt. Using the COR1 coronagraph I was able to observe many such features between 1.5 - 2.0 solar radii, a height below what would normally be expected if these features originate at the top of the streamer belt as suggested by the earlier observations.