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之线深度是指太阳表面Fe I吸收线6173.3 Å处的真实连续光谱与核心强度之间的强度差。HMI在六个位置对6173.3 Å处的Fe I吸收线进行采样。假定“纯净”的太阳Fe I线型为高斯分布，且HMI滤波器的滤波透射曲线为狄拉克δ函数，可以计算Fe I线的首项和第二项傅里叶系数，并据此估算多普勒速度。通过“重建”太阳线，利用多普勒移位λo、线宽σ和线深度Id的估计值，得到连续光谱强度Ic的估算值（代理值）。更详细的信息请参考http://jsoc.stanford.edu/relevant_papers/observables.pdf。