Fifty Years of 3He-Rich Events

Abstract

The early 1970s saw a new and surprising feature in the composition of solar energetic particles (SEPs), resonant enhancements up to 10,000-fold in the ratio 3He/4He that could even make 3He dominant over H in rare events. It was soon learned that these events also had enhancements in the abundances of heavier elements, such as a factor of ∼10 enhancements in Fe/O, which was later seen to be part of a smooth increase in enhancements vs. mass-to-charge ratio A/Q from H to Pb, rising by a factor of ∼1000. These events were also associated with streaming 10–100 keV electrons that produce type III radio bursts. In recent years we have found these “impulsive” SEP events to be accelerated in islands of magnetic reconnection from plasma temperatures of 2–3 MK on open field lines in solar jets. Similar reconnection on closed loops traps the energy of the particles to produce hot (>10 MK), bright flares. Sometimes impulsive SEP intensities are boosted by shock waves when the jets launch fast coronal mass ejections. No single theory yet explains both the sharp resonance in 3He and the smooth increase up to heavier elements; two processes seem to occur. Sometimes the efficient acceleration even exhausts the rare 3He in the source region, limiting its fluence.

Energy Spectra vs. Element Abundances in Solar Energetic Particles and the Roles of Magnetic Reconnection and Shock Acceleration

“…The abundance of He, or He/C or He/O, seems to be partly affected by FIP-dependent factors arising from the uniquely high FIP = 24.6 eV of He (e.g. Reames, 2017Reames, , 2021dLaming, 2009Laming, , 2015. While H/He is an obvious comparison of dominant elements, studying H/He alone mixes the factors that modify H with those that modify He, making them very difficult to disentangle.…”

“…Temerin and Roth, 1992;Tylka, 1999, 2003). Acceleration of small "impulsive" SEP events appears to occur at sites of magnetic reconnection (Drake et al, 2009) on open magnetic-field lines in solar jets (Bučík et al, 2018a(Bučík et al, , 2018b, or on closed field lines in flares (Mandzhavidze, Ramaty, and Kozlovsky, 1999;Murphy et al, 1991, Murphy, Kozlovsky, andShare, 2016), producing distinctive particle enhancements of 3 He (e.g., Serlemitsos and Balasubrahmanyan, 1975;Temerin and Roth, 1992;Mason, 2007;Bučík, 2020;Reames, 2021d) and of heavy elements (Mason et al, 1986;Reames, Meyer, and von Rosenvinge, 1994) up to Au and Pb Reames and Ng, 2004;Mason et al, 2004;Reames, Cliver, and Kahler, 2014a, 2014b. Larger "gradual" SEP events are accelerated by shock waves driven out from the Sun by wide, fast coronal mass ejections (CMEs; Kahler et al, 1984;Mason, Gloeckler, and Hovestadt, 1984;Reames, Barbier, and Ng, 1996;Zank, Rice, and Wu, 2000;Cliver, Kahler, and Reames, 2004;Lee, 2005;Lee, Mewaldt, and Giacalone, 2012;Desai and Giacalone, 2016).…”

Second-Class Citizen in the Heliophysics Community

“…Those events are generally small and impulsive, with source regions in coronal flares and jets. The 3 He acceleration process was first explained by absorption of electromagnetic ion cyclotron waves, but currently favored (Reames, 2021) is magnetic reconnection in confined coronal volumes, which may account for upper limits to the observed 3 He event fluence distribution observed at one AU (Ho et al, 2005; Figure 4). The 3 He acceleration process appears to occur differently from that of 4 He (Ho et al, 2019) and may even completely strip a coronal source region of all 3 He ions (Reames, 2021).…”

Solar activities associated with 3He-rich solar energetic particle events observed by Solar Orbiter

“…Since the Wang et al (2006) paper, it has been proclaimed that coronal jets are responsible for 3 He-rich SEP events, even though the original paper primarily discussed the importance of the interaction between the base of an open field line and a nearby bipole. Jets have been emphasized in some of the review articles on 3 He-rich SEP events (e.g., Mason, 2007;Bučík, 2020;Reames, 2021). It is not difficult to find clear examples of jets in EUV or soft X-rays temporally correlated with 3 He-rich SEP events (Nitta et al, 2006(Nitta et al, , 20082015;Wang et al, 2006;Bučík et al, 2014Bučík et al, , 2018.…”