Astronomers from the Center for Solar-Terrestrial Research (NJIT-CSTR) at
New Jersey Institute of Technology have detailed radio observations of an
amazing aurora-like display that occurs 40,000 km above a sunspot, which is
a relatively cold and dark patch on the sun. Their findings were published
in the journal Nature Astronomy.
The new radio emission, according to researchers, has properties in common
with auroral radio emissions that are frequently observed in planetary
magnetospheres, including those surrounding Earth, Jupiter, and Saturn, as
well as some low-mass stars.
According to Sijie Yu, a scientist at NJIT-CSTR and main author of the
paper, the discovery provides fresh insights into the formation of powerful
solar radio bursts and may open new paths for studying comparable events in
distant stars with huge starspots.
"We've detected a peculiar type of long-lasting polarized radio bursts
emanating from a sunspot, persisting for over a week," Yu added. This is not
like the normal, fleeting solar radio bursts that usually persist for a few
minutes or even hours. This is a fascinating finding that might change how
we understand stellar magnetic dynamics."
Famous auroral light displays, such as the Aurora Borealis or Aurora
Australis, can be seen across the sky in Earth's polar regions. These
displays are caused by solar activity disrupting Earth's magnetosphere,
which makes it easier for charged particles to precipitate to the polar
regions where the magnetic field converges and interact with nitrogen and
oxygen atoms in the upper atmosphere. These electrons, which are
accelerating toward the north and south poles, can produce powerful radio
broadcasts at frequencies of a few hundred kHz.
The newly reported solar radio emissions, according to Yu's team, are
different from previously identified solar radio noise storms in both
spectrum and temporal aspects. They were found over a large sunspot patch
that was momentarily growing where magnetic fields on the sun's surface are
very high.
"Our spatially, temporally and spatially resolved analysis suggests that
they are due to the electron-cyclotron maser (ECM) emission, involving
energetic electrons trapped within converging magnetic field geometries," Yu
said.
"The cooler and intensely magnetic areas of sunspots provide a favorable
environment for the ECM emission to occur, drawing parallels with the
magnetic polar caps of planets and other stars and potentially providing a
local solar analog to study these phenomena."
"However, unlike the Earth's auroras, these sunspot aurora emissions occur
at frequencies ranging from hundreds of thousands of kHz to roughly 1
million kHz—a direct result of the sunspot's magnetic field being thousands
of times stronger than Earth's."
Co-author Rohit Sharma of the University of Applied Sciences Northwestern
Switzerland (FHNW) said, "Our observations reveal that these radio bursts
are not necessarily tied to the timing of solar flares either." "Instead,
sporadic flare activity in nearby active regions seems to pump energetic
electrons into large-scale magnetic field loops anchored at the sunspot,
which then power the ECM radio emission above the region."
Yu defines the "cosmic lighthouse effect" as the result of rotational
modulation of the "sunspot radio aurora" occurring in time with the solar
rotation.
"As the sunspot traverses the solar disk, it creates a rotating beam of
radio light, similar to the modulated radio aurora we observe from rotating
stars," Yu said. Since this sunspot radio aurora is the first of its kind to
be detected, retrospective analysis is the next stage in our process. Our
goal is to ascertain whether any of the solar bursts that have been
previously observed may be examples of this recently discovered
emission."
Though weaker, the solar radio emissions are comparable to stellar auroral
emissions previously detected, which may imply that certain radio bursts
observed in different stellar settings might originate from starspots on
colder stars, similar to sunspots.
"This finding is some of the most convincing proof of radio ECM emissions
from the sun that we have observed. The features are similar to those found
on our planets and other far-off stars, so we thought this model would work
for other stars that have starspots as well," co-author and assistant
professor of physics Bin Chen of NJIT-CSTR stated.
According to the study, astrophysicists may need to reconsider their
present models of stellar magnetic activity in light of the most recent
discovery, which links the behavior of our sun to the magnetic activities of
other stars.
"We're beginning to piece together the puzzle of how energetic particles
and magnetic fields interact in a system with the presence of long-lasting
starspots, not just on our own sun but also on stars far beyond our solar
system," Surajit Mondal, a solar researcher at NJIT,
Dale Gary, a renowned professor of physics at NJIT-CSTR, continued, "By
comprehending these signals from our own sun, we can better interpret the
powerful emissions from the most common star type in the universe, M-dwarfs,
which may reveal fundamental connections in astrophysical phenomena."
The finding was obtained by the study team using broadband dynamic radio
imaging spectroscopic data from the Karl G. Jansky Very Large Array, with
collaboration from Tim Bastian of the National Radio Astronomy Observatory
and Marina Battaglia from FHNW.
Provided by
New Jersey Institute of Technology
