Radio Sounding Determination of Two‐Dimensional Plasma Structure Velocity

The Mars spacecraft Tianwen‐1 transmitted radio signals to Earth for its orbit determination. The frequencies of the radio signals exhibited sharp variations when an ICME passed through the Mars‐Earth signal path on 18 June 2021. These sharp variations were observed at different ground stations at different times. This was most probably caused by increased plasma electron density in the ICME sheath crossing the signal paths of different ground stations successively. The time‐lag information of the frequency variations is used to infer the two‐dimensional sky‐plane velocity of the corresponding plasma structure in this study. The result is compared with the in situ observations from the nearest spacecraft, STEREO‐A, and shows good consistency. The validated radio sounding method for two‐dimensional velocities of plasma structures is beneficial for characterizing complex flow geometry and transverse dynamics, and can also contribute to better space weather forecasting in the future.