The Depiction of Coronal Structure in White-Light Images |
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Authors: | Huw Morgan Shadia Rifai Habbal Richard Woo |
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Institution: | (1) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, U.S.A.;(2) Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 238-725, Pasadena, CA, 91109, U.S.A. |
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Abstract: | The very steep decrease in density with heliocentric distance makes imaging of coronal density structures out to a few solar
radii challenging. The radial gradient in brightness can be reduced using numerous image processing techniques, thus quantitative
data are manipulated to provide qualitative images. We introduce a new normalizing-radial-graded filter (NRGF): a simple filter
for removing the radial gradient to reveal coronal structure. Applied to polarized brightness observations of the corona,
the NRGF produces images which are striking in their detail. Total-brightness, white-light images include contributions from
the F corona, stray light, and other instrumental contributions which need to be removed as effectively as possible to properly
reveal the electron corona structure. A new procedure for subtracting this background from LASCO C2 white-light, total-brightness
images is introduced. The background is created from the unpolarized component of total-brightness images and is found to
be remarkably time-invariant, remaining virtually unchanged over the solar cycle. By direct comparison with polarized-brightness
data, we show that the new background-subtracting procedure is superior in depicting coronal structure accurately, particularly
when used in conjunction with the NRGF. The effectiveness of the procedures is demonstrated on a series of LASCO C2 observations
of a coronal mass ejection (CME). |
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