A secret behind the workings of sunquakes – seismic activity on the Sun during solar flares – might be hidden beneath the solar surface.
These earthquake-like events release acoustic energy in the form of waves that ripple along the Sun’s surface, like waves on a lake, in the minutes following a solar flare – an outburst of light, energy, and material seen in the Sun’s outer atmosphere.
Scientists have long suspected that sunquakes are driven by magnetic forces or heating of the outer atmosphere, where the flare occurs. These waves were thought to dive down through the Sun’s surface and deep into its interior. But new results, using data from NASA’s Solar Dynamics Observatory, have found something different.
In July 2011, SDO observed a sunquake with unusually sharp ripples emanating from a moderately strong solar flare. Scientists were able to track the waves that caused these ripples back to their source, using a technique called helioseismic holography. This technique, which used SDO’s Helioseismic and Magnetic Imager to measure how the solar surface was moving, has previously been used to track acoustic waves from a variety of other sources in the Sun.
Instead of the waves traveling into the Sun from above, the scientists saw the surface ripples of a sunquake emerging from deep beneath the solar surface right after a flare occurred. The results, published in the journal Astrophysical Journal Letters, found the acoustic source was around 700 miles (1,100 kilometers) below the surface of the Sun – not above the surface as previously thought.
The scientists believe that these waves were driven by a submerged source, which was in turn somehow triggered by the solar flare in the atmosphere above. The new findings might help explain a long-standing mystery about sunquakes: why some of their characteristics look remarkably different from the flares that trigger them.
The scientists still haven’t identified exactly what mechanism actually causes sunquakes, though the results do provide the clue that their origins likely lurk beneath the surface. The scientists plan to continue searching for a mechanism by looking at other sunquakes to see if they have similarly submerged sources.
Reference: “Submerged Sources of Transient Acoustic Emission from Solar Flares” by Charles Lindsey, J. C. Buitrago-Casas, Juan Carlos Martínez Oliveros, Douglas Braun, Angel D. Martínez, Valeria Quintero Ortega, Benjamín Calvo-Mozo and Alina-Catalina Donea, 21 September 2020, Astrophysical Journal Letters.