Characterizing High-Energy Solar Proton Events with Energies Below and Above 100 MeV

We analyzed 58 high-energy proton events that occurred during the years 1996 – 2022. In 32 out of the 58 (55%) events, the proton energies extended up to \(\sim 68\) MeV but did not reach 100 MeV. In the remaining 26 events, the proton energies exceeded 100 MeV. We studied the differences in the characteristics of these proton events and their associations with solar and interplanetary phenomena to improve understanding proton sources and acceleration processes.

The coronal mass ejections (CMEs) associated with \(>100\) MeV proton events appeared to be, on average, more energetic than those associated with \(< 100\) MeV proton events. The peak and integrated fluxes (fluence) of the soft X-ray (SXR) flares were higher in > 100 MeV proton events, but there was almost no difference in the rise times of the flares. In a major part of the \(> 100\) MeV proton events, protons were released over the rise phase of the SXR flares, whereas in most of the \(<100\) MeV events the proton releases occurred after the peak of the SXR flares. We established limits for the CME speed V_CME and SXR peak flux F_pk or total fluence F_i, which helped us to distinguish the events in the two groups. Solar eruptions with V_CME \(> 1000\) km s⁻¹ and F\(_{\mathrm{pk}} > 5 \cdot 10^{-5} \) W m⁻² had a high probability to produce proton events of \(> 100\) MeV. On the other hand, eruptions with V\(_{\mathrm{CME}} > 900\) km s⁻¹ and F\(_{i} <5 \cdot 10^{-4} \) J m⁻² and eruptions with V\(_{ \mathrm{CME}} < 900\) km s⁻¹ irrespective of the SXR total fluence were very likely to produce proton events of \(< 100\) MeV.

All proton events were associated with decametric Type III radio bursts, and most of them had Type II bursts associations either in metric or decametric–hectometric (DH) wavelengths or both. Both metric- and DH-Type II emissions were observed in 50% of \(<100\) MeV proton events while they were observed in 88% of \(>100\) MeV events. Our analysis showed that protons in most of the \(>100\) MeV events were released low in the corona (\(\leq 3.0\) R_⊙) before the onsets of the DH-Type II radio bursts. Conversely, protons in most of the \(<100\) MeV events were released higher in the corona (\(>3\) R_⊙) and after the DH-Type II onsets.

We conclude that protons in most of the \(> 100\) MeV events are accelerated either by the flare reconnection processes or by shocks low in the corona and could undergo reacceleration higher in the corona in CME shocks manifested in DH-Type II radio emission. In the \(<100\) MeV events, protons are mainly accelerated in CME shocks at coronal heights \(>3\) R_⊙.