On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal

The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe i absorption line at 6173 . We use the output of a high-resolution, 3D, time-dependent, radiation-hydrodynamic simulation based on the CO (5) BOLD code to calculate profiles F(lambda,x,y,t) for the Fe i 6173 line. The emerging profiles F(lambda,x,y,t) are multiplied by a representative set of HMI filter-transmission profiles R (i) (lambda, 1a parts per thousand currency signia parts per thousand currency sign6) and filtergrams I (i) (x,y,t; 1a parts per thousand currency signia parts per thousand currency sign6) are constructed for six wavelengths. Doppler velocities V (HMI)(x,y,t) are determined from these filtergrams using a simplified version of the HMI pipeline. The Doppler velocities are correlated with the original velocities in the simulated atmosphere. The cross-correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal is formed rather low in the solar atmosphere. The same analysis is performed for the SOHO/MDI Ni i line at 6768 . The MDI Doppler signal is formed slightly higher at around 125 km. Taking into account the limited spatial resolution of the instruments, the apparent formation height of both the HMI and MDI Doppler signal increases by 40 to 50 km. We also study how uncertainties in the HMI filter-transmission profiles affect the calculated velocities.