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J.L. Lean, Y.-M. Wang, J.T. Mariska (Naval Research Laboratory), L.W. Acton (Montana State University)
Magnetic fields that emerge in the solar photosphere and extend upwards into the corona are associated with coronal heating. Some studies have determined empirically that coronal brightness depends directly on photospheric field strength, whereas others relate the brightness to the length of the loops or to the sheering of opposite polarity fields. We use the potential field source surface (PFSS) model of Wang and Sheeley (ApJ, 392, 310, 1992) to investigate the applicability of a range of quantitative associations between photospheric magnetic fields and the global brightness of the non-flaring soft X-ray corona, recorded in full disk X-rays images made by the SXT on Yohkoh. The model extrapolates all photospheric magnetic field lines, in both active regions and smaller scale features, into the corona. For an assigned coronal temperature of 1.5E6 K, the model determines coronal density by assuming hydrostatic equilibrium along each closed field line and using adopted scaling laws to relate the footpoint density to the magnetic field and/or loop length. Integrating the brightness along the line of sight then permits direct simulation of the independently measured SXT full disk coronal images. With the NSO Carrington magnetic field maps as input, the PFSS simulations can account for 85 global X-ray corona during the six years from 1992 to 1997. This agreement is achieved using a constant coronal temperature and a function that depends on both the absolute strength of the photospheric magnetic field footprints, and on the inverse loop length. Despite the overall good agreement of the simulations and observations, significant differences occur during some Carrington rotations. Simulations that utilize inputs from three independent ground-based observatories (NSO, WSO and MWO) can also at times differ significantly from each other. NASA Office of Space Science has funded this work.
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