A comprehensive study of cavities on the Sun: Structures and Evolution

Abstract:

Coronal cavities are often observed as circular or elliptical-shaped, darkned regions above the solar limb in EUV coronal images. They are believed to be regions of lower density relative to the surrounding corona. The cavity surrounds the prominence: prior to the eruption and as an aspect of CMEs. We used SDO/AIA limb synoptic maps, constructed from annuli above the solar limb; best show cavities in the 211Å, 193Å and 171Å passbands. The prominence associated with each cavity is best seen in the 304Å synoptic maps. We observed 429 cavities between May 2010 - Feb 2015. We examined correlations between each cavity's height, width and length. Our findings showed that around 38% of cavities were prolate, 27% oblate and 35% circular in shape. The lengths of the cavities ranged from 0.06-2.9 R_¤. When a cavity is longer than 1.5 R_¤ it has a narrower height range (0.1-0.3 R_¤), whereas when the cavity was shorter than 1.5 R_¤, it had a wider height range (0.07-0.5 R_¤). We find that the overall 3-D topology of the long stable cavities can be characterized as a long tube with an elliptical cross section. We also noticed that the circular and oblate cavities are longer in length than prolate cavities. We also studied the physical mechanisms behind the cavity drift towards the pole and found it to be tied to the meridional flow. Finally, by observing the evolution cavity regions using SDO/HMI surface magnetic field observations, we found cavity belt formed near the polar coronal hole boundary. Results showed that the cavity belt migrated towards higher latitude with time and the cavity belt disappeared after the polar magnetic field reversal. This result shows that cavity evolution provides new insight into the solar cycle. Moreover, we selected 30 random cavities from our catalog and studied their morphology, thermal properties, linear polarization signatures, precursors of eruption, and underlying magnetic field and tried to interpret the shapes and stability of the cavities.