Astrophysical Journal—Jacob Harris, Mausumi Dikpati, Ian Hewins, Sarah E. Gibson, Scott W. McIntosh, Subhamoy Chatterjee, Thomas Kuchar
Left panel: all Coronal Holes centroids (CH-centroids) analysed during the entire solar cycle 23. Positive (negative) CH-centroids are displayed in blue-filled (red-filled) circles, and straight line fits for each legacy number have been displayed in solid (dashed) black lines. The thick black line represents the average slope, calculated for all CH-centroids that recur for 7 or more CRs. The thick green line represents the slope obtained from the speed due to local differential rotation. Middle panel: enlarged inset of the region marked within two horizontal strips in left panel, for CRs between 1949 and 1990, namely during the late rising phase of cycle 23. Right panel: stack plot for the same period as in middle panel for CRs 1949-1990. Magenta-circled CH-centroids in left and middle panels indicate anomalous slopes.
Features at the Sun's surface and atmosphere are constantly changing due to its magnetic field. The McIntosh Archive provides a long-term (45 yr) record of these features, digitized from hand-drawn synoptic maps by Patrick McIntosh. Utilizing this data, we create stack plots for coronal holes, i.e., Hovmöller-type plots of latitude bands, for all longitudes, stacked in time, allowing tracking of coronal hole movement. Using a newly developed two-step method of centroid calculation, which includes a Fourier descriptor to represent a coronal hole's boundary and calculate the centroid by the use of Green's theorem, we calculate the centroids of 31 unique, long-lived equatorial coronal holes for successive Carrington rotations during the entire solar cycle 23, and estimate their slopes (time versus longitude) as the coronal holes evolve. We compute coronal hole centroid drift speeds from these slopes, and find an eastward (prograde) pattern that is actually retrograde with respect to the local differential rotation. By discussing the plausible physical mechanisms which could cause these long-lived equatorial coronal holes to drift retrograde, we identify either classical or magnetically modified westward-propagating solar Rossby waves, with a speed of a few tens to a few hundreds of meters per second, to be the best candidate for governing the drift of deep-rooted, long-lived equatorial coronal holes. To explore plausible physics of why long-lived equatorial coronal holes appear few in number during solar minimum/early rising phase more statistics are required, which will be studied in future.