Characteristics of source location and solar cycle distribution of the strong solar proton events (≥ 1000 pfu) from 1976 to 2018

We studied the source locations and solar cycle distribution of strong solar proton events (≥ 1000 pfu) measured at the Earth from 1976 to 2018. There were 43 strong solar proton events (SPEs) during this period. 27.9 per cent of the strong SPEs were ground level enhancement (GLE) events. We detect more strong SPEs coming from the western hemisphere. The strong SPEs were distributed in the region of [E90-W90], extreme SPEs (≥10000 pfu) appeared within the longitudinal area from E30 to W75, while the SPEs with peak fluxes ≥ 20000 pfu concentrated in the range from E30 to W30 and were always accompanied by super geomagnetic storms (Dst ≤−250 nT). The northern and southern hemispheres of the Sun have 23 and 20 strong SPEs, respectively. The ranges S0–S19 and N0–N19 have 13 and 11 strong SPEs, respectively. S20–S45 and N20–N45 have 7 and 12 strong SPEs, respectively, indicating that the N-S asymmetry of strong SPEs mainly occurred in the areas with a latitude greater than 20○ of the two hemispheres of the Sun. The statistical results showed that 48.8 per cent, 51.2 per cent, and 76.7 per cent of the strong SPEs appeared during the rising phase, declining phase, and in the period from two years before to the three years after the solar maximum, respectively. The number of strong SPEs during a solar cycle has a poor correlation with the solar cycle size.