Scientists have made detailed radio observations of an unusual aurora-like phenomenon occurring 40,000 km above a relatively dark and cool area of the Sun known as a solar spot. Using the Karl G. Jansky Very Large Array, a team from the New Jersey Institute of Technology’s Center for Solar-Terrestrial Research (NJIT-CSTR) observed polarized radio bursts emanating from a solar spot that lasted over a week.

Lead author Sijie Yu, a scientist at NJIT-CSTR, says this discovery provides new insights into the origin of intense solar radio bursts. It may also help understand similar phenomena on distant stars with large starspots. We detected an unusual type of long-lasting polarized radio bursts emerging from a solar spot, unlike typical solar radio bursts that usually last minutes or hours, says Yu. This is an exciting find that could change our understanding of stellar magnetic processes.

Aurora displays visible in the polar skies of Earth, like the Northern and Southern Lights, occur when solar activity disturbs Earth’s magnetosphere. This allows charged particles to rain down towards the magnetic field convergence at the poles. There, the particles excite oxygen and nitrogen atoms in the upper atmosphere, emitting intense low-frequency radio around hundreds of kHz.

The team claims the newly observed solar spot radio emissions differ spectrally and temporally from known solar radio storms. Their spatially, temporally and spectrally resolved analysis suggests electron cyclotron maser emission (ECM), where energetic electrons become trapped in converging magnetic field geometries. Solar spots’ colder, stronger magnetic regions provide an environment suitable for ECM, analogous to planetary magnetic polar caps and stellar phenomena.

However, unlike Earth’s auroras, these solar spot auroras emit from hundreds of kHz to around 1 MHz due to the spot’s magnetic field being thousands of times stronger than Earth’s. Co-author Rohit Sharma of the Northwest Switzerland University of Applied Sciences adds the bursts are not necessarily linked to solar eruptions, but sporadic flaring in nearby active regions may pump energetic electrons into large-scale magnetic loops anchored to the spot, powering ECM emission above it.

The researchers believe the “solar spot radio aurora” exhibits rotational modulation in sync with the Sun’s rotation, creating what Yu describes as a “cosmic lighthouse effect.” As the spot traverses the Sun’s disk, it sweeps a rotating beam of radio light, akin to rotationally modulated radioauroras seen in other stars.

The solar emissions, though weaker, resemble stellar auroral emissions previously observed, suggesting starspots on cooler stars may similarly source some radio bursts in stellar environments. This is some of the clearest ECM radio evidence we’ve seen from the Sun, with features resembling those in our planets and distant stars, says co-author Bin Chen of NJIT-CSTR.

The team argues this last idea linking our Sun’s behavior to other stars’ magnetic activities may prompt astrophysicists to re-examine current stellar magnetic models. We’re piecing together how energetic particles and magnetic fields interact in systems with long-lasting starspots, not just our Sun but stars far beyond, says NJIT solar researcher Surajit Mondal.

By understanding our own Sun’s signals, we can better interpret powerful emissions from the most common type of star in the universe, M dwarfs, that may reveal fundamental astrophysical connections, adds NJIT-CSTR Distinguished Professor Dale Gary. The discovery helps unravel fundamental mysteries of stellar and solar phenomena.


Sources

New Jersey Institute of Technology | Yu, S., Chen, B., Sharma, R. et al. Detection of long-lasting aurora-like radio emission above a sunspot. Nat Astron (2023). doi.org/10.1038/s41550-023-02122-6


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