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Dopplergrams

Dopplergrams are maps of solar surface velocity. The dopplergram data is located in the DRMS series hmi.V_45s and hmi.V_720s

Keywords:

All the FITS header keywords that appear in the dopplergram data are explained here. Any combination of FITS header keywords CAMERA and T_REC is unique for the both DRMS series hmi.V_45s and hmi.V_720s. Further explanation on WCL keyword descriptions can be found in this paper.

Algorithm For Calculation:

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. The MDI-like algorithm for calculating the dopplergrams is available here.

Processing:

The link to the HMI pipeline processing is here.
The status of the pipeline processing can be checked here.

Calibration:

The wavelength dependence calibration tests for magnetograms are outlined here. The image quality calibration methods are outlined in this paper. The polarization calibration methods are outlined here.

Status Notes and/or Known Problems:

Look-up tables and polynomial correction have been applied to the Dopplergrams. All the Line-of-Sight Observables show fluctuations in the values with a twenty four hour cadence. This is an instrumental effect.









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