Detection of Low-Latitude Plumes in the Outer Corona by ULYSSES Radio Ranging Measurements

Abstract

Low-latitude plumes have been detected beyond the field of view of coronagraphs in the range of 23--42 R0 in an equatorial coronal hole by Ulysses radio ranging measurements conducted in 1991 at 13 and 3.6 cm wavelengths. Exploiting the ranging measurement features of high sensitivity, wide dynamic range, high precision, high stability, and long duration, the plumes have been identified by the similarity between their transverse path-integrated density profiles and those of polar plumes observed in white-light measurements of the inner corona. Like the polar plumes, the low-latitude plumes are consistent with radial expansion and show a variation in path-integrated density of +/-10%. These results suggest that plumes are a feature common not exclusively to polar coronal holes but to coronal holes in general.