Soothing Massage of HMI Magnetic Field Data

The Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) has measured solar polarization at six wavelengths across the Fe I 6173 spectral line with one arc second resolution nearly every 90 or 135 seconds since May 2010, and the Stokes parameters are determined every 720 seconds over the full disk. The quality of the filtergrams is remarkably uniform, but the inverted and disambiguated magnetic field values are sometime incorrect or show small systematic variations with time and space. Estimates of the numerical uncertainties are provided for each pixel, but, for some kinds of analysis, having a smoother and more uniform time series can be useful. This report describes methods that can be used to 'massage' the observations to improve consistency and appearance and minimize unwanted variability. Effects considered include issues related to field inversion, disambiguation, instrument sensitivity, optical distortion, and systematic errors due to spacecraft velocity. The resulting 'corrections' typically depend on assumptions about the behavior of the solar magnetic field, so care must be taken when using such results.