High-Angular-Resolution Observations and Analysis of Solar Wind H-ENAs at Mars Using Tianwen-1/MINPA Data

Energetic neutral atoms (ENAs) are generated by charge exchange process during collisions between energetic ions and neutral particles. Due to their electrical neutrality, ENAs are unaffected by electromagnetic fields and can travel along ballistic trajectories. Compared with in situ ion measurements, ENA observations enable large-scale remote imaging and provide an important means of understanding the interactions between solar wind plasma and planets. However, their angular resolution and field of view are generally very limited, which severely restricts research based on ENA observations. In this study, we propose a scanning observation and data-processing method that provides finer angular sampling of ENA measurements during spacecraft maneuvering and use it to investigate the angular distribution of H-ENAs. Using hydrogen ENA (H-ENA) data from the Mars Ion and Neutral Particle Analyzer (MINPA) aboard China’s Tianwen-1 spacecraft, we obtained an angular sampling interval of approximately 0.1∘ during scanning, compared with the instrument’s intrinsic polar angular response width of 9.7∘. The results show that the H-ENA population exhibits a velocity of 342.2±81 km/s and a temperature of 4.2±0.5 eV, whereas the upstream solar wind H+ have a velocity of 352.2±3.2 km/s and a temperature of 3.3±0.02 eV. The angular distribution of H-ENAs is highly consistent with that of the upstream solar wind H+. These observations are consistent with solar wind H+ as an important source of H-ENAs at Mars and with charge exchange as the production process. This study obtained a finely sampled angular distribution of solar wind H-ENAs and provides a method for future planetary ENA observations.