Abstract
As the observational signature of the footprints of solar magnetic field lines open into the heliosphere, coronal holes provide a critical measure of the structure and evolution of these lines. Using a combination of Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope (SOHO/EIT), Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), and Solar Terrestrial Relations Observatory/Extreme Ultraviolet Imager (STEREO/EUVI A/B) extreme ultraviolet (EUV) observations spanning 1996 – 2015 (nearly two solar cycles), coronal holes are automatically detected and characterized. Coronal hole area distributions show distinct behavior in latitude, defining the domain of polar and low-latitude coronal holes. The northern and southern polar regions show a clear asymmetry, with a lag between hemispheres in the appearance and disappearance of polar coronal holes.
Highlights
Coronal holes are the observational signatures of regions of open solar magnetic field
Through the use of Solar and Heliospheric Observatory (SOHO)/Extreme ultraviolet Imaging Telescope (EIT), Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), and Solar Terrestrial Relations Observatory (STEREO)/EUVI A/B extreme ultraviolet (EUV) data from May 1996 until August 2014, coronal hole boundaries have been tracked over the entirety of Solar Cycle and a good portion of Cycle
This extensive range allows for a detailed analysis of coronal hole evolution in all latitudes, and for a comparison of the magnetic open flux, which is directly measured from the coronal holes, from the past cycle to the current cycle
Summary
Coronal holes are the observational signatures of regions of open solar magnetic field. More recent work by McIntosh et al (2013) explored the hemispheric asymmetry of photospheric magnetism throughout Cycle and the early stages of Cycle While He I 10,830 Å observations of coronal holes can provide accurate results in polar regions, there is discrepancy at lower latitudes (Kahler, Davis, and Harvey, 1983; Schrijver and De Rosa, 2003; Malanushenko and Jones, 2005). McIntosh et al (2014) showed that low-latitude coronal holes migrate toward the equator as the cycle progresses, whereas during solar minimum, holes exist primarily in polar regions, where EUV bright points and coronal green line emission is sparse These areas are of crucial importance in the solar magnetic activity cycle.
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