Correction of MDI Full-Disk Magnetograms for Limb Angle Effects by the Assumption of Average Magnetic Uniformity

Details Correction of MDI Full-Disk Magnetograms for Limb Angle Effects by the Assumption of Average Magnetic Uniformity Correction of MDI Full-Disk Magnetograms for Limb Angle Effects by the Assumption of Average Magnetic Uniformity

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh11a0375r&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH11A-0375

Physics

1650 Solar Variability (7537), 7524 Magnetic Fields, 7529 Photosphere, 7538 Solar Irradiance

Scientific paper

Analysis of magnetic fields on the sun is crucial to understanding the generation of solar active regions, plage, and the supergranular network and their contributions to Total Solar Irradiance (TSI) variation. The Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO) satellite has measured full-disk Stokes-V magnetograms along with cotemporal continuum images throughout Solar Cycle 23. These pairs of images can be used to study continuum irradiance changes correlated to magnetogram signal over the entire disk and over most of Cycle 23. However the MDI magnetogram signal must be corrected for line-of-sight angle variations. Typically a 1/mu correction factor (where mu is the cosine of the line-of-sight angle to the surface normal) is applied across the entire disk. However near-limb measurements where mu approaches zero are siginificantly over-corrected by a simple 1/mu factor. We demonstrate a newly developed geometric correction function for MDI magnetograms based on the assumption that the distribution of Magnetic Flux Density (MFD) in the active latitudes should exhibit the same behavior no matter what angle it is viewed from. We examine a series of 106 magnetograms in the time period from 01-January-2001 to 30-April-2001. This method was successful in correcting MDI magnetograms to reasonable MFD in the limbs, with a minimal number of overcorrected points in the extreme limb, an improvement upon the typical approach of 1/mu. The resulting distributions of MFD were more uniform over 50 bins of mu and can be more successfully used to study continuum irradiance variations as a function of MFD across the entire disk. This research was partially supported by LWS TR&T Grant NNG05GK46G.

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