3He-rich solar energetic particles (SEP) events are characterized by a peculiar elemental composition with rare species like 3He or ultra-heavy ions tremendously enhanced over the solar system abundances. We report on 3He-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 3He and 4He especially in cases of a low 3He to 4He ratio. Therefore, we developed a data-driven detection algorithm to find time periods during which a 3He 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 3He and 4He. As a result, we present 3He and 4He fluences as well as 3He/4He fluence ratios for the 144 periods in 6 energy channels in the range from 160 to 1280 keV nucleon−1.The 3He fluence distributions show mean (and median) values decreasing with increasing energy, ranging from <InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math> <mn>5.84</mn> <mo>⋅</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>4</mn> </mrow> </msup> </math></EquationSource> <EquationSource Format="TEX">$5.84\cdot 10^{4}$</EquationSource> </InlineEquation> to <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math> <mn>0.57</mn> <mo>⋅</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>4</mn> </mrow> </msup> </math></EquationSource> <EquationSource Format="TEX">$0.57\cdot 10^{4}$</EquationSource> </InlineEquation> (and <InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math> <mn>1.77</mn> <mo>⋅</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>4</mn> </mrow> </msup> </math></EquationSource> <EquationSource Format="TEX">$1.77\cdot 10^{4}$</EquationSource> </InlineEquation> to <InlineEquation ID="IEq4"> <EquationSource Format="MATHML"><math> <mn>0.22</mn> <mo>⋅</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>4</mn> </mrow> </msup> </math></EquationSource> <EquationSource Format="TEX">$0.22\cdot 10^{4}$</EquationSource> </InlineEquation>) particles/(cm2 srMeV nucleon−1).The 3He/4He 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.

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

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2026年5月25日

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Marlon Köberle,
Lars Berger,
Radoslav Bučík,
Bernd Heber,
Glenn M. Mason,
Linghua Wang

摘要

³He-rich solar energetic particles (SEP) events are characterized by a peculiar elemental composition with rare species like ³He or ultra-heavy ions tremendously enhanced over the solar system abundances. We report on ³He-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 ³He and ⁴He especially in cases of a low ³He to ⁴He ratio. Therefore, we developed a data-driven detection algorithm to find time periods during which a ³He 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 ³He and ⁴He. As a result, we present ³He and ⁴He fluences as well as ³He/⁴He fluence ratios for the 144 periods in 6 energy channels in the range from 160 to 1280 keV nucleon⁻¹.

The ³He fluence distributions show mean (and median) values decreasing with increasing energy, ranging from $5.84 \cdot 10^{4}$ $5.84\cdot 10^{4}$ to $0.57 \cdot 10^{4}$ $0.57\cdot 10^{4}$ (and $1.77 \cdot 10^{4}$ $1.77\cdot 10^{4}$ to $0.22 \cdot 10^{4}$ $0.22\cdot 10^{4}$ ) particles/(cm² srMeV nucleon⁻¹).

The ³He/⁴He 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.

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

摘要

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