Solar photospheric velocities measured in space: A comparison between SO/PHI-HRT and SDO/HMI

The Polarimetric and Helioseismic Imager (SO/PHI) on board Solar Orbiter is a spectropolarimeter scanning the Fe i line at 617.3 nm, providing data of the solar photosphere. The same line is sampled by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and many other ground-based instruments. In this paper our aim is to assess the consistency between the line-of-sight (LoS) velocity measurements from the two instruments. Reliable measurements of upflows and downflows from SO/PHI are crucial and unique when Solar Orbiter is facing the far side of the Sun. Also, a combination of measurements from two vantage points to study horizontal flows must rely on consistent observations. For this purpose, we compare the LoS velocity measured by SO/PHI's High Resolution Telescope (SO/PHI-HRT) and SDO/HMI on 29 March 2023, when Solar Orbiter was crossing the Sun--Earth line at a distance of 0.39 au from the Sun. Because such co-alignments are rare, this configuration offered an almost unique opportunity to directly compare data products from the two telescopes. The data are aligned and remapped to allow a pixel-by-pixel comparison of the whole time series of 4 hours. Temporal and spatial variations are considered for a direct combination of the measurements. The LoS velocity distributions are evaluated and a clear linear relation is found between the two instruments with a slope of 0.96 and a correlation of 92%. We find that the signals form at similar heights, with a separation of 9±12 km, which is larger than previous estimates. A close-up look at the penumbra of a sunspot and its Evershed flow is presented. We conclude that the signals inferred by and show very good agreement and high correlation when instrumental effects and large-scale velocities on the Sun are properly accounted for.