[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P10] Comparison of CNA frequency spectra observed by spectroral riometer: substorms, solar flares, and solar proton events during the May 2024 storm

*Tomotaka Tanaka¹, Yasunobu Ogawa^1,2, Yoshimasa Tanaka^1,2,3, Akira Kadokura^2,3, Mizuki Fukizawa², Kiyoka Murase², Keisuke Hosokawa⁴, Shin-ichiro Oyama^2,5, Antti Kero⁶ (1.The Graduate University for Advanced Studies, 2.National Institute of Polar Research, 3.Joint Support-Center for Data Science Research, 4.The University of Electro-Communications, 5.Institute for Space-Earth Environmental Research, ISEE, 6.University of Oulu)

Keywords:spectral riometer, substorm, solar flare, proton

A spectral riometer measures cosmic noise absorption (CNA) in the frequency (20-55 MHz) and is expected to have an ability to estimate the altitude profile of the electron density especially in the lower ionosphere. Basically, the magnitude of CNA is expected to be proportional to the −2 power of the observed frequency when the electron density enhancement occurs at higher altitudes (>70 km), while the power index would be larger than -2 when the enhancement occurs at altitude lower than 70 km. However, the usage of these data should be discussed as the method of analysis has not been fully established.
During the May 2024 solar magnetic storm event, the spectral riometers in Kilpisjärvi (KIL, 69.07°N 20.75°E) observed a number of CNA enhancements. We found the CNA enhancements were mainly caused by three different precipitation events: substorms, solar flares, and solar protons, which make different altitude profiles of ionization. As the power index was thought to be different up to the tpes of events, we investigated the index of each events separately and found that the index tend to increse from -2 to larger in order of ionization altitude profiles depending on the type of event as expected. This result strongly supports that the spectral riometer is capable of detecting the ionization at low altitudes (< 70km).
In this presentation, we will show the results of a statistical analysis of the power index of the CNA spectrum and discuss appropriate method of analysing the data.