Transformation of vector magnetograms and the problems associated with the effects of perspective and the azimuthal ambiguity

Abstract

Off-center vector magnetograms which use all three components of the measured field provide the maximum information content from the photospheric field and can provide the most consistent potential field independent of the viewing angle by defining the normal component of the field. The required transformations of the magnetic field vector and the geometric mapping of the observed field in the image plane into the heliographic plane have been described. Here we discuss the total transformation of specific vector magnetograms to detail the problems and procedures that one should be aware of in analyzing observational magnetograms. The effect of the 180-deg ambiguity of the observed transverse field is considered as well as the effect of curvature of the photosphere. Specific results for active regions AR 2684 (23 September, 1980) and AR 4474 (26 April, 1984) from the Marshall Space Flight Center Vector Magnetograph are described which point to the need for the heliographic projection in determining the field structure of an active region.