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Paper: Limits to the Radiative Asymmetry of the Quiet Solar Disk
Volume: 383, Subsurface and Atmospheric Influences on Solar Activity
Page: 253
Authors: Livingston, W.; Sheeley, N.R. Jr.
Abstract: Precise data on the uniformity of photospheric radiation over the solar disk seems not to exist. Such may be needed for the future detection of planets crossing solar-like stellar disks, for example. To obtain this information we have made monochromatic scans along the central meridian of the quiet Sun using single element detectors which do not require “flat fielding.” The scans were in continua and selected Fraunhofer lines ranging from 3129 to 46880 Å; the observational epoch was near solar minimum: Oct 2006 to Feb 2007. The meridian was chosen to avoid rotational Doppler shifts. We extract the asymmetry between the northern and southern hemispheres and this is our main product. In the near IR and visible continuum, averaging over granulation and discounting sunspots, such asymmetry is as low as 0.01%; 0.005% at 34168 Å on 8 Feb 2007. In the violet and UV this increases to 1%. In the cores of medium strength photospheric lines and in chromospheric lines the asymmetry is up to 15%. Faculae are the probable source of our measured quiet disk asymmetries, and the continuum at 34168 Å is favorable for this reason. Line core scans are in general flatter than continuum scans because they sample thinner, higher layers of the atmosphere, where the temperature gradient is less. Obviously it would be desirable to expand on the temporal coverage.
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