SDO/HMI line-of-sight Dopplergram

Dopplergrams are maps of solar surface velocity. The HMI (Camera 2) instrument takes 72 filtergrams to construct a single dopplergram. Six filtergrams are taken in six positions across the nominal 6173.3 Å spectral line; each filtergram is taken in two polarization states (right circularly polarized (RCP), or Stokes I-V, and left circularly polarized (LCP), or I+V). Assuming that the absorption line is a Gaussian and that the HMI filter transmission profiles are delta functions, the Fourier coefficients can be calculated and are then used to estimate the line-of-sight magnetic field strength B.

多普勒图是太阳表面速度的映射。HMI（相机2）仪器通过采集72个滤光片图像来构建一个单一的多普勒图。在名义上的6173.3 Å光谱线周围，六个位置各采集一个滤光片图像；每个滤光片图像分别在两个偏振状态下采集（右旋圆偏振（RCP），或斯托克斯I-V，以及左旋圆偏振（LCP），或I+V）。假设吸收线为高斯型，并且假设HMI滤光片传输轮廓为δ函数，可以计算出傅里叶系数，随后用于估计视线方向上的磁场强度B。