Solar Energetic Particle Events and Associated Type II Radio Bursts from Different Source Regions

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Abstract
Large solar energetic particle (SEP) events are thought to originate from the shocks driven by fast coronal mass ejections (CMEs) and thus are generally accompanied by type II radio bursts. However, a significant proportion of type II radio bursts is not accompanied by SEP events. To study the relationship between SEPs and type II radio bursts and the associated physical mechanisms, we statistically analyze 43 SEP halo CMEs and 131 non-SEP halo CMEs observed from 2010 to 2024, and check the related properties of type II radio bursts and the solar source region. We find that nearly all SEP events and approximately two-thirds of non-SEP events are accompanied by type II radio bursts. Type II radio bursts associated with SEP events usually have longer durations and lower ending frequencies. The starting frequency exhibits a clear source region dependence, being highest for “single active region (AR),” intermediate for “multiple ARs,” and lowest for “outside of ARs.” Furthermore, the spectra of both protons and electrons exhibit a similar softening trend in the three types of source regions. Joint analysis of spectra and type II radio bursts reveals that the proton spectra index has a good anticorrelation with the starting frequency of the type II radio bursts. Our statistical results have important implications for the mechanisms behind SEP acceleration.

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