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 10^4 5. 84 ⋅ 10 4 to 0. 57 10^4 0. 57 ⋅ 10 4 (and 1. 77 10^4 1. 77 ⋅ 10 4 to 0. 22 10^4 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.
Köberle et al. (Fri,) studied this question.