Correction of Offset in MDI/SOHO Magnetograms |
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Authors: | Liu Yang Xuepu Zhao Hoeksema J Todd |
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Institution: | (1) W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA , 94305-4085, U.S.A |
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Abstract: | Shutter noise induces a small random shift of the zero point in full-disk magnetograms obtained by the Michelson Doppler Imager
(MDI) instrument aboard SOHO. In this paper, we develop a method to remove this offset by fitting the distribution of the
magnetic field strength with a Gaussian function (Ulrich et al., 2002). We also discover a systematic error in the five-minute magnetograms that are the sum of five individual magnetograms
computed on-board; this error can be removed together with the offset. The mean solar magnetic field and synoptic frames derived
from corrected magnetograms show significant improvement. Standard synoptic charts benefit from reduced noise and elimination
of systematic errors in the individual magnetograms. This indicates that this correction is effective and necessary. |
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