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A NOTE ON SATURATION SEEN IN THE MDI/SOHO MAGNETOGRAMS View all abstracts by submitter

Yang Liu   Submitted: 2007-02-08 12:16

A type of saturation is sometimes seen in sunspot umbrae in SOHO/MDI 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 {it on-board data processing} and the {it 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 {it 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 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.

Authors: Y. Liu, A.A. Norton, P.H. Scherrer
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2007-02-09 12:04
Go to main E-Print page    Geoeffectiveness of halo coronal mass ejections  The influence of the internal structuring of coronal loops on the properties of their damped transverse oscillations  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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