Vikas Sonwalkar, a professor emeritus, and Amani Reddy, an assistant professor, have discovered this new wave, which channels lightning energy into the ionosphere at lower latitudes and subsequently into the magnetosphere. The energy, after entering the ionosphere, is reflected back by its lower boundary-located around 55 miles above the Earth's surface-in the opposite hemisphere.
Prior research suggested that lightning energy entering the ionosphere at low latitudes remained trapped within it, preventing it from reaching the radiation belts. These belts are layers of charged particles that encircle Earth, held in place by the planet's magnetic field.
"We as a society are dependent on space technology," Sonwalkar noted. "Modern communication and navigation systems, satellites, and spacecraft with astronauts aboard encounter harmful energetic particles of the radiation belts, which can damage electronics and cause cancer."
He further added, "Having a better understanding of radiation belts and the variety of electromagnetic waves, including those originating in terrestrial lightning, that impact them is vital for human operations in space."
The newly discovered wave type, which Sonwalkar and Reddy have named the "specularly reflected whistler," emits a whistling sound when converted into audio. In contrast, lightning energy entering the ionosphere at higher latitudes is transmitted to the magnetosphere as a different wave, known as a magnetospherically reflected whistler, which reflects one or more times within the magnetosphere.
The ionosphere, a region of Earth's upper atmosphere rich in ions and free electrons, is ionized by solar radiation and cosmic rays. This layer is essential for radio communication, as it reflects and modifies radio waves. Meanwhile, the magnetosphere is a protective region of space surrounding Earth, formed by the planet's magnetic field, which shields the atmosphere and technology from the harmful particles of the solar wind.
Their research demonstrates that both types of whistlers-specularly reflected and magnetospherically reflected-are present in the magnetosphere.
The authors utilized plasma wave data from NASA's Van Allen Probes, which operated from 2012 to 2019, along with lightning data from the World Wide Lightning Detection Network. They developed a wave propagation model that incorporated specularly reflected whistlers, revealing a doubling in the lightning energy reaching the magnetosphere.
Analysis of plasma wave data from the Van Allen Probes confirmed that specularly reflected whistlers are a prevalent phenomenon within the magnetosphere.
Since most lightning occurs at low latitudes-regions known for frequent thunderstorms-the researchers suggest that specularly reflected whistlers likely carry a larger share of lightning energy to the magnetosphere compared to magnetospherically reflected whistlers.
The influence of lightning-generated whistler waves on radiation belt dynamics and their application in remotely sensing magnetospheric plasma have been studied since the 1950s.
Both Sonwalkar and Reddy are part of the Department of Electrical and Computer Engineering at UAF's College of Engineering and Mines, with Reddy also affiliated with UAF's Geophysical Institute.
Research Report:Specularly reflected whistler: A low-latitude channel to couple lightning energy to the magnetosphere
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University of Alaska Fairbanks
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