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Paper: Spectral Line Selection for HMI
Volume: 358, Solar Polarization 4
Page: 193
Authors: Norton, A.A.; Pietarila Graham, J.D.; Ulrich, R.K.; Schou, J.; Tomczyk, S.; Liu, Y.; Lites, B.W.; Lopez Ariste, A.; Bush, R.I.; Socas-Navarro, H.; Scherrer, P.H.
Abstract: We present information on two spectral lines, Fe I 6173 Å and Ni I 6768 Å, 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 Landé 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 I. The Fe I line was recommended, and ultimately chosen, for use in HMI.
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