Joint Science Operations Center (JSOC)
The HMI Linedepth refers to the difference in intensity between the "true"continuum and intensity at the core of the the Fe I absorption line at 6173.3 Å on the surface of the sun. The linedepth (Ld) data is in the DRMS series hmi.Ld_45s and hmi.Ld_720s
All the FITS header keywords that appear in the linedepth data are explained here. Any combination of FITS header keywords CAMERA and T_REC is unique for the both DRMS series hmi.Ld_45s and hmi.Ld_720s. Further explanation on WCL keyword descriptions can be found in this paper.
HMI samples the Fe I absorption line at 6173.3 Angstrom at six points. Assuming that the "pure" solar Fe I line profile is a Gaussian and the HMI filter filter transmission profiles are delta functions,the first and second Fourier coefficients of the of the Fe I line can be calculated and an estimate of the linedepth is made. More details can be found here.
The link to the HMI pipeline processing is here. The status of the pipeline processing can be checked here.
Look-up tables and polynomial correction have *NOT* been applied to the Linedepth data.
All the Line-of-Sight Observables show fluctuation in the values with a twenty four hour cadence. This is an instrumental effect.
