Particle accelerator region revealed in a solar flare

Solar flares are among the most violent explosions in our solar system, but despite their immense energy — equivalent to a hundred billion atomic bombs detonating at once — physicists have still not been able to answer exactly how these sudden bursts on the sun are able to carry particles back to Earth. launch, nearly 93 million miles away, in less than an hour.

Now, in a study published June 8 in Nature, Researchers at the New Jersey Institute of Technology (NJIT) have pinpointed the precise location where solar flares accelerate charged particles to nearly the speed of light.

The new findings, made possible by observations of an X-class solar flare in 2017 by NJIT’s Expanded Owens Valley Solar Array (EOVSA) radio telescope, have revealed a highly efficient particle accelerator located at the tip of the eruption’s brightest point in the outermost region. the sun’s atmosphere, called the flare’s “cusp region,” where the explosion’s surrounding plasma is converted into high-energy electrons.

Researchers say the discovery of the region, measured at nearly twice the volume of Earth, could open new doors for investigating fundamental processes of particle acceleration that are ubiquitous in the universe.

“The findings in this study help explain the long-standing mystery of how solar flares can produce so much energy in just seconds,” said Gregory Fleishman, corresponding author of the paper and distinguished research professor of physics at NJIT’s Center for Solar-Terrestrial Research. . “The solar flare unleashes its power in a much larger area of ​​the sun than expected by the classical model of solar flares. While others have postulated that it should, this is the first time the specific size, shape and location of this important area has been determined. identified, and the efficiency of the energy conversion to particle acceleration within the flare was measured.”

The discovery follows separate 2020 studies published in Science and Natural Astronomywhere EOVSA’s detailed snapshots of the flare and changes in the sun’s magnetic field — taken at hundreds of radio frequencies simultaneously — initially gave the NJIT team a clue as to its location.

“Our recent studies suggested that the flare cusp might be the location where such high-energy electrons are produced, but we weren’t sure,” explains Bin Chen, an associate professor at NJIT and co-author of the paper. “We had originally discovered a magnetic bottle-like structure at the site that contained an overwhelmingly large number of electrons compared to anywhere else in the flare, but now with the new measurements from this study, we can say with more confidence that this is the particle from the flare. is accelerator.”

Using EOVSA’s unique microwave imaging capabilities, the team was able to measure the energy spectrum of electrons at hundreds of locations of an X-class solar flare caused by a reconfiguration of magnetic field lines along the sun’s surface on September 10, 2017.

“EOVSA’s spectral imaging gave us a comprehensive map of the flare’s thermal plasma as it evolved second-by-second. But to our surprise, we found a mysterious hole in the thermal plasma map that started developing on the eve of the torch,” said Gelu Nita, NJIT research professor and co-author of the paper. “More than that, as thermal particles disappeared in the region, the hole was then densely filled with non-thermal, high-energy particles.”

The team’s analysis revealed an incredibly efficient energy conversion process in the solar flare’s particle accelerator, rapidly releasing intense energy from the sun’s magnetic fields and transferring it to kinetic energy in the region.

“We wondered how efficient this energy conversion process would be… how many particles in this area would be accelerated beyond the thermal energy of the explosion?” added Sijie Yu, study co-author and NJIT assistant research professor. “Using extreme ultraviolet data from the sun, we confirmed that virtually no particles remained in the region at thermal energies below a few million Kelvin, consistent with the EOVSA measurement that the particles had all accelerated to non-thermal energies of more than 20 keV, or nearly 100 million Kelvin.”

The team now says these latest findings could help scientists study fundamental questions in particle physics that are not possible on Earth, as well as provide new insights into how such high-energy particles from the sun could affect Earth during future space weather events.

“An important aspect of this study is that it draws theorists’ attention to the precise location where most of the energy release and particle acceleration occurs, and provides quantitative measurements to guide numerical models,” said Dale Gary, distinguished professor and director of the NJIT. EOVSA. “However, to extend our measurements to much broader flare regions and weaker but more frequent flare events, we are developing a new generation, solar-powered special radio array called the Frequency Agile Solar Radio Telescope, which will be at least 10 times larger. and orders of magnitude more powerful.”

“We still want to investigate the physical mechanism that drives particle acceleration in solar flares. But future studies should take into account what we now know about these massive explosions — both the main energy release in the cusp region and the 100% efficiency with which charged particle acceleration occurs,” said Fleishman. “These findings call for a major overhaul of the models we use to study solar flares and their impact on Earth.”

This research was supported by grants from the National Science Foundation.


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