Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh23a1624f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH23A-1624
Physics
7507 Chromosphere, 7529 Photosphere, 7536 Solar Activity Cycle (2162)
Scientific paper
We use observations from the solar aspect sensor of RHESSI to characterize the large-scale temperature variation across the solar disk. Previous observations have suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band at 670 nm. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its nominal solar pointing. We reassemble these line images into synoptic images with a relatively low time cadence but an almost full coverage of more than six years. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. The analysis establishes limits on the quadrupole dependence of brightness (temperature) on position angle, a crucial unknown in our precise measurement of the solar oblateness.
Fivian Martin D.
Hudson Hugh S.
Lin Robert P.
Zahid Jabran H.
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