Scientists uncover aurora-like radio emission above a sunspot

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.