Astronomers have detected extraordinary radio emissions resembling polar auroras at an altitude of 40,000 kilometers above a sunspot. Although sharing some characteristics with Earth’s auroras, the newly identified phenomenon differs in intensity and duration. Similar events may occur on other stars with significant sunspots. Exploring these occurrences could enhance our understanding of stellar geomagnetic mechanisms.
On Earth, polar auroras (both boreal and austral) manifest when solar activity disrupts the magnetosphere, leading to the precipitation of charged particles toward polar regions where magnetic fields converge. Interaction with oxygen and nitrogen atoms in the upper atmosphere produces dazzling, colorful displays. The acceleration of these particles can result in intense radio emissions, typically in the range of a few hundred kilohertz. Similar auroral radio emissions have been observed on other planets, including Jupiter and Saturn.
However, the newly detected solar radio emissions are notably intense, originating in regions with exceptionally high magnetic fields. Given that a sunspot’s magnetic field is thousands of times more potent than Earth’s, the emitted frequencies can reach up to one million kilohertz. Additionally, these emissions differ both spectrally and temporally from previously observed solar radio bursts. This discovery provides valuable insights into the unique characteristics of solar magnetic phenomena.
“We’ve detected a peculiar type of long-lasting polarized radio bursts emanating from a sunspot, persisting for over a week,” explains the lead author of the study, Sijie Yu, in a statement from the New Jersey Institute of Technology. “This is quite unlike the typical, transient solar radio bursts typically lasting minutes or hours. It’s an exciting discovery that has the potential to alter our comprehension of stellar magnetic processes,” he adds.
Exploring solar magnetism is crucial for understanding its magnetic activity, internal dynamics, and influence on planets. This understanding could then be extrapolated to other stellar systems, including potentially habitable exoplanets. Observing radio emissions provides a unique insight into the physical parameters governing these phenomena.
Phenomena Potentially Applicable to Other Stars
In the course of their study, Yu and his team utilized wide-band dynamic radio imaging observations from the Karl G. Jansky Very Large Array radio telescope (located in New Mexico). As reported in the Nature Astronomy journal, the analyzed radio emissions occurred at an altitude of 40,000 kilometers above a sunspot. The findings suggest that these emissions are attributable to electron-cyclotron maser (ECM) emissions, involving highly energetic electrons trapped in converging magnetic fields. The colder and more intensely magnetic regions of sunspots provide a conducive environment for ECM. Consequently, ECM can offer direct measurements of the magnetic field intensity on the solar surface.
Radio ECM (Electromagnetic Continuum) bursts have previously been observed in the planetary magnetospheres. However, the highly polarized nature, broad spectral band, and intense radiation of this type of burst set it apart. Similar emissions of varying degrees and relatively long duration have been detected in low-mass stars. However, no persistent ECM radio emission has been observed thus far at the level of the sun.
The experts in the new study have also noted that the observed radio bursts were not necessarily synchronized with solar flares. Instead, eruptions occurring in the surrounding active regions seem to draw energetic electrons from large-scale magnetic loops anchored to the sunspot. These loops then fuel the radio emission known as ECM.
On the other hand, researchers suggest that the rotation of sunspot auroras would be modulated and synchronized with that of the star, creating a “cosmic lighthouse effect.” “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,” explains Yu. Moreover, since the emission characteristics are comparable to those of other, more distant stars, the results could be applied to those with spots. Indeed, like sunspots, stellar spots on other stars of similar size (to the Sun) could be the source of intense radio bursts detected in their environments.
“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,” says Surajit Mondal, a co-author of the study and also from the New Jersey Institute of Technology. These findings provide a new avenue for deciphering key habitability parameters of other planetary systems.