Magnetic Fields from SOHO MDI Converted to the Mount Wilson 150 Foot Solar Tower Scale

Abstract

In order to permit the construction of long-duration time series dependent on the Sun's magnetic field, this paper presents a detailed cross-correlation between sets of simultaneous magnetograms from the Mount Wilson Observatory (MWO) and the Michelson Doppler Imager (MDI) aboard the SOHO spacecraft. The MWO 150 foot (45.72 m) solar tower telescope magnetogram data are for the Fe I 525.0 nm and Ni I 676.8 nm lines, and the MDI data are level 1.8 magnetograms also for the Ni I 676.8 nm spectral line. In these comparisons, we apply a saturation correction factor to the MWO 525.0 nm fields prior to the derivation of the MDI scale factor. Data from 1997 March to 2002 August are used for this work. We have found that the ratio of MWO Fe I 525.0 nm magnetograms over MDI magnetograms is about 1.7, and it is a function of the center-to-limb angle. Moreover, there are differences between the west-side and the east-side ratios, and these differences may come from the angle dependence of the Michelson filters in the MDI instrument. The MDI tuning changes, on the other hand, are not associated with significant jumps in the derived scale factor ratio. The average scale factors should be adequate for the construction of MDI images closely comparable to those of the saturation-corrected long-duration MWO 525.0 nm sequence.