Radio Absorption in the Nightside Ionosphere of Mars During Solar Energetic Particle Events

• Published in: Space Weather Vol. 21; no. 12
• Main Authors: Harada, Y., Nakamura, Y., Sánchez‐Cano, B., Lester, M., Terada, N., Leblanc, F.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.12.2023; American Geophysical Union (AGU)
• Subjects: Absorption; Energetic particles; Ionization; Ionosphere; Ionospheric absorption; Lower ionosphere; Manned Mars missions; Mars; Mars atmosphere; Mars exploration; Mars ionosphere; Mars surface; Mathematical models; Modelling; Numerical models; Planetary ionospheres; radio absorption; Radio communications; Radio frequency; Radio waves; Sciences of the Universe; solar energetic particles; Solar flares; Space weather; Spacecraft; Surface chemistry
• ISSN: 1542-7390; 1539-4964; 1542-7390
• DOI: 10.1029/2023SW003755

Characterization, understanding, and prediction of the Martian radio environment are of increasing importance to the forthcoming human exploration of Mars. Here we investigate 3–5 MHz radio absorption in the nightside ionosphere of Mars caused by enhanced ionization at <100 km altitudes during solar energetic particle (SEP) events. We conduct a quantitative analysis of radio absorption and SEP flux data that have been accumulated by two spacecraft currently orbiting Mars, thereby demonstrating that radio absorption is clearly correlated with SEP fluxes. A comparison of the observations with radio absorption properties predicted by a numerical model indicates that the relative temporal changes, radio frequency dependence, and SEP energy dependence of the observed radio absorption are in agreement with the model prediction. Meanwhile, the model systematically overestimates the radio absorption in the ionosphere by a factor of 3.7. We explore several sources of uncertainty, including the electron‐neutral collision frequency, absolute sensitivity of the SEP instrument, and limited transport of SEPs to the atmosphere, but the ultimate cause of the systematic discrepancy between the measured and modeled radio absorption is yet to be identified. Further efforts should be put into the development of a comprehensive and observationally validated model of radio absorption in the Martian ionosphere to assist the future crew and spacecraft activities on the surface of Mars. Plain Language Summary Radio waves are widely used for communication, positioning, and navigation in our society. As human exploration of Mars progresses in the coming decades, radio technology will be in use for crew and spacecraft activities on the surface of Mars. On Earth, high‐frequency radio communication is sometimes disrupted due to enhanced radio absorption in the lower ionosphere arising from solar flares. The ionosphere of Mars also causes strong radio absorption during space weather events as observed in the nightside ionosphere during solar energetic particle (SEP) precipitation. To assess the effects of ionospheric absorption on radio communication at Mars, we need to develop a reliable numerical model that is validated against observations. We initiate this effort by comparing observations with predictions from a state‐of‐the‐art numerical model. The model successfully reproduces important aspects of the observed radio absorption during SEP events, namely relative temporal changes, radio frequency dependence, and SEP energy dependence. However, the modeled and observed absorption levels systematically differ by a factor of 3.7, reasons for which have not been fully understood. Further research is required to improve and validate the model if we are to characterize the radio communication environment prior to the future arrival of humans to Mars. Key Points Observations indicate a clear correlation between solar energetic particle fluxes and radio absorption in the nightside ionosphere of Mars The observed properties of radio absorption are well reproduced by a numerical model except for a factor of 3.7 systematic discrepancy Further model development and validation will pave the way for accurate prediction of the radio environment on the Martian surface