SwRI scientists source solar emissions with largest-ever concentration of rare helium isotope

SAN ANTONIO — April 9, 2025 —The NASA/ESA Solar Orbiter recently recorded the highest-ever concentration of a rare helium isotope (³He) emitted from the Sun. A Southwest Research Institute-led team of scientists sought the source of this unusual occurrence to better understand the mechanisms that drive solar energetic particles (SEPs) that permeate our solar system. SEPs are high-energy, accelerated particles including protons, electrons and heavy ions associated with solar events like flares and coronal mass ejections.

“This rare isotope, which is lighter than the more common ⁴He by just one neutron, is scarce in our solar system — found at a ratio of about one ³He ion per 2,500 ⁴He ions,” said SwRI’s Dr. Radoslav Bucik, lead author of a paper describing this phenomenon. “However, solar jets appear to preferentially accelerate ³He to high speeds or energies, likely due to its unique charge-to-mass ratio.”

Bucik said the mechanism behind this acceleration remains unknown, but it can typically boost ³He abundance by up to 10,000 times its usual concentration in the Sun’s atmosphere — an effect unparalleled in any other known astrophysical setting. Incredibly, in this case Solar Orbiter recorded a 200,000-fold enhancement of ³He. In addition to its great abundance, the ³He was accelerated to significantly higher speeds than heavier elements.

The SwRI team pinpointed the origin of the ³He emissions. NASA’s Solar Dynamics Observatory (SDO) 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. Despite its tiny size, the jet was clearly linked to the SEP event, Bucik said.

“Surprisingly, the magnetic field strength in this region was weak, more typical of quiet solar areas rather than active regions,” he added. “This finding supports earlier theories suggesting that ³He enrichment is more likely in weakly magnetized plasma, where turbulence is minimal.”

Additionally, this event stands out as one of the rare cases where ion enhancements do not follow the usual pattern. Typically, events like these exhibit greater abundance of heavy ions such as iron. But in this case, iron was not increased. Instead, carbon, nitrogen, silicon and sulfur were significantly more abundant than expected. Scientists have observed only 19 similar events in the past 25 years, highlighting the rarity and puzzling nature of this phenomenon.

While the Parker Solar Probe was in a favorable location, it was too far away to detect the event, Bucik notes. This highlights the importance of spacecraft operating closer to the Sun, to detect more of these small, intriguing events and offering valuable insights into the acceleration mechanisms of this least understood energetic particle population in our solar system.

Access this Astrophysical Journal paper, “Origin of unusual composition of ³He-rich solar energetic particles,” at: https://iopscience.iop.org/article/10.3847/1538-4357/adb48d.

For more information, visit https://www.swri.org/markets/earth-space/space-research-technology/space-science/heliophysics.