Authors: Isaac Kofi Asante

The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory performs full disk observations of the Sun in the Fe I 6173Å absorption line. Filtergrams produced from HMI at 6 different wavelengths and two polarizations are transmitted to the ground for conversion to photospheric observables using available Line of Sight (LOS) algorithms. Utilizing realistic 3D RMHD StellarBox simulations for this work, we investigate the accuracy of the velocities derived with the HMI LOS algorithm with respect to the ring diagram helioseismology analysis. The Doppler velocities of the Fe I 6173Å spectral line modeled for the considered StellarBox simulations are computed using the bisector method and compared with velocities retrieved with the HMI LOS algorithm for different angles with respect to the solar disk center (0deg – 75deg). Our results show substantial agreement in the velocities derived with the HMI LOS algorithm and the bisector method for cases close to the disk center (0 deg – 30 deg), as well as for the same structure and contrast of the rings. However, away from the disk center and towards the limb, disparities emerge within the power spectral densities obtained by both velocities. However, away from the disk center  and towards the limb (> 30 deg), disparities emerge within the power spectral densities obtained by both velocities, and HMI LOS velocities lead to less-contrast ring structures. We also discuss how changing resolution affects the velocities derived with both methods.