Hardness of precipitating particle energy spectrum revealed by spectral riometer: the May 2024 Extreme Space Weather Event

This study investigated whether the frequency spectra of cosmic noise absorption (CNA), observed by the spectral riometer in Kilpisjärvi, Finland, can indicate the hardness of the energy spectrum of precipitating particles. CNA data have traditionally been used to evaluate electron density enhancements in the mesosphere. When observed at multiple frequencies, CNA can be approximated as 𝐶𝑁𝐴 ∝ 𝑓-n, where 𝑓 is the radio frequency of cosmic noise and 𝑛 is referred to as spectral index. Specifically, a smaller 𝑛 corresponds to electron enhancement at lower altitudes, and consequently a harder particle energy spectrum. In this study, variation in spectral indices was analyzed for three natural events, i.e., substorms, solar flares, and solar proton events, which were identified using geomagnetic data and satellite-based proton and X-ray observations. The CNA spectra were fitted with a one-minute resolution to ensure statistical reliability. The frequency distributions of the spectral index during these event types were centered around 2.0. For substorms, the spectral index in the morning sector was smaller than those in the evening and night sectors, consistent with previous studies, which found that harder electron precipitation tends to occur at that time. The smallest index also suggested that high-energy electrons caused electron density enhancement below 59 km altitude. During solar proton events, variations in proton flux hardness observed by the satellite correlated well with changes in the CNA spectral index. The results demonstrated that the spectral index derived from CNA is a valid indicator of particle spectral hardness in both substorms and solar proton events. This study presents the first observational evidence that spectral riometers can retrieve ionization information below 70 km altitude, a region previously difficult to monitor continuously.