Migration of Low Latitude Long Lived Coronal Holes in the McA: An Analysis of Rossby Waves Based on Coronal Hole Drift Speeds
Abstract
By developing a novel centroid-calculation technique, we analyze long-term McIntosh Archive data to compute the centroids of long-lived coronal holes (CH) in the latitude bands of +10 to -10. The technique involves a two-step algorithm for computing the CH-centroids: (i) Fast Fourier Transform to determine the surface area that represents a coronal hole in a specified latitude-band; (ii) Green's theorem to convert the surface integral to a line-integral along the hole boundary. After building a Hovmoller-type (longitude-time) diagram for these CH-centroids, we estimate their latitude-longitude drift patterns with time. For solar cycle 23, we find that their spatio-temporal drift is not determined: by the local differential rotation; instead a large retrograde longitudinal speed of 100-150 m/s overpowers the local differential rotation speed, causing the resultant drift-speed of these CH-centroids in longitude with time. We reason that Rossby waves are the most plausible candidates to cause the retrograde drift patterns of these deep-rooted, long-lived equatorial coronal holes.
We are now exploring how Rossby wave speeds, derived from drift speeds of coronal holes, varies from cycle to cycle, specifically from solar cycle 20 to 23. Notably solar cycles 20 and 23 are weak and solar cycles 21 and 22 are strong. If there is a correlation between the strength of the solar cycles and the Rossby wave speeds this will point to magnetized Rossby waves being the true cause local coronal hole drift speeds. If there is a random relationship, hydrodynamic Rossby waves or a different mechanism could be causing the drift speeds. We are also exploring ways to improve upon the centroid calculation technique, both in terms of accuracy and computation time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMSH22A..07H