Enigmatic Radio Emissions Resurface Above a Sunspot, Reshaping Stellar Understanding

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A group of scientists has revealed prolonged radio emissions emanating from a sunspot, reminiscent of phenomena observed in the polar regions of celestial bodies like planets and certain stars. This discovery has the potential to revolutionize our comprehension of intense stellar radio bursts. Image Credit: Sijie Yu

Researchers at the New Jersey Institute of Technology have documented extraordinary, long-lasting radio emissions originating from a sunspot, offering fresh insights into magnetic phenomena associated with the Sun and other stars.

In a study recently published in the journal Nature Astronomy, a team of astronomers affiliated with the New Jersey Institute of Technology’s Center for Solar-Terrestrial Research (NJIT-CSTR) has presented a detailed account of radio observations capturing an extraordinary aurora-like phenomenon occurring at an altitude of 40,000 kilometers above a relatively dark and cold region on the Sun, known as a sunspot.

Distinctive Features of the Novel Radio Emission

Experts point out that this novel radio emission shares characteristics with the auroral radio emissions commonly observed in the magnetospheres of planets such as Earth, Jupiter, Saturn, and certain low-mass stars.

This discovery offers fresh insights into the origins of intense solar radio bursts and opens up new avenues for understanding similar occurrences in distant stars characterized by large starspots. These findings were outlined by the study’s lead author, Sijie Yu, a scientist at NJIT-CSTR.

Unique Aspects of the Sunspot Radio Emissions

Yu states, “We’ve detected a peculiar type of long-lasting polarized radio bursts originating from a sunspot, persisting for over a week. This is in stark contrast to the typical, transient solar radio bursts that usually last for mere minutes or hours. It’s an exciting discovery that has the potential to alter our comprehension of stellar magnetic processes.”

Comparison With Earth’s Auroras

The famous auroral displays visible in Earth’s polar regions, such as the Aurora Borealis and Aurora Australis, occur when solar activities disrupt Earth’s magnetosphere, allowing charged particles to precipitate into the polar regions where they interact with oxygen and nitrogen atoms in the upper atmosphere. These accelerated electrons generate intense radio emissions at frequencies around a few hundred kilohertz as they move toward the north and south poles.

Yu’s team asserts that the newly observed solar radio emissions, which were detected over a large sunspot region characterized by strong magnetic fields on the Sun’s surface, differ from previously known solar radio noise storms in terms of their spectral and temporal characteristics.

Mechanism Behind the Sunspot Radio Emissions

Yu explains, “Our analysis, which provides spatial, temporal, and spectral resolution, suggests that these emissions are the result of electron-cyclotron maser (ECM) radiation, involving energetic electrons trapped within converging magnetic field configurations. The cooler and highly magnetic sunspot regions create a favorable environment for ECM radiation, drawing parallels with the magnetic polar caps of planets and other stars. This potentially provides a local solar analogue for studying these phenomena.”

“However, unlike Earth’s auroras, these sunspot aurora emissions occur at frequencies ranging from hundreds of thousands of kilohertz to approximately one million kilohertz, a direct consequence of the sunspot’s magnetic field being thousands of times stronger than Earth’s.”

Rohit Sharma, a scientist from the University of Applied Sciences Northwestern Switzerland (FHNW) and a co-author of the study, adds, “Our observations reveal that these radio bursts are not necessarily linked to the timing of solar flares. Instead, sporadic flare activity in nearby active regions seems to inject energetic electrons into large-scale magnetic field loops anchored at the sunspot, which subsequently fuel the ECM radio emissions above the region.”

The “sunspot radio aurora” is believed to exhibit rotational modulation synchronized with the Sun’s rotation, creating what Yu describes as a “cosmic lighthouse effect.”

“As the sunspot traverses the solar disk, it generates a rotating beam of radio light, akin to the modulated radio auroras observed in rotating stars,” Yu notes. “Since this sunspot radio aurora represents the first detection of its kind, our next step involves retrospective analysis. We aim to determine if some of the previously recorded solar bursts could be instances of this newly identified emission.”

Potential Implications for Astrophysical Models

Although the solar radio emissions are less potent, they bear a resemblance to stellar auroral emissions observed in the past. This suggests that starspots on cooler stars, akin to sunspots, could serve as sources for certain radio bursts observed in various stellar environments.

Bin Chen, an associate professor of physics at NJIT-CSTR and a co-author of the study, remarks, “This observation provides some of the clearest evidence of radio ECM emissions originating from the Sun. The characteristics resemble some of those observed on our planets and other distant stars, prompting us to consider the possibility that this model could potentially be applied to other stars with starspots.”

The team asserts that this recent insight, which draws connections between the Sun’s behavior and magnetic activities in other stars, could prompt astrophysicists to reconsider their current models of stellar magnetic activity.

Surajit Mondal, a solar researcher at NJIT, says, “We are gradually assembling the puzzle of how energetic particles and magnetic fields interact within a system featuring long-lasting starspots, not only on our Sun but also on stars located far beyond our solar system.”

Dale Gary, a distinguished professor of physics at NJIT-CSTR, adds, “By comprehending these signals originating from our Sun, we can gain a deeper understanding of the potent emissions from the most abundant type of star in the universe, M-dwarfs. This knowledge may unveil fundamental connections in astrophysical phenomena.”

Reference: “Detection of long-lasting aurora-like radio emission above a sunspot” by Sijie Yu, Bin Chen, Rohit Sharma, Timothy S. Bastian, Surajit Mondal, Dale E. Gary, Yingjie Luo, and Marina Battaglia, published on November 13, 2023, in Nature Astronomy.
DOI: 10.1038/s41550-023-02122-6

The research team, in collaboration with Marina Battaglia from FHNW, Tim Bastian from the National Radio Astronomy Observatory, and Yingjie Luo from the University of Glasgow, employed broadband dynamic radio imaging spectroscopy observations conducted using the Karl G. Jansky Very Large Array to make this groundbreaking discovery.

Frequently Asked Questions (FAQs) about solar radio emissions

More about solar radio emissions

• Nature Astronomy Journal
• New Jersey Institute of Technology
• Karl G. Jansky Very Large Array
• Electron-Cyclotron Maser (ECM) Radiation
• Aurora Borealis and Aurora Australis