University of Minnesota
School of Physics & Astronomy


New physics from the radiation belt

Cindy Cattell
Professor Cindy Cattell with a model of the STEREO space probe
Patrick O'Leary

Professor Cindy Cattell of the School of Physics and Astronomy is part of the NASA STEREO Waves collaboration. STEREO consists of two space-based observatories - one ahead, "A" of Earth in its orbit, the other trailing behind,"B".

While STEREO is principally meant to make stereoscopic images of the Sun and solar storms, Cattell has seen important new results about the Earth’s radiation belt, which have not previously been probed with modern wave instruments. NASA hopes to study the belts further because they pose a substantial danger to satellites and because they provide an excellent laboratory for studying relativistic particle acceleration in space plasmas. (See Research Spotlight on Radiation Belt Storm Probes.) Cattell has already found that STEREO observations have yielded surprising new physics.

Based on earlier observations of waves, theoretical models of the radiation belts were developed that showed that one type of electromagnetic wave, whistler waves with small amplitudes (~1 mV/m), required tens of hours to accelerate electrons to relativistic energies (1 MeV and above). During one passage through the dawn radiation belt, STEREO discovered that the whistler waves had amplitudes that were more than two orders of magnitude larger (>250 mV/m), sometimes saturating the detector. Cattell and physics undergraduate Kris Kersten used a simulation running on a Minnesota Supercomputer Institute computers to demonstrate that waves of that amplitude could accelerate electrons to relativistic energies in a fraction of a second rather than in tens of hours. The acceleration is a coherent nonlinear process rather than a stochastic ('drunkard's walk') one. On one of its perigee passes, STEREO A came through the radiation belt and observed no large waves. Shortly after, as STEREO B passed through on a similar trajectory, a "substorm" (an explosive energy release process in the Earth's magnetic tail) was occurring. It was on this pass that STEREO B recorded the extraordinarily large whistler waves. Whistler waves are so named because they were first heard on radios during World War One after lightning strikes. The whistler waves heard by STEREO were not caused by lightning but were excited by lower energy electrons injected into the radiations belts by the substorm.

Cattell said that it was "somewhat of a fluke" that the STEREO Waves instrument was turned on as it moved through the belts. "Normally our instruments wouldn’t even be turned on until the satellites reached their prime locations in the solar wind." With the knowledge that STEREO would be passing through interesting regions near the Earth during the four passes required to place STEREO A and B in their desired orbits, Associate Program Director Keith Goetz pushed for turning on the WAVES instruments before the passage through the radiation belts. Cattell said that Goetz primarily designed and built the instrument, which continuously samples the waveforms (electric field versus time) and records and transmits to the ground only the largest (and thus dynamically significant) waves. Plasma wave instruments flown on previous satellites in the radiation belts were not capable of recognizing the most significant waves. Previous waveform observations used random sampling and with limited data storage capability it is not surprising that the very large amplitude waves were not seen.

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