Washington, March 6 : Scientists have solved a 40-year-old puzzle by identifying the origin of the intense radio waves in the Earth's upper atmosphere that control the dynamics of the Van Allen radiation belts.
These belts consist of high-energy electrons that can damage satellites and spacecraft and pose a risk to astronauts performing activities outside their spacecraft.
Beginning in the late 1960s, spacecraft observations of wideband electromagnetic noise at frequencies below a few kilohertz established the presence of a steady, incoherent noise band in the frequency range between 200 Hz and 1 kHz.
This emission was dubbed "plasmaspheric hiss" because of its unstructured nature, its spectral resemblance to audible hiss and its confinement to the plasmasphere, a dense plasma region around the Earth.
The origin of these low-frequency radio waves was earlier thought to be lightning or instabilities from a plasma.
But, new research by the UCLA Department of Atmospheric and Oceanic Sciences determines the source to be an intense electromagnetic wave type called "chorus," which energizes electrons and was initially thought to be unrelated to the hiss.
"That chorus waves are the dominant source of plasmaspheric hiss was a complete surprise," said Jacob Bortnik, one of the researchers.
According to Bortnik, "Here, we show that a different wave type, called chorus, can propagate into the plasmasphere from tens of thousands of kilometers away and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day-night asymmetry in intensity, its association with solar activity and its spatial distribution."
"The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere, while hiss depletes these electrons at lower equatorial altitudes," he added.
Hiss tends to be confined inside of the plasmasphere, and chorus outside of it. While chorus occurs outside the plasmasphere, it leaks inside of it.
"A better understanding of plasmaspheric hiss will help scientists to more accurately model the behavior of the high-energy electrons in the Van Allen radiation belts and thus improve their forecasts of space conditions," said Bortnik.