Scientists Solve the Mystery of Powerful Radio Waves in Earth's Upper Atmosphere

This so-called plasmaspheric hiss or low-frequency radio waves which have been baffling the scientific community for over four centuries originate in the Van Allen radiation belts surrounding the Earth, and may pose grave danger to the operation of satellites in orbit around our planet or to astronauts during spacewalks. The Van Allen radiation belts have been discovered in the middle of the 1960's with the help of the US Explorer-1 satellite, almost by accident.

They consist of highly energetic charged elementary particles, trapped within powerful magnetic fields originating from Earth. Aside of protecting us by the action of deadly charged particles streaming towards the surface of the Earth, the Van Allen belt also seem to be the origin of powerful electromagnetic bursts of energy called 'chorus', during a phase when electrons are being energized by the magnetic field. Previously, scientists believed that these low-frequency radio emission are determined by natural phenomenons such as the appearance of lighting or even by plasma instabilities inside the belts themselves.

University of California researcher Jacob Bortnik, however, has proven otherwise. He writes that there are multiple theories that claim to have the power to explain the appearance of such phenomenons, but often they only offer partial explanation for the observed properties. Bortnik has shown that plasmasphere thousands of kilometers above the surface of the planet could easily evolve into a low-frequency radio emission.

Further still, Bortnik was able to create a model that accurately describes the broad range of emitted frequencies, plus the variation in radio frequency power in relation to the day-night asymmetry and solar activity. The chorus is basically the process through which these electrons are being energized inside the Van Allen belt, while the actual emission of radio frequencies occurs in the lower altitudes of the Earth's equatorial regions.

The frequencies spread throughout a range between 200 Hz and 1 kHz, and is called plasmaspheric hiss, being relatively similar to a sound hiss confined to the dense plasma structure that surrounds the Earth. Bortnik's work was originally related to the chorus, albeit he soon found out that the chorus and the hiss itself were tightly connected, thus began wondering whether the chorus determines the radio emission. Soon after, he found out that by modeling the chorus and the hiss would not only be confined to the region inside the plasmasphere, but it could also determine the emission of highly energetic electrons in the surrounding space.



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