A team of NASA-funded scientists recently discovered long-lasting radio signals above sunspots — regions of the Sun’s surface where the magnetic field is so strong that surface temperatures are reduced and surrounding atmospheric pressure decreases, creating dark spots on the solar surface.
The team’s results show the radio signals are emanating from the sunspots in a way similar to that of auroras on Earth. The radio signals were detected around 40,000 kilometers above a sunspot. They are the first of their kind to be observed at the Sun, with previous long-lasting radio signals only being observed on other planets or stars.
“This sunspot radio emission represents the first detection of its kind,” said lead author Sijie Yu of the New Jersey Institute of Technology.
The dark spot in the upper-left region of the Sun in this image is a sunspot. One of the long-lasting radio bursts discovered by Yu et al. was found above this sunspot. (Credit: NASA/Solar Dynamics Observatory)
The discovery of the long-lasting radio signals at the Sun will allow scientists to better understand not only our star but also how distant stars function by learning more about the processes behind the radio signals and how they are produced. The Sun regularly emits short radio bursts that can last anywhere from just a few minutes to a few hours.
However, the radio bursts observed by Yu et al. lasted for over a week — significantly longer than the Sun’s normal short radio bursts. The team performed their observations with the Karl G. L=Jansky Very Large Array (VLA) in New Mexico.
In addition to their long-lasting nature, the radio bursts also feature spectra and polarization characteristics similar to that of the auroras produced by Earth’s atmosphere and magnetic field. On Earth (as well as other planets like Jupiter and Saturn), auroras are produced by the interaction between solar particles and the magnetic field, wherein the solar particles are pulled and accelerated toward the poles where magnetic field lines converge.
As these solar particles accelerate toward the poles, they produce very intense radio signals at frequencies of a few hundred kilohertz. When the solar particles finally reach the planet’s atmosphere, they slam into atoms within the atmosphere and emit light. This emission of light is an aurora.
After collecting their observations with VLA, Yu et al. performed an analysis of the data to learn more about the characteristics and nature of the long-lasting radio burst found above the sunspots. Their results showed that the sunspot radio bursts are likely being produced via a process similar to that of the production of auroras. Energetic electrons get trapped within the magnetic field being emitted from the sunspot. After getting captured in the field, the particles are pulled and accelerated downward to the sunspot, where the magnetic field lines converge — creating radio signals.
Unlike Earth, Jupiter, and Saturn’s auroras, though, the sunspot radio bursts occur at much, much higher frequencies of roughly one million kilohertz — a stunning jump from the normal hundreds of thousands of kilohertz that Earth’s auroras occur at.
“That’s a direct result of the sunspot’s magnetic field being thousands of times stronger than Earth’s,” said Yu.
In addition to being observed on several different planets, these long-lasting radio bursts have also been observed at some types of low-mass stars. The discovery of these long-lasting bursts at the Sun provides the option for aurora-like radio emissions to originate from large starspots as well as from aurora in the polar regions of the stars.
“The discovery excites us as it challenges existing notions of solar radio phenomena and opens new avenues for exploring magnetic activities both in our Sun and in distant stellar systems,” Yu explained.
Throughout the following weeks and months, Yu et al. will re-investigate other solar radio bursts to determine if any are similar to the long-lasting bursts discovered with VLA.
“We aim to determine if some of the previously recorded solar bursts could be instances of this newly identified emission,” Yu said.
“NASA’s growing heliophysics fleet is well suited to continue to investigate the source regions of these radio bursts. For example, the Solar Dynamics Observatory continually monitors the Sun’s active regions, which likely give rise to this phenomenon,” said heliophysicist and solar radio researcher Natchimuthuk Gopalswamy of NASA’s Goddard Space Flight Center.
(Lead image: An artist’s impression of the long-lasting radio emissions above a sunspot. Credit: Sijie Yu, New Jersey Institute of Technology)
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