A Note on Saturation Seen in the MDI/SOHO Magnetograms |
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| Authors: | Y Liu A A Norton P H Scherrer |
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| Institution: | (1) W.W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085, USA;(2) National Solar Observatory, 950 N. Cherry Ave., Tucson, AZ 85719, USA |
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| Abstract: | A type of saturation is sometimes seen in sunspot umbrae in MDI/SOHO magnetograms. In this paper, we present the underlying
cause of such saturation. By using a set of MDI circular polarization filtergrams taken during an MDI line profile campaign
observation, we derive the MDI magnetograms using two different approaches: the on-board data processing and the ground data processing, respectively. The algorithms for processing the data are the same, but the former is limited by a 15-bit lookup table. Saturation
is clearly seen in the magnetogram from the on-board processing simulation, which is comparable to an observed MDI magnetogram taken one and a half hours before the campaign data. We analyze
the saturated pixels and examine the on-board numerical calculation method. We conclude that very low intensity in sunspot
umbrae leads to a very low depth of the spectral line that becomes problematic when limited to the 15-bit on-board numerical
treatment. This 15-bit on-board treatment of the values is the reason for the saturation seen in sunspot umbrae in the MDI
magnetogram. Although it is possible for a different type of saturation to occur when the combination of a strong magnetic
field and high velocity moves the spectral line out of the effective sampling range, this saturation is not observed. |
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