Sources of Solar Protons in the Events of February 24–25 and July 16–17, 2023

Abstract

From the beginning of January 2021 to the end of August 2023, the radiation monitor of the Spektr-RG spacecraft registered three enhancements in the count rate, which exceed the background variations during the solar activity cycle and have a comparable maximum value. These enhancements are associated with solar proton events (SPEs) from the flares X1.0 on October 28, 2021; M6.3 on February 25, 2023; and M5.7 on July 17, 2023. Using the example of these events, as well as smaller SPEs from the flares M3.7 on February 24, 2023, and M4.0 on July 16, 2023, threshold criteria for “proton” flares are discussed. In powerful SPEs, the contribution of solar protons to the radiation dose can exceed the total contribution of galactic cosmic rays (GCR) over a sufficiently long period of time. Therefore, such SPEs are sources of increased radiation hazard and require prediction based on real-time observations. It was shown that, in these five flares, thresholds were overcome according to three criteria: plasma temperature >12 MK (soft X-ray source), duration (>5 min) of microwave or hard X-ray (HXR) radiation (electron acceleration >100 keV), and height of flare development process >60 Mm (radio emission at plasma frequencies <610 MHz). The arrival of the first solar protons >100 MeV to the Earth’s orbit was expected no earlier than 10 min relative to the beginning of HXR or microwave radiation, i.e., could have been predicted in advance. To study the relationship between solar flares and SPEs, we used data from the anticoincidence shield of the spectrometer on INTEGRAL (ACS SPI), which is an effective but uncalibrated detector of HXR >100 keV and protons >100 MeV, as well as patrol observations of radio emission at fixed frequencies (Radio Solar Telescope Network). It is noted that the X2.2 (N25E64) flare on February 17, 2023 satisfied all three “protonity” criteria and could become the source of a powerful SPE near the Earth in a case of favorable location on the Sun. In the M8.6 (N27W29) flare on February 28, 2023, the third criterion was not met, and it did not lead to an SPE as expected (it developed in a plasma with a density >2.5 × 10¹⁰ cm^–3 and plasma frequency >1415 MHz).