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Upper atmosphere mystery solved by UCLA researchers

University of California, Los Angeles (UCLA) researchers have conducted detailed modeling to reveal that characteristics of natural very low—frequency radio waves known as chorus in the Earths upper atmosphere are primarily responsible for the observed relativistic electron build-up showing that radial diffusion is not responsible for the observed acceleration during an atmospheric storm. This newfound knowledge and procedural understanding will influence our activities throughout the universe.


atmosphereNew research published in the journal Nature resolves decades of scientific controversy over the origin of the extremely energetic particles known as ultra-relativistic electrons in the Earths near-space environment and is likely to influence our understanding of planetary magnetospheres throughout the universe.

Discovering the processes that control the formation and ultimate loss of these electrons in the Van Allen radiation belts — the rings of highly charged particles that encircle the Earth at a range of about 1,000 to 50,000 kilometers above the planets surface — is a primary science objective of the recently launched NASA Van Allen Probes mission. Understanding these mechanisms has important practical applications, because the enormous amounts of radiation trapped within the belts can pose a significant hazard to satellites and spacecraft, as well astronauts performing activities outside a craft.

Ultra-relativistic electrons in the Earths outer radiation belt can exhibit pronounced variability in response to activity on the sun and changes in the solar wind, but the dominant physical mechanism responsible for radiation-belt electron acceleration has remained unresolved for decades. Two primary candidates for this acceleration have been inward radial diffusive transport and local stochastic acceleration by very low-frequency plasma waves.

In research published Dec. 19 in Nature, lead author Richard Thorne, a distinguished professor of atmospheric and oceanic sciences in the UCLA College of Letters and Science, and his colleagues report on high-resolution satellite measurements of high-energy electrons during a geomagnetic storm on Oct. 9, 2012, which they have numerically modeled using a newly developed data-driven global wave model.

atmosphere2Their analysis reveals that scattering by intense, natural very low—frequency radio waves known, as chorus in the Earths upper atmosphere, is primarily responsible for the observed relativistic electron build-up.

The teams detailed modeling, together with previous observations of peaks in electron phase space density reported earlier this year by Geoff Reeves and colleagues in the journal Science, demonstrates the remarkable efficiency of natural wave acceleration in the Earths near-space environment.

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