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| An illustration showing EOVSA capturing a pulsating radio burst from a solar flare. Credit: Sijie Yu of NJIT/CSTR; Yuankun Kou of NJU; NASA SDO/AIA |
A solar radio burst with a signal pattern, akin to that of a heartbeat, has
been pinpointed in the Sun's atmosphere, according to a new study.
In findings published in the journal Nature Communications, an international
team of researchers has reported uncovering 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.
Researchers say the study's findings could help scientists better understand
the physical processes behind the energy release of solar flares—the solar
system's most powerful 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 intense bursts of radio waves from the Sun, which are
often associated with solar flares and have been known to feature signals
with repeating patterns.
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 routinely observes the Sun in a wide range of microwave frequencies
over 1 to 18 gigahertz (GHz) and is sensitive to radio radiation emitted by
high-energy electrons in the Sun's atmosphere, which are energized in 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).
The team identified a strong quasi-periodic pulsation (QPP) signal at the
base of the electric current sheet stretching more than 25,000 kilometers
through the eruption's core flaring region where opposing magnetic field
lines approach each other, break and reconnect, generating intense energy
powering 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."
Using the unique microwave imaging capabilities of EOVSA, the team was able
to measure the energy spectrum of electrons at the two radio sources in this
event.
"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 other corresponding author of the
paper and professor of astronomy Xin Cheng at NJU, with observations of soft
X-ray emission from the solar flares observed by NOAA's GOES satellite,
which measures the 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 team's analysis showed there are magnetic islands, or bubble-like
structures that form in the current sheet, quasi-periodically moving toward
the flaring region.
"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.
Reference:
Yuankun Kou et al, Microwave imaging of quasi-periodic pulsations at flare
current sheet, Nature Communications (2022).
DOI: 10.1038/s41467-022-35377-0
Tags:
Space & Astrophysics