SDO/HMI line-of-sight Dopplergram, 45-Second Data

Dopplergrams are maps of solar surface velocity. The HMI (Camera 2) instrument takes 72 filtergrams to construct a single dopplergram. Six filtergrams are taken in six positions across the nominal 6173.3 Å spectral line; each filtergram is taken in two polarization states (right circularly polarized (RCP), or Stokes I-V, and left circularly polarized (LCP), or I+V). Assuming that the absorption line is a Gaussian and that the HMI filter transmission profiles are delta functions, the Fourier coefficients can be calculated and are then used to estimate the line-of-sight magnetic field strength B.