Source of Solar Energetic Particles with the Largest ³He Enrichment Ever Observed

(Solar Orbiter Nugget #59 by R. Bučík¹, G. M. Mason², S. M. Mulay³, G. C. Ho¹, R. F. Wimmer-Schweingruber⁴, J. Rodríguez-Pacheco⁵)

1. Introduction

On October 24, 2023, ESA/NASA’s Solar Orbiter recorded the highest-ever enrichment of ³He in solar energetic particles. This rare isotope, which is lighter than the more common ⁴He by just one neutron, is scarce in our Solar System— typically found at a ratio of about one ³He ion per 2,500 ⁴He ions [1]. However, solar jets appear to preferentially accelerate ³He to high energies, likely due to its unique charge-to-mass ratio [2]. The mechanism behind this acceleration remains unknown, but it can boost ³He abundance by up to 10,000 times its usual concentration in the Sun’s atmosphere [3] — an effect not seen in any other known astrophysical setting.

2. Results

At the time of the observation, Solar Orbiter was about halfway between the Earth and the Sun (0.47 astronomical units, or AU, from the Sun). The spacecraft recorded solar energetic particles with a ³He enrichment reaching an astonishing factor of 200,000 compared to its abundance in the solar atmosphere (see Figure 1). Notably, only ³He was significantly accelerated, while heavier elements attained much lower energies.

NASA’s Solar Dynamics Observatory (SDO), at Earth’s distance from the Sun (1 AU), provided high-resolution images of a small solar jet at the edge of a coronal hole—a region where magnetic field lines open into interplanetary space (Figure 1 a,c,d). Despite its tiny size, the jet was clearly linked to the solar energetic particle event. Surprisingly, the magnetic field strength in this region was weak, typical of quiet solar areas rather than active regions. This finding supports earlier theories suggesting that ³He enrichment is more likely to occur in weakly magnetized plasma, where turbulence is minimal [4, 5].

Additionally, this event stands out as one of the rare cases where heavy ions do not follow the usual mass-dependent enhancement pattern [6, 7]. Typically, events like these exhibit enhancements of heavy ions such as iron, but in this case, iron was not enhanced. Instead, carbon, nitrogen, silicon, and sulfur were significantly more abundant than expected. Only 19 events of this kind have been documented in the past 25 years, highlighting the rarity and puzzling nature of this phenomenon [8, 9, 10]. While the cause of these unusual enhancements remains unclear, we suggest that the temperature distribution in the source may play a key role. The Parker Solar Probe was in a favorable location, but at 0.66 AU from the Sun, it was still too far to detect the event. 

Figure 1. (a) The Sun in extreme ultraviolet (EUV), as observed by SDO at the time of energetic ion release from the source region. The blue arrow marks the source (a small bright point) located at the edge of the coronal hole, outlined by the red contour. (b) He-mass histogram for ions accelerated to speeds greater than 9,000 km/s. (c) A zoomed-in view of the solar surface magnetic field in the source region, enclosed by yellow rectangle. Black and white indicate strong magnetic field, while gray represents weak fields. (d) A differential EUV image showing the jet, marked by the blue arrow.  

3. Conclusions

This rare event, marked by extreme ³He enrichment and unusual heavy ion patterns, offers new clues about how weak magnetic fields and source temperature contribute to the generation of solar energetic particles. Spacecraft such as Solar Orbiter operating closer to the Sun, may detect more of these small but intriguing events—offering valuable insights into the acceleration mechanisms of this very poorly understood energetic particle population in our Solar System.

For more details see the associated paper, [11, Bucik et al., ApJ 981 178 (2025)].

Affiliations
(1) Southwest Research Institute, San Antonio, TX 78238, USA
(2) Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA
(3) School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
(4) Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, 24118 Kiel, Germany
(5) Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain

References

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[2] Bučík R. 2020 SSRv 216 24. https://doi.org/10.1007/s11214-020-00650-5
[3] Mason G. M. 2007 SSRv 130 231. https://doi.org/10.1007/s11214-007-9156-8
[4] Liu S., Petrosian V. and Mason G. M. 2004 ApJL 613 L81. https://doi.org/10.1086/425070
[5] Liu S., Petrosian V. and Mason G. M. 2006 ApJ 636 462. https://doi.org/10.1086/497883
[6] Mason G. M., Mazur J. E., Dwyer J. R. et al. 2004 ApJ 606 555. https://doi.org/10.1086/382864
[7] Reames D. V. and Ng C. K. 2004 ApJ 610 510. https://doi.org/10.1086/421518
[8] Mason G. M. et al. 2016 ApJ 823 138. https://doi.org/10.3847/0004-637X/823/2/138
[9] Mason G. M. et al. 2023 ApJ 957 112. https://doi.org/10.3847/1538-4357/acf31b
[10] Bučík R. et al. 2023 A&A 673 L5. https://doi.org/10.1051/0004-6361/202345875
[11] Bučík R. et al. 2025 ApJ 981 178. https://doi.org/10.3847/1538-4357/adb48d
 

Acknowledgements

R.B. acknowledges support by NASA grant Nos. 80NSSC21K1316 and 80NSSC22K0757. S.M.M. acknowledges support from the UK Research and Innovation's Science and Technology Facilities Council under grant award numbers ST/T000422/1 and ST/X000990/1. The Suprathermal Ion Spectrograph (SIS) is a European facility instrument funded by ESA under contract number SOL.ASTR.CON.00004 with CAU. We thank ESA and NASA for their support of the Solar Orbiter and other missions whose data were used in this paper. Solar Orbiter post-launch work at JHU/APL and the Southwest Research Institute is supported by NASA contract NNN06AA01C and at CAU by German Space Agency (DLR) grant No. 50OT2002. The UAH team acknowledges financial support by the Spanish Ministerio de Ciencia, Innovacion y Universidades MCIU/AEI Project PID2019 – 104863RBI00/AEI/10.13039/501100011033.