MEASUREMENTS OF EUV CORONAL HOLES AND OPEN MAGNETIC FLUX

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 $2-5 \times 10^{22}$ 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 $2-4 \times 10^{22}$ 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.

[1]  A. Gilbert,et al.  THE FIRST STRAY LIGHT CORRECTED EXTREME-ULTRAVIOLET IMAGES OF SOLAR CORONAL HOLES , 2011, 1110.3052.

[2]  P. Riley,et al.  Comparison of Heliospheric In Situ Data with the Quasi-steady Solar Wind Models , 2007 .

[3]  J. Zirker.  Coronal holes and high‐speed wind streams , 1977 .

[4]  C. J. Wolfson,et al.  The Solar Oscillations Investigation - Michelson Doppler Imager , 1995 .

[5]  C. J. Wolfson,et al.  The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) , 2011 .

[6]  J. T. Hoeksema,et al.  A New Method for Polar Field Interpolation , 2011 .

[7]  Y.-M. Wang,et al.  Coronal Holes and Open Magnetic Flux , 2009 .

[8]  J. G. Timothy,et al.  Extreme-ultraviolet observations of coronal holes - Initial results from Skylab , 1974 .

[9]  J. T. Hoeksema,et al.  The Helioseismic and Magnetic Imager (HMI) Investigation for the Solar Dynamics Observatory (SDO) , 2012 .

[10]  P. Riley.  An Alternative Interpretation of the Relationship between the Inferred Open Solar Flux and the Interplanetary Magnetic Field , 2007 .

[11]  S. Cranmer,et al.  Ultraviolet Coronagraph Spectrometer Observations of a High-Latitude Coronal Hole with High Oxygen Temperatures and the Next Solar Cycle Polarity , 2001 .

[12]  C. Schrijver,et al.  Photospheric and heliospheric magnetic fields , 2003 .

[13]  E. Parker.  Dynamics of the Interplanetary Gas and Magnetic Fields , 1958 .

[14]  R. H. Levine.  Open magnetic fields and the solar cycle , 1982 .

[15]  E. Cliver,et al.  Coronal dimmings and energetic CMEs in April‐May 1998 , 2000 .

[16]  Yang Liu,et al.  Comparison of Line-of-Sight Magnetograms Taken by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager and Solar and Heliospheric Observatory/Michelson Doppler Imager , 2012 .

[17]  S. Cranmer.  Coronal Holes , 2009, Living reviews in solar physics.

[18]  D. Mackay,et al.  The Sun’s Global Photospheric and Coronal Magnetic Fields: Observations and Models , 1999, 1211.6545.

[19]  K. L. Harvey,et al.  Spatial structure of the solar wind and comparisons with solar data and models , 1998 .

[20]  Scott H. Hawley,et al.  The Magnetic Nature of Coronal Holes , 1996, Science.

[21]  Angelos Vourlidas,et al.  FIRST MEASUREMENTS OF THE MASS OF CORONAL MASS EJECTIONS FROM THE EUV DIMMING OBSERVED WITH STEREO EUVI A+B SPACECRAFT , 2009 .

[22]  C. J. Wolfson,et al.  Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) , 2008 .

[23]  Yunchun Jiang,et al.  Formation of Transient Coronal Holes during the Eruption of a Quiescent Filament and its Overlying Sigmoid , 2008, 0901.0784.

[24]  K. O. Kiepenheuer,et al.  Structure and Development of Solar Active Regions , 1968 .

[25]  Allen S. Krieger,et al.  The structure and evolution of coronal holes , 1975 .

[26]  N. R. Sheeley,et al.  On potential field models of the solar corona , 1992 .

[27]  H. P. Jones,et al.  Differentiating Coronal Holes from the Quiet Sun by He 1083 nm Imaging Spectroscopy , 2004 .

[28]  Edward J. Smith,et al.  Sources of the solar wind at solar activity maximum , 2002 .

[29]  J. Harvey,et al.  Comparison of coronal holes observed in soft X-ray and He i 10 830 Å spectroheliograms , 1983 .

[30]  Larisza D. Krista,et al.  Automated Coronal Hole Detection Using Local Intensity Thresholding Techniques , 2009, 0905.1814.

[31]  P. Gazis,et al.  Solar cycle changes in coronal holes and space weather cycles , 2002 .

[32]  J. Harvey,et al.  Coronal holes and solar magnetic fields , 1979 .

[33]  J. Wilcox,et al.  A model of interplanetary and coronal magnetic fields , 1969 .

[34]  K. L. Harvey,et al.  Polar Coronal Holes During Cycles 22 and 23 , 2002 .

[35]  D. Mackay,et al.  A nonpotential model for the Sun's open magnetic flux , 2010, 1006.4011.

[36]  J. Linker,et al.  The Structure and Dynamics of the Corona—Heliosphere Connection , 2012 .

[37]  W. Neupert,et al.  EIT: Extreme-ultraviolet Imaging Telescope for the SOHO mission , 1995 .