Measurements of Euv Coronal Holes and Open Magnetic Flux

Abstract

Coronal holes are regions on the Sun's surface that map the foot-prints of open magnetic field lines. We have developed an automated routine to detect and track boundaries of long-lived coronal holes using full-disk EUV images obtained by SoHO:EIT, SDO:AIA, and STEREO:EUVI. We measure coronal hole areas and magnetic flux in these holes, and compare the measurements with calculations by the PFSS model. It is shown that, from 1996 through 2010, the total area of coronal holes measured with EIT images varies between 5% and 17% of the total solar surface area, and the total unsigned open flux varies between 2-5×10 22 Mx. The solar cycle dependence of these measurements are similar to the PFSS results, but the model yields larger hole areas and greater open flux than observed by EIT. The AIA/EUVI measurements from 2010-2013 show coronal hole area coverage of 5-10% of the total surface area, with significant contribution from low latitudes, which is under-represented by EIT. AIA/EUVI have measured much enhanced open magnetic flux in the range of 2-4×10 22 Mx, which is about twice the flux measured by EIT, and matches with the PFSS calculated open flux, with discrepancies in the location and strength of coronal holes. A detailed comparison between the three measurements (by EIT, AIA-EUVI, and PFSS) indicates that coronal holes in low latitudes contribute significantly to the total open magnetic flux. These low-latitude coronal holes are not well measured with either the He I 10830 line in previous studies, or EIT EUV images; neither are they well captured by the static PFSS model. The enhanced observations from AIA/EUVI allow a more accurate measure of these low latitude coronal holes, and their contribution to open magnetic flux.

Real-Time Solar Wind Prediction Based on SDO/AIA Coronal Hole Data

“…The algorithm searches for a dominantlocal minimum in the intensity distribution for the EUV image. On average it lies between 55 ± 15 digital numbers (DN), consistent with the findings of Lowder et al (2014). If the algorithm is not able to detect a well defined minimum in the distribution, which is the case for images where 5% of the whole solar disc is covered by coronal hole pixels, the threshold is set to 55 DNs.…”

“…The distribution of the solar wind is also decisive for deriving the propagation behavior of CMEs . The relationship between the size and location of CHs in the corona and the solar wind parameters and geomagnetic effects measured at 1 AU has been investigated by several groups (Nolte et al, 1976;Robbins, Henney, and Harvey, 2006;Vršnak, Temmer, and Veronig, 2007a,b;Luo et al, 2008;Abramenko, Yurchyshyn, and Watanabe, 2009;Obridko et al, 2009;de Toma, 2011;Verbanac et al, 2011;Lowder et al, 2014). In the early phases of coronal holes identification, CHs were visually tracked by experienced observers (Harvey and Recely, 2002;McIntosh, 2003).…”

“…In the early phases of coronal holes identification, CHs were visually tracked by experienced observers (Harvey and Recely, 2002;McIntosh, 2003). Recently, several groups tried to automate the process for the identification and detection of coronal holes using different approaches such as perimeter tracing (Kirk et al, 2009), fuzzy clustering (Barra et al, 2009), multichannel segmentation (Delouille, Barra, and Hochedez, 2007), edge-based segmentation (Scholl and Habbal, 2008), intensity thresholding (Krista and Gallagher, 2009;de Toma, 2011;Rotter et al, 2012) and magnetic track-boundaries (Lowder et al, 2014). One way to model the physical parameters of the solar wind are MHD models of the corona and heliosphere, using synoptic solar magnetic field maps as input, such as ENLIL (Odstrcil and Pizzo, 2009).…”

Three-Year Global Survey of Coronal Null Points from Potential-Field-Source-Surface (PFSS) Modeling and Solar Dynamics Observatory (SDO) Observations

“…Several studies have found the distribution of magnetic null points in PFSS model fields (Cook, Mackay, and Nandy, 2009;Platten et al, 2014). The PFSS is, however, a theoretical model field known to depart from the field of the real solar corona (Lowder et al, 2014). One measure of the fidelity of the model could be obtained by comparing the number of null points predicted by the model to the number observed in coronal observations.…”

Segmentation of Coronal Holes Using Active Contours Without Edges

“…Krista and Gallagher (2009) use magnetic flux skewness to distinguish CHs from filaments, a threshold consistency test for robustness of their intensity thresholds, and solar wind data from ACE and STEREO to compare to CH locations. Lowder et al (2014) extend the method of Krista and Gallagher (2009) to use data from multiple instruments, study CH persistence, and compare CH magnetic flux to potential field source surface models. Caplan, Downs, and Linker (2016) propose point-spread function decovolution and limb brightening correction to normalize for instrumental variation and use a two-threshold region growing approach for segmentation of CHs and study CH persistence in SDO/AIA and STEREO/EUVI images.…”