Magnetograms are maps of the observed solar magnetic field in the photosphere. The line-of-sight component of the field can be accurately measured as it evolves in time over the full visible disk of the Sun. The HMI instrument exploits the Zeeman effect, which allows the strength of the line-of-sight component of the magnetic field to be determined by measurements of the spectral line in circularly polarized light.
HMI makes two independent measurements of the line-of-sight component of the photospheric magnetic field. One is collected every 45 seconds with the HMI Doppler camera. The other is computed every 720 seconds using filtergrams recorded by the Vector Field camera. The spatial resolution is 1 arc second (half arc-second pixels) and the full disk images are collected on a 4096**2 detector. The noise level is nominally between 5 and 10 Gauss. HMI really measures flux density in Mx/cm2 in each pixel.
hmi.M_720s magnetograms are computed every 12 minutes (720 seconds) by combining registered filtergrams obtained over a 1260 second time interval. The images provide a full-disk low-noise image of the Sun.
hmi.M_45s magnetograms are derived every 45 seconds. They provide a full disk image of the magnetic field and are best used to investigate rapidlly evolving field structures.
A sequence of polarized filtergrams is taken at six wavelength positions across the nominal 6173.3 Å spectral line.
The hmi.M_45s magnetograms are obtained with CAMERA 1. This is the normal HMI Doppler camera and the magnetic field is derived from the difference of Dopplergrams obtained in right and left circularly polarized light computed from an interleaved sequence of 12 filtergrams that takes 45 seconds to complete. The MDI-like algorithm for calculating the Dopplergrams is described here.
CAMERA 2 provides the filtergrams for the 720s magnetograms. The vector field camera observes the same wavelengths, but collects additional polarizations in a 135-second sequence. Using only the circularly polarized exposures, registered filtergrams collected during several sequences are combined every 12 minutes to produce hmi.M_720s.
The link to the HMI pipeline processing is here.
The status of the pipeline processing can be checked here.
Precise calibration of magnetic field measurements is problemmatic because of instrumental characteristics (e.g. spatial resolution, scattered light, and filter characteristics) and because of the inherent complexity of the solar magnetic field in space and time, of the emission of radiation in the solar atmosphere, and of the interpretation of the measured polarization signals.
The wavelength dependence calibration tests for Dopplergrams are outlined here. The image quality calibration methods are outlined in this paper. The polarization calibration methods are outlined here.
Look-up tables and polynomial correction have been applied to the line-of-sight magnetic field. All of the Line-of-Sight Observables show fluctuations in the values with a twenty four hour period. This is an instrumental effect due to the SDO orbit.
The Primary Keywords are T_REC (the time of the observation) and CAMERA (the HMI light path). The FITS header keywords for the magnetogram data are explained here. WCS coordinate keyword descriptions can be found in this paper.