According to a new study, a solar radio burst with a signal pattern similar to a heartbeat has been detected in the Sun’s atmosphere.
An multinational team of researchers published their findings in the journal Nature Communications, revealing the source location of a radio signal coming from within a C-class solar flare more than 5,000 kilometers above the Sun’s surface.
The findings of the study, according to the researchers, could help scientists better understand the physical mechanisms underlying the energy release of solar flares, the solar system’s most intense explosions.
“The discovery is unexpected,” said Sijie Yu, the study’s corresponding author and astronomer affiliated with NJIT’s Center for Solar-Terrestrial Research. “This beating pattern is important for understanding how energy is released and is dissipated in the Sun’s atmosphere during these incredibly powerful explosions on the Sun. However, the origin of these repetitive patterns, also called quasi-periodic pulsations, has long been a mystery and a source of debate among solar physicists.”
Solar radio bursts are powerful bursts of radio waves from the Sun that are frequently associated with solar flares and have been shown to have recurrent patterns.
The appearance of magnetic islands within the long-stretched current sheet plays a key role in tweaking the energy release rate during this eruption. Such a quasi-periodic energy release process leads to a repeating production of high-energy electrons, manifesting as QPPs in the microwave and soft X-ray wavelengths.
Professor Xin Cheng
The team was able to uncover the source of these pattern signals after studying microwave observations of a solar flare event on July 13, 2017, captured by NJIT’s radio telescope called the Expanded Owens Valley Solar Array (EOVSA), which is located at Owens Valley Radio Observatory (OVRO), near Big Pine, Calif.
EOVSA frequently examines the Sun at microwave frequencies ranging from 1 to 18 gigahertz (GHz) and is sensitive to radio radiation released by high-energy electrons in the Sun’s atmosphere that are accelerated by solar flares.
“From EOVSA’s observations of the flare, the team revealed radio bursts featuring a signal pattern repeating every 10-20 seconds, like a heartbeat,” according to study leading author Yuankun Kou, a Ph.D. student at Nanjing University (NJU).
A strong quasi-periodic pulsation (QPP) signal was found at the base of an electric current sheet stretching more than 25,000 kilometers through the eruption’s core flaring region, where opposing magnetic field lines approach, break, and reconnect, generating intense energy that powers the flare.
But surprisingly, Kou says they discovered a second heartbeat in the flare.
“The repeating patterns are not uncommon for solar radio bursts,” Kou said. “But interestingly, there is a secondary source we did not expect located along the stretched current sheet that pulses in a similar fashion as the main QPP source.”
“The signals likely originate from quasi-repetitive magnetic reconnections at the flare current sheet,” added Yu. “This is the first time a quasi-periodic radio signal located at the reconnection region has been detected. This detection can help us to determine which of the two sources caused the other one.”
The team was able to detect the energy spectrum of electrons at the two radio sources in this event using EOVSA’s unique microwave imaging capabilities.
“EOVSA’s spectral imaging gave us new spatially and temporally resolved diagnostics of the flare’s nonthermal electrons. … We found the distribution of high-energy electrons in the main QPP source vary in phase with that of the secondary QPP source in the electronic current sheet,” said Bin Chen, associate professor of physics at NJIT and co-author of the paper. “This is a strong indication that the two QPPs sources are closely related.”
Continuing their investigation, the team members combined 2.5D numerical modeling of the solar flare, led by the paper’s other corresponding author and NJU professor of astronomy Xin Cheng, with observations of soft X-ray emission from solar flares measured by NOAA’s GOES satellite, which measures soft X-ray fluxes from the Sun’s atmosphere in two different energy bands.
“We wanted to know how the periodicity occurs in the current sheet,” said Cheng. “What is the physical process driving the periodicity and how is it related to the formation of the QPPs?”
The investigation revealed that magnetic islands, or bubble-like structures, emerge in the current sheet and move quasi-periodically toward the flaring zone.
“The appearance of magnetic islands within the long-stretched current sheet plays a key role in tweaking the energy release rate during this eruption,” explained Cheng. “Such a quasi-periodic energy release process leads to a repeating production of high-energy electrons, manifesting as QPPs in the microwave and soft X-ray wavelengths.”
Ultimately, Yu says the study’s findings cast fresh light on an important phenomenon underlying the reconnection process that drives these explosive events.
“We’ve finally pinpointed the origin of QPPs in solar flares as a result of periodic reconnection in the flare current sheet. This study prompts a reexamination of the interpretations of previously reported QPP events and their implications on solar flares.”
Additional co-authors of the paper include NJU researchers Yulei Wang and Mingde Ding, as well as Eduard P. Kontar at the University of Glasgow. This research was supported by grants from the National Science Foundation.
