Determination of 3He-Rich Periods Measured by the Suprathermal Ion Telescope on STEREO-A During Solar Cycle 24

Abstract 3 He-rich solar energetic particles (SEP) events are characterized by a peculiar elemental composition with rare species like 3 He or ultra-heavy ions tremendously enhanced over the solar system abundances. We report on 3 He-rich SEP periods measured by the Suprathermal Ion Telescope (SIT) on board the Solar Terrestrial Relations Observatory Ahead (STEREO-A) beginning in 2007 until 2020, covering the whole Solar Cycle (SC) 24. The mass resolution capabilities of the SIT do not allow to easily distinguish between 3 He and 4 He especially in cases of a low 3 He to 4 He ratio. Therefore, we developed a data-driven detection algorithm to find time periods during which a 3 He enhancement can be statistically determined. With this method we were able to identify a total of 144 periods, which likely include all periods in SC 24, that can be resolved by the SIT. We lay a special emphasis on the quantification of 3 He and 4 He. As a result, we present 3 He and 4 He fluences as well as 3 He/ 4 He fluence ratios for the 144 periods in 6 energy channels in the range from 160 to 1280 keV nucleon −1 . The 3 He fluence distributions show mean (and median) values decreasing with increasing energy, ranging from 5.84· 10⁴ 5.84 ⋅ 10 4 to 0.57· 10⁴ 0.57 ⋅ 10 4 (and 1.77· 10⁴ 1.77 ⋅ 10 4 to 0.22· 10⁴ 0.22 ⋅ 10 4 ) particles/(cm 2 srMeV nucleon −1 ). The 3 He/ 4 He fluence ratios show a clear tendency to increase with increasing energies, with means ranging between 0.34 at 193 keV nucleon −1 and 1.09 at the 772 keV nucleon −1 channel, reaching an apparent plateau above 546 keV nucleon −1 .