SDO/HMI Linedepth Data

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.

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 at http://jsoc.stanford.edu/relevant_papers/observables.pdf.

HMI所测量的Linedepth是指太阳表面Fe I吸收线在6173.3 Å处的真实连续谱强度与核心强度之间的强度差。HMI在六个位置采样该Fe I吸收线。在假定“纯净”的太阳Fe I线形为高斯分布，且HMI滤波器的传输特性为δ函数的条件下，可计算出Fe I线的第一、二傅里叶系数，并据此估算多普勒速度。通过对多普勒移位λo、线宽σ和线深度Id的估计，重构太阳线的强度，从而获得连续谱强度Ic的估算（代理值）。更多详细信息请参考http://jsoc.stanford.edu/relevant_papers/observables.pdf。