3He-Rich Solar Energetic Particles: Solar Sources

Abstract

3 He-rich solar energetic particles (SEPs), showing up to a 10,000-fold abundance enhancement of rare elements like 3 He or ultra-heavy nuclei, have been a puzzle for more than 50 years. One reason for the current lack of understanding of 3 He-rich SEPs is the difficulty resolving the source regions of these commonly occurring events. Since their discovery, there has been strong evidence that 3 Herich SEP production is associated with flares on the Sun. Anomalous abundances of 3 He-rich SEPs have been attributed to a unique acceleration mechanism that must routinely operate at flare sites. Flares associated with 3 He-rich SEPs have been often observed in jet-like forms indicating an acceleration in magnetic reconnection involving field lines open to interplanetary space. Owing to a fleet of spacecraft around the Sun, providing a greatly improved resolution of solar imaging observations, 3 He-rich SEP sources are now explored in unprecedented detail. This paper outlines the current understanding of 3 He-rich SEPs, mainly focusing on their solar sources.

The long period of3He-rich solar energetic particles measured by Solar Orbiter 2020 November 17–23

“…The events are highly associated (>95%) with type III radio bursts (e.g., Reames & Stone 1986;Nitta et al 2006), the emission generated by ∼10-100 keV outward streaming electrons. Solar sources of 3 He-rich SEPs have been associated with extreme ultraviolet (EUV) jets (Bučík 2020, and references therein), suggesting acceleration via magnetic reconnection involving field lines open to interplanetary space (Reames 2002). Progress in understanding 3 He-rich SEPs has been hampered by the low intensities and short durations of these events.…”

Fifty Years of 3He-Rich Events

“…Reconnection is often triggered by large-scale waves moving across the corona (Bučík et al, 2016). It is now possible to consider the nature of the associated jets directly for many 3 He-rich events (Bučík et al, 2018a(Bučík et al, , 2018b(Bučík et al, , 2021see review;Bučík, 2020). It has even been possible to compare the temperatures derived from the extreme ultraviolet (EUV) images of coronal source regions of 24 solar jets associated with 3 He-rich events (Bučík et al, 2021) with the abundancederived temperatures from SEPs (Reames et al, 2014b).…”

Impulsive SEP Events (and Flares)

“…Narrow, slower CMEs are, in fact, associated with jets (Kahler et al 2001;Bučík et al 2018a, b). Sources of 3 He-rich events have been reviewed recently by Bučík (2020). Any correlation between impulsive-SEP abundances and CMEs is difficult to quantify, but Fig.…”

“…4.18 Difference images show change in intensity in the LASCO C2 coronagraph for narrow CMEs associated with impulsive SEP events (Kahler et al 2001 # AAS) 2013; Lee et al 2015). However, such models do not yet include any related particle acceleration (see review of solar jets by Raouafi et al 2016) although observations now clearly associate 3 He-rich SEP events directly with helical jets (Bučík et al 2018a, b;Bučík 2020). These SEP events are often associated with active-region coronal holes (Innes et al 2016), i.e.…”