German researchers have discovered huge, vortex-shaped waves on the sun. As the team led by Laurent Gizon of the Max Planck Institute for Solar System Research (MPS) in the journal Nature Astronomy reports, this is a similar phenomenon to the terrestrial Rossby waves that form in the Earth’s atmosphere and oceans. The solar waves propagate opposite to the direction of rotation of the sun, have a lifespan of several months and maximum amplitudes at the equator of the sun.
That Rossby waves also occur on stars was postulated decades ago. “Solar Rossby waves have very small amplitudes and periods of several months, making them extremely hard to spot,” Gizon said. For their study, the researchers have evaluated a record of the Heliospheric and Magnetic Imager (HMI) of NASA’s Solar Dynamics Observatory (SDO), which is the result of six years of observation.
“The HMI images have a sufficiently high spatial resolution to track the movement of the granules on the visible surface of the Sun,” said Björn Löptien, lead author of the study. The granules are comparatively small convection particles that are around 1,500 kilometers long on the sun’s surface. Thanks to the movement of the granules, the researchers were able to deduce the much larger turbulences associated with the Rossby waves. Using helioseismological methods, the solar Rossby waves could be studied in the sun’s interior at depths up to 20,000 kilometers.
“Overall, we find large, vortex-shaped waves on the Sun that move against the rotation, and it’s completely unexpected that these waves can only be seen in the equatorial regions,” says Gizon. The wave patterns are stable for several months. For the first time, the researchers were able to determine the relationship between the frequency and wavelength of the waves and thus unambiguously identify them as Rossby waves. Gizon: “Solar Rossby waves are gigantic, their wavelengths are comparable to the sun’s radius.” They are an integral part of the Sun’s internal dynamics, contributing half of the sun’s kinetic energy.