CME-related Large Decreases in the Differential Phase Delay of Tianwen-1 DOR Signals

Differential phase delay is calculated for the differential one-way range (DOR) signals transmitted by Tianwen-1, the first Chinese Mars spacecraft that entered into the Mars orbit on 2021 February 10. Large decreases in the differential phase delay are identified in the DOR signals received by ground stations on 2021 March 23 and June 18. The decreases indicate sizable increases of the total electron content (TEC) along the DOR signal path between Tianwen-1 and the ground stations. The TEC increases are estimated to be 85 and 175 TEC units on 2021 March 23 and June 18, respectively. Evidence shows that they are caused by the sheath regions ahead of the coronal mass ejections (CMEs) that traversed the signal path on both days. The results represent the first observations of CME-related structures by the DOR signals and demonstrate the potential of DOR signals in remote sensing the interplanetary plasma structures in the solar wind.