Possible presence of cool electron atmosphere at Mercury diagnosed from whistler-mode wave characteristics

Cold plasma is the most fundamental component of plasma physics and reflects planetary atmospheres through ionization processes. However, cold electrons at Mercury remain poorly understood due to the difficulty of measuring low-energy electrons (< 100 eV), which are often contaminated by spacecraft charging and secondary electrons. We investigated the possible presence of cold and cool (< 100 eV) electrons at Mercury estimated from diagnostics using whistler-mode wave propagation. We found that the possible mid-latitude wave propagation observed during BepiColombo’s 4th Mercury flyby, characterized by a low power attenuation, cannot be reproduced in the absence of cold and cool electrons. This sensitive diagnosis is consistent with whistler-mode hiss wave characteristics in the Earth’s plasmasphere, which holds rich cold electrons. Contrary to the long-held belief that Mercury holds no cold electrons due to its tenuous atmosphere under the strong solar wind dynamical pressure, our results possibly suggest the importance of previously unknown ionization and generation processes contributing to the cold electron population at Mercury. The possibility regarding the presence of cool electron atmosphere at Mercury provides insights into the evolution of the planetary atmosphere, and also should be a new scientific target after Mercury orbital insertion in 2026 with full wave and plasma suite observations.