When high-speed wind blows over stagnant air masses in the atmosphere, the turbulent mixing results in a striking “breaking wave” cloud pattern. These are called Kelvin-Helmholtz waves, and a very similar thing happens in space with high-energy solar wind plasma. But those waves can perturb Earth’s magnetic field, or the magnetosphere, which shields us from cosmic radiation. Those swirls open the gate to potentially harmful effects of space weather. The findings were published in Nature Communications this week.
Named after 19th-century scientists Lord William Thomson Kelvin and Hermann von Helmholtz, these Kelvin-Helmholtz instability waves can be found throughout the universe -- creating these distinctive patterns in clouds and water surfaces everywhere, from your summer paddling pool to Jupiter’s atmosphere. The appearance of these “breaking waves” is created by velocity shear: Two fluids, like water and wind, interacting at different speeds can create different pressures at the back and front ends of the wave.
"In clouds, you see it because the lower atmosphere is more stagnant and you have a higher speed wind going over it, which creates that distinctive swirl pattern. The phenomenon is really ubiquitous in nature,” University of New Hampshire’s Shiva Kavosi explains in a news release. “Often, the waves are present in the atmosphere but not visible if there are no clouds. In that case, pilots cannot see them and aircraft may experience severe and unexpected turbulence."
While ubiquitous, ultra low-frequency Kelvin-Helmholtz waves weren’t considered a common mechanism for changing the dynamics of the magnetosphere, until now. Kavosi and University of New Hampshire’s Joachim Raeder surveyed seven years of data from NASA’s five-satellite THEMIS mission, which has crossed the magnetopause (the edge of Earth’s magnetosphere) over a thousand times.
The duo found that Kelvin-Helmholtz waves occur at the magnetopause about 19% of the time -- that’s much more frequently than they thought.
“This is significant because whenever the edge of Earth's magnetosphere, the magnetopause, gets rattled it will create waves that propagate everywhere in the magnetosphere, which in turn can energize or de-energize the particles in the radiation belts,” Raeder explains. These waves allow particles from solar wind to enter the magnetosphere, producing oscillations that change the energy levels of the planet’s radiation belts and potentially threatening spacecraft and Earth-based technologies.