Spectral Line Selection for HMI

摘要

We present information on two spectral lines, Fe i 6173 A and Ni 6768 A, that were candidates for use in the Helioseismic and Magnetic Imager (HMI) instrument. Both Fe i and Ni i profiles have clean continuum and no blends that threaten performance. The higher Lan& factor of Fe I means its operational velocity range in regions of strong magnetic field is smaller than for Ni i. Fe I performs better than Ni i for vector magnetic field retrieval. Inversion results show that Fe I consistently determines field strength and flux more accurately than the Ni i line. Inversions show inclination and azimuthal errors are recovered to 2° above 600 Mx/cm2 for Fe i and above 1000 Mx/cm2 for Ni 1. The Fe I line was recommended, and ultimately chosen, for use in HMI.