Scientists discover the phenomenon that affects the Earth’s radiation belts

Two University of Alaska Fairbanks scientists have discovered a new type of “whistle,” an electromagnetic wave that carries a significant amount of lightning energy into Earth’s magnetosphere.

The research was published today in Advances in science.

Vikas Sonwalkar, a professor emeritus, and Amani Reddy, an assistant professor, discovered the new type of wave. The wave carries lightning energy, which enters the ionosphere at low latitudes, into the magnetosphere. The energy is reflected upward from the lower boundary of the ionosphere, at about 55 miles altitude, into the opposite hemisphere.

It was previously believed, the authors write, that lightning energy entering the ionosphere at low latitudes remained trapped in the ionosphere and therefore was not reaching the radiation belts. The belts are two layers of charged particles that surround the planet and are held in place by the Earth’s magnetic field.

“We as a society are dependent on space technology,” Sonwalkar said. “Modern communication and navigation systems, satellites and spacecraft with astronauts on board encounter harmful energetic particles of the radiation belts, which can damage electronics and cause cancer.

“Having a better understanding of the radiation belts and the variety of electromagnetic waves, including those originating from terrestrial lightning, that affect them is vital to human operations in space,” he said.

Sonwalkar and Reddy’s discovery is a type of whistle they call a “speculatively reflected whistle.” Whistles produce a hissing sound when played through a speaker.

Lightning energy entering the ionosphere at higher latitudes reaches the magnetosphere as another type of whistle called a magnetospherically reflected whistle, which undergoes one or more reflections within the magnetosphere.

The ionosphere is a layer of the Earth’s upper atmosphere characterized by a high concentration of ions and free electrons. It is ionized by solar radiation and cosmic rays, making it conductive and crucial for radio communication because it reflects and modifies radio waves.

The Earth’s magnetosphere is a region of space surrounding the planet and created by the Earth’s magnetic field. It provides a protective barrier that prevents most solar wind particles from reaching the atmosphere and harming life and technology.

Sonwalkar and Reddy’s research shows that both types of whistles — specularly reflected whistles and magnetospherically reflected whistles — coexist in the magnetosphere.

In their research, the authors used plasma wave data from NASA’s Van Allen Probes, which launched in 2012 and operated until 2019, and lightning data from the Global Lightning Detection Network.

They developed a wave propagation model that, when considering speculatively reflected whistles, showed a doubling of lightning energy reaching the magnetosphere.

Review of plasma wave data from the Van Allen Probes showed that specularly reflected whistles are a common magnetospheric phenomenon.

Most lightning occurs in low latitudes, which are tropical and subtropical regions prone to developing thunderstorms.

“This implies that specularly reflected plumes probably carry a larger fraction of the lightning energy into the magnetosphere compared to that carried by magnetospherically reflected plumes,” Sonwalkar said.

The influence of lightning-generated whistle waves on radiation belt physics and their use in remote sensing of magnetospheric plasma have been investigated since the 1950s.

Sonwalkar and Reddy are with the Department of Electrical and Computer Engineering in the UAF College of Engineering and Mines. Reddy is also associated with the UAF Institute of Geophysics.

Sonwalkar and Reddy’s research is supported by grants from the National Science Foundation and NASA EPCoR, the Established Program to Stimulate Competitive Research.