Statistical characteristics of solar energetic proton events from January 1997 to June 2005

Abstract

We have made a statistical study of 163 solar proton events (SPEs) associated with X-ray flares, coronal mass ejections (CMEs) and radio type II bursts during January 1997–June 2005. These SPEs were categorized by the peak fluxes of >10 MeV solar protons into three groups. There are 37 large SPEs with fluxes of more than 100 protons cm −2 s −1 sr −1, 34 moderate SPEs with flux ranges of 10–100 protons cm −2 s −1 sr −1 and 92 minor SPEs with flux ranges of 1–10 protons cm −2 s −1 sr −1. To understand the determinant of solar proton events, we have examined the association of these SPEs with X-ray flares, CMEs and radio type II emissions from metric to decametric-hectometric (DH) wave ranges. The primary results from this study are: (1) most SPEs (112/163) corresponded to the solar flares favorably located at solar western hemisphere and the center of the activity source region tended to shifted to the west with increasing of the solar proton fluxes; (2) there seems a longitudinal cutoff for each group of SPEs, which also moves toward west with increasing of the solar proton flux; (3) each SPE observed at Earth was associated with a fast (average speed ∼1228 km s −1) and wide (average angle width of 266°) CME; (4) the percentage of these SPEs associated with metric (DH) type II burst increased from 54% (42%) to 81% (100%). Overall, The most intensive SPEs are more likely to be produced by major flares located near central meridian of the Sun and shock waves driven by very fast halo CMEs ( v ⩾ 1600 km s −1). This suggested that CME-driven shock acceleration is a necessary condition for large SPEs production.