NJIT researchers discover long-hidden source of gamma rays unleashed by solar flares
In findings published in Nature Astronomy, scientists at NJIT's Center for Solar-Terrestrial Research (NJIT-CSTR) have pinpointed a previously unknown class of high-energy particles in the Sun's upper atmosphere responsible for generating the long-puzzling radiation signals observed during major solar flare events for decades.
The signals were traced back to a localized region in the solar corona during a powerful X8.2-class flare that erupted on September 10, 2017, where trillions upon trillions of particles were measured at energies of several million electron volts (MeV) - hundreds to thousands of times more energetic than typical flare particles and moving near the speed of light.
Researchers believe these particles generate gamma rays through a process known as bremsstrahlung - a mechanism in which lightweight charged particles, such as electrons, emit high-energy light when they collide with material in the Sun's atmosphere.
The team says the discovery fills critical gaps in our understanding of solar flare physics and could improve models of solar activity that ultimately enhance space weather forecasting.
"We knew solar flares produced a unique gamma-ray signal, but that data alone couldn't reveal its source or how it was generated," said Gregory Fleishman, NJIT-CSTR research professor of physics and lead author of the study. "Without that crucial information, we couldn't fully understand the particles responsible or evaluate any potential impact on our space weather environment. By combining gamma-ray and microwave observations from a solar flare, we were finally able to solve this puzzle."
To find the source, the NJIT team combined observations of the 2017 flare from NASA's Fermi Gamma-ray Space Telescope and NJIT's Expanded Owens Valley Solar Array (EOVSA), a state-of-the-art radio telescope array in California.
Fermi provided crucial measurements of high-energy gamma-ray emissions during the flare, while EOVSA delivered spatially resolved microwave imaging that captured the signatures of accelerated particles in the solar corona.
By analyzing these datasets together, the team identified a distinct region in the solar atmosphere - called Region of Interest 3 (ROI 3) - in addition to two previously studied areas, ROI 1 and ROI 2, where microwave and gamma-ray signals converged.
This convergence pointed to a unique population of particles energized to MeV levels.
"Unlike the typical electrons accelerated in solar flares, which usually decrease in number as their energy increases, this newly discovered population is unusual because most of these particles have very high energies, on the order of millions of electron volts, with relatively few lower-energy electrons present," explained Fleishman.
Using advanced modeling, the team linked the energy distribution of these particles directly to the observed gamma-ray spectrum, pointing to bremsstrahlung emission - high-energy light usually produced when electrons collide with solar plasma - as the elusive source of the gamma-ray signals.
Fleishman also says their observations within ROI 3 - located near regions of significant magnetic field decay and intense particle acceleration - support long-standing theories about how solar flares accelerate particles to extreme energies and sustain them.
"We see clear evidence that solar flares can efficiently accelerate charged particles to very high energies by releasing stored magnetic energy. These accelerated particles then evolve into the MeV-peaked population we discovered," said Fleishman.
For now, Fleishman says key questions remain about these extreme particle populations.
Future observational insights could soon come from NJIT's Expanded Owens Valley Solar Array (EOVSA), currently being upgraded to EOVSA-15. This project, led by NJIT-CSTR professor of physics and EOVSA director Bin Chen - a co-author on the study - is funded by the National Science Foundation and will enhance the array with 15 new antennas and advanced ultra-wideband feeds.
"One big unknown is whether these particles are electrons or positrons," Fleishman said. "Measuring the polarization of microwave emissions from similar events could provide a definitive way to tell them apart. We expect to gain this capability soon with the EOVSA-15 upgrade."
The team's study, "Solar Flare Hosts MeV-peaked Electrons in a Coronal Source," was supported by funding from the National Science Foundation and NASA.
Research Report:Megaelectronvolt-peaked electrons in a coronal source of a solar flare