Achieving Consistent Vector Magnetic Field Measurements from SDO/HMI

NASA's Solar Dynamics Observatory (SDO) is delivering vector magnetic field observations of the full solar disk with unprecedented temporal and spatial resolution; however, the satellite is in a highly inclined geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3 km/s over a day which introduces significant orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated that the orbital artifacts contaminate all spatial and temporal scales in the data and developed a procedure for mitigating these artifacts in the Doppler data obtained from the Milne-Eddington inversions in the HMI Pipeline. Simultaneously, we have found that the orbital artifacts may be introduced by inaccurate estimates for the free-spectral ranges (FSRs) of the optical elements in HMI. We describe our approach and attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical elements of HMI within their measurement uncertainties of ±1%.