An Update of the Automatic Detection of Polar Coronal Holes in the EUV
Abstract
An automated detection method of polar coronal holes was presented by Kirk et al. in 2009. This method, called perimeter tracking, uses a series of 171, 195, and 304 Å full disk images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO to measure the perimeter of polar coronal holes as they appear on the limbs. We have updated the previous time series, which ranged from mid 1996 through 2007, up to the most recently available EIT data in 2010, and refined the detection parameters, adjusting them to fit the data more precisely and accurately. Additionally, we increased our analysis to utilize all available EIT images rather than one image per day in the previous work. We find the wavelength average of the northern hole is about 5.5% of the solar surface area while the southern hole average measures about 4.5% during 1996 in the revised data set. These values are somewhat larger then we previously reported due to the improvements in the algorithm. We also find that in 2010, the northern and southern hole areas are about 3.0% and 3.5% of the total solar surface, respectively. The north and south polar hole areas are now noticeably smaller in the recent minimum than they were at the beginning of cycle 23. This is especially true in the northern hemisphere. We will compare these polar hole areas with the total coronal hole area found using EIT synoptic maps over the same time series and wavelengths.
- Publication:
-
Eddy Cross-Disciplinary Symposium on Sun-Climate Research
- Pub Date:
- October 2010
- Bibcode:
- 2010eddy.confE...1H