The HMI continuum data refers to map of the continuum intensity of the solar spectrum in the region of the Fe I absorption line at 6173 Å on the surface of the sun. The continuum data is available in the DRMS series hmi.Ic_45s and hmi.Ic_720s
All the FITS header keywords that appear in the Continuum data are explained here. Any combination of FITS header keywords CAMERA and T_REC is unique for the both DRMS series hmi.Ic_45s and hmi.Ic_720s. Further explanation on WCL keyword descriptions can be found in this paper.
HMI samples the Fe I absorption line at 6173.3 Å 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 Doppler Velocity can be made. An estimate (proxy) for the continuum intensity Ic is obtained by "reconstructing" the solar line from the estimates of the Doppler shift λo, the linewidth σ, and the linedepth Id. More details can be found here.
All the Line-of-Sight Observables show fluctuation in the values with a twenty four hour cadence. This is an instrumental effect.