Xuepu Zhao, J. Todd Hoeksema and Phil H. Scherrer
The Whole Sun Month (WSM) equatorial coronal hole or "elephant's trunk" crossed the visible solar disk between 22 and 31 August, 1996. The hole was observed by a variety of imaging instruments, including MDI, EIT, CDS, SUMER, and UVCS on SOHO, SXT on Yohkoh, and the 10830 instrument at Kitt Peak. The hole boundaries observed by various instruments are not the same. There are differences not only in shape and size, but also in the hole's evolution, because different instruments are sensitive to different physical properties that depend on height. By extrapolating the observed photospheric field into the corona, we determine the location of the open field region with height and compare it with the observed boundaries of the coronal hole. We use both MDI and WSO magnetograms in our calculation to see how using high resolution MDI data improves the boundary determination. Because of the 96-minute cadence of MDI magnetograms, it may be possible to determine whether rapid changes of the detailed photospheric field affect the evolution of coronal hole boundary.
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