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Twin probes discover 'zebra stripe' pattern in Earth's inner radiation belt

Jenny Winder, News Writer
Mar 22, 2014, 7:00 UTC

Sen—Scientists, using data from the twin NASA Van Allen Probes, have discovered a new, persistent pattern in Earth's inner radiation belt.

The probes, which launched on August 30, 2012 as the Radiation Belt Storm Probes, were re-named in honour of physicist James Van Allen who, in 1958, discovered the radiation belts encircling our planet. The Van Allen Probes mission goal is to shed light on how and why radiation levels in the belts change with time.

The radiation belts are dynamic, doughnut-shaped regions around our planet, extending high above the atmosphere, made up of high-energy particles trapped by Earth's magnetic field. Radiation levels across the belts are affected by solar activity, such as solar storms, and can ebb and flow. During active conditions, radiation levels can dramatically increase, which can create hazardous space weather conditions that harm orbiting spacecraft and endanger humans in space.

A cutaway model of the radiation belts with the two satellites flying through them. Image credit: NASA.

Data from the Van Allen Probes Ion Composition Experiment (RBSPICE) on board each of the twin spacecraft revealed that the highly energized population of electrons of the inner radiation belt is organized into very structured arrays that resemble slanted zebra stripes.

The pattern is produced by the slow rotation of Earth, which was previously considered incapable of affecting the motion of radiation belt particles, which have velocities that approach the speed of light.

Scientists had previously believed that increased solar wind activity was the primary force behind any patterns in our planet's radiation belts. But these zebra stripes were shown to be visible even during low solar wind activity, which prompted a search for an alternate explanation of how they form.

"It is because of the unprecedented high energy and temporal resolution of our energetic particle experiment, RBSPICE, that we now understand that the inner belt electrons are, in fact, always organized in zebra patterns," said Aleksandr Ukhorskiy of the Johns Hopkins Applied Physics Laboratory (APL), co-investigator on RBSPICE and lead author of the paper. "Furthermore, our modeling clearly identifies Earth's rotation as the mechanism creating these patterns. It is truly humbling, as a theoretician, to see how quickly new data can change our understanding of physical properties."

An example of energetic electron spectra, measured on June 18, 2013, by NASA's twin Van Allen Probes. Image credit: Aleksandr Ukhorskiy, APL.

Because of the tilt in Earth's magnetic field axis, the planet's rotation generates an oscillating, weak electric field that permeates through the entire inner radiation belt. Ukhorskiy suggested this analogy: "If the inner belt electron populations are viewed as a viscous fluid, these global oscillations slowly stretch and fold that fluid, much like taffy is stretched and folded in a candy store machine," he said in a statement.

This stretching and folding process results in the striped pattern observed across the entire inner electron belt, extending from above Earth's atmosphere (about 500 miles above the planet's surface) up to roughly 8,000 miles.

"This finding tells us something new and important about how the universe operates," said Barry Mauk of APL, Van Allen Probes project scientist and an author of the paper.

"The new results reveal a new large-scale physical mechanism that can be important for planetary radiation belts throughout the solar system. An instrument similar to RBSPICE is now on its way to Jupiter on NASA's Juno mission, and we will be looking for the existence of zebra stripe-like patterns in Jupiter's radiation belts."

The twin probes are named in honour of physicist James Van Allen who, in 1958, discovered the radiation belts encircling our planet.