A Note on Saturation Seen in the MDI/SOHO Magnetograms

Details A Note on Saturation Seen in the MDI/SOHO Magnetograms A Note on Saturation Seen in the MDI/SOHO Magnetograms

Mar 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007soph..241..185l&link_type=abstract

Solar Physics, Volume 241, Issue 1, pp.185-193

Physics

17

Scientific paper

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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