Notes on the HMI Noise Mask Used in Vector Field Disambiguation
The average vector magnetic field intensity measured by HMI in weak-field regions varies across the disk, with Sun-spacecraft velocity, and with HMI tuning. The noise is important for setting the threshold for disambiguation of the vector field. This is described in the Vector Field Pipeline Overview paper.
For each tuning of the HMI instrument, a map is made of the average weak-field intensity. A separate map is made for each 100 m/s interval in Sun-SDO velocity. The map is projected onto Chebychev polynomials and stored in the data series hmi.lookup_chebycoef_bnoise, as specified in the keyword MASKINFO.
The reconstructed noise mask is interpolated to the specific velocity for each disambiguation of the field. See the noise reconstruction code for implementation.
Notes and Changes
For some data calculated between December 2013 and May 2014 MASKINFO will point to a different data series, hmi.lookup_ChebyCoef_TWindow. Those maps are now included in hmi.lookup_chebycoef_bnoise
Additional information can be found at Noise Mask Summary