HMI Data Series hmi.M_45s - Line-of-Sight Magnetic Field - 45 second
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HMI magnetograms are constructed from a series of filtergrams obtained at 6 wavelengths in right and left circular polarizations.
The filtergrams for each polarization determine a velocity and the difference in the two velocities is the magnetic field.
The 45s magnetograms use filtergrams from the HMI Doppler camera as part of the standard 'velocity' sequence.
The algorithm interpolates in time to the center of the 45s interval using 135 seconds of data.
The noise level is approximately 15 G (more precisely it is a flux density measured in maxwell's per square cm).