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.

Line-of-sight magnetic field strength