Solar radio burst with a signal pattern similar to a heartbeat have been detected in the Sun’s atmosphere according to the researchers. A multinational team of researchers published their results in the journal Nature Communications, revealing the source location of a radio signal emanating from inside a C-class solar flare more than 5,000 kilometres above the Sun’s surface.
The results of the study, according to the researchers, might help scientists better understand the physical mechanisms behind the energy release of solar flares—the solar system’s most intense explosions.
“The finding is surprising,” said Sijie Yu, the study’s corresponding author and an astronomer at NJIT’s Center for Solar-Terrestrial Studies. “This beating pattern is critical for understanding how energy is generated and lost in the Sun’s atmosphere during these tremendously intense solar outbursts. Yet, the origin of these repeated patterns, also known as quasi-periodic pulsations, has long been a subject of mystery and controversy among solar physicists.”
Solar radio bursts are powerful bursts of radio waves from the Sun that are often connected with solar flares and have been shown to have recurrent patterns.
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., the team was able to determine the source of these pattern signals.
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.
According to research lead author Yuankun Kou, a Ph.D. student at Nanjing University, the team discovered radio bursts with a signal pattern repeating every 10-20 seconds, “like a heartbeat” (NJU).
A significant quasi-periodic pulsation (QPP) signal was found near the base of an electric current sheet running more than 25,000 kilometres across the eruption’s core flaring zone, where opposing magnetic field lines approach, break, and rejoin, releasing immense energy that powers the flare.
Nevertheless, Kou claims that they detected a second pulse in the flare.
“Repeating patterns are frequent in solar radio bursts,” Kou said. “Yet, there is an unexpected secondary source placed along the stretched current sheet that pulses in the same manner as the primary QPP source.”
“The signals are most likely caused by quasi-repetitive magnetic reconnections at the flare current sheet,” Yu stated. “This is the first time a quasi-periodic radio signal has been observed in the reconnection zone. This detection may assist us in determining which of the two sources is responsible for the other.”
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.
“The spectrum imagery provided by EOVSA provided us with novel geographically and temporally detailed diagnostics of the flare’s nonthermal electrons. In the electronic current sheet, we discovered that the distribution of high-energy electrons in the primary QPP source varies in phase with that of the secondary QPP source “Bin Chen, an associate professor of physics at NJIT and co-author of the work, said. “There is solid evidence that the two QPPs sources are connected.”
Continuing their investigation, the team members combined 2.5D numerical modelling 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 discover how periodicity arises in the present sheet,” Cheng said. “What is the physical mechanism behind periodicity, and how does it relate to QPP formation?”
The investigation revealed that magnetic islands, or bubble-like structures, emerge in the current sheet and move quasi-periodically towards the flaring zone.
“The development of magnetic islands inside the long-stretched current sheet plays an important role in adjusting the energy release rate during this eruption,” Cheng stated. “Such a quasi-periodic energy release mechanism results in the repeated creation of high-energy electrons, which appear as QPPs in microwave and soft X-ray wavelengths.”
Moreover, Yu claims that the results of the research provide new insight on a crucial aspect underlying the reconnection process that causes these explosive occurrences.
“We’ve now found the source of QPPs in solar flares: periodic reconnection in the flare current sheet. This research calls for a rethinking of past explanations of QPP occurrences and their consequences for solar flares.”
Yulei Wang and Mingde Ding of NJU, as well as Eduard P. Kontar of the University of Glasgow, are also co-authors on the article. The National Science Foundation provided funding for this study.
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