Line Shape Changes and Doppler Measurements in Solar Active Regions. I. A Method for Correcting Dopplergrams from SOHO MDI

Abstract

We present a method to correct MDI high-resolution Dopplergrams in active regions for systematic observational errors due to the spectral line shape changes. We use a measurement campaign with the Advanced Stokes Polarimeter to investigate the influence of line shape changes in active regions on MDI Dopplergram calibration. An estimate of the width of the Ni I λ6768 absorption line used by MDI to measure velocity provides a linear correction to the standard calibration. This correction can be largely explained by the Zeeman broadening of the absorption line.