Van Allen Probes Provide Insight Into Electron Acceleration In Earth's Radiation Belts

April Flowers for redOrbit.com – Your Universe Online

Data from NASA’s Van Allen Probes mission has helped researchers resolve decades of scientific uncertainty over the origin of ultra-relativistic electrons in Earth’s near space environment. The findings, published in Nature, are likely to influence our understanding of planetary magnetospheres throughout the universe.

One of the primary science objectives of the Van Allen Probes is to understand the processes that control the formation and ultimate loss of such relativistic electrons. Such understanding would have important practical applications because of the enormous amounts of radiation trapped within the two Van Allen radiation belts. The belts, which were discovered above Earth’s upper atmosphere in 1958 by James Van Allen, consist of high-energy electrons and protons and can pose a significant hazard to satellites and spacecraft, as well to astronauts performing activities outside a spacecraft.

In response to activity on the sun, and changes in the solar wind, such electrons in the Earth’s outer radiation belt can exhibit pronounced increases in intensity. The dominant physical mechanism responsible for such radiation belt electron acceleration, however, has remained unresolved for decades.

Scientists have constructed two primary theories for electron acceleration, one external and one internal. From outside the belts, scientists developed a theoretical process known as inward radial diffusive transport. From inside, in contrast, they hypothesize  that the electrons are undergoing strong local acceleration from very low frequency plasma waves. The very nature of the wave acceleration also creates controversies. Scientists debate if it is stochastic – that is, a linear and diffusive process – or is it non-linear and coherent?

The high-resolution measurements made by the Van Allen Probes were used by Richard Thorne, UCLA professor of Atmospheric and Oceanic Sciences, to suggest that local acceleration is at work. Thorne and his team analyzed observations of high-energy electrons during a geomagnetic storm of Oct. 9, 2012, along with a data-driven global wave model, revealing that linear, stochastic scattering by intense, natural very low-frequency radio waves — known as chorus waves — in Earth’s upper atmosphere can account for the observed relativistic electron build-up.

“The successful point-by-point comparison of radiation belt features observed by the Van Allen Probes with the predictions of the state of the art model developed by Richard Thorne and his group dramatically demonstrates the significance of in situ particle acceleration within Earth’s radiation belts,” said David Sibeck, mission scientist for the Van Allen Probes at NASA’s Goddard Space Flight Center.

These findings, combined with previous observations of peaks in electron phase space density reported earlier this year in Science by Geoff Reeves at Los Alamos National Laboratory and his colleagues, reveals the remarkable efficiency of natural wave acceleration in Earth’s near space environment. The findings demonstrate that radial diffusion was not responsible for the observed acceleration during this storm, said Thorne, a scientist at the University of California at Los Angeles.

Thorne said that the local wave acceleration process is a universal physical process and should also be effective in the magnetospheres of Jupiter, Saturn and other magnetized plasma environments in the cosmos The new detailed analysis from Earth, according to Thorne, will influence future modeling of other planetary magnetospheres.

“This new finding is of paramount importance to unlocking the multitude of processes behind particle behavior in the belts,” says Barry Mauk, project scientist for the Van Allen Probes at the Johns Hopkins Applied Physics Laboratory. “To have one of the primary science objectives of the mission met within just over a year of launch is a testament to the quality and quantity of the data the instruments on the probes are gathering, and to the teams analyzing them.”