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Measurements of EUV Coronal Holes and Open Magnetic Flux  

Chris Lowder   Submitted: 2014-02-25 15:36

Coronal holes are regions on the Sun's surface that map the footprintsof open magnetic field lines. We have developed an automated routineto detect and track boundaries of long-lived coronal holes usingfull-disk extreme-ultraviolet (EUV) images obtained by SOHO/EIT,SDO/AIA, and STEREO/EUVI. We measure coronal hole areas and magneticflux in these holes, and compare the measurements with calculations bythe potential field source surface (PFSS) model. It is shown that,from 1996 through 2010, the total area of coronal holes measured withEIT images varies between 5% and 17% of the total solar surface area,and the total unsigned open flux varies between (2-5) imes1022Mx. The solar cycle dependence of these measurements is similar to thePFSS results, but the model yields larger hole areas and greater openflux than observed by EIT. The AIA/EUVI measurements from 2010-2013show coronal hole area coverage of 5%-10% of the total surface area,with significant contribution from low latitudes, which isunder-represented by EIT. AIA/EUVI have measured much enhanced openmagnetic flux in the range of (2-4) imes1022 Mx, which isabout twice the flux measured by EIT, and matches with the PFSScalculated open flux, with discrepancies in the location and strengthof coronal holes. A detailed comparison between the three measurements(by EIT, AIA-EUVI, and PFSS) indicates that coronal holes in lowlatitudes contribute significantly to the total open magnetic flux.These low-latitude coronal holes are not well measured with either theHe I 10830 line in previous studies, or EIT EUV images; neither arethey well captured by the static PFSS model. The enhanced observationsfrom AIA/EUVI allow a more accurate measure of these low-latitudecoronal holes and their contribution to open magnetic flux.

Authors: Lowder, C., Qiu, J., Leamon, R. & Liu, Y.

Publication Status: ApJ (accepted)
Last Modified: 2014-02-26 10:52
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