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Rewriting the Book on the Radiation Belts
UNH investigators prepare for a NASA dual-satellite mission that will make the first simultaneous, comprehensive measurements of Earth’s radiation belts

AT THE DAWN OF THE SPACE AGE in 1958, NASA launched the Explorer 1 satellite with a simple instrument on board built by James Van Allen of the University of Iowa. When the 30-pound spacecraft passed through a region of Earth’s magnetosphere it encountered bands of nasty, high-energy radiation and Van Allen’s instrument – a Geiger counter built with tubes like an old television set – was flooded with particles.

The “Van Allen radiation belts” – an invisible torus of energetic particles held captive by Earth’s magnetic field – had been discovered. The inner belt was 400 to 4,000 miles up while the outer belt was 9,000 to 15,000 miles above the Equator. Both belts curved inward toward the magnetic poles. It was the first major scientific discovery of the new era of space exploration.

van allen belts  
This image shows the inner and outer Van Allen radiation belts as colored bands for purposes of illustration, but the sizes of the belts are closer to their real dimensions.
Courtesy of Geoff Reeves/Los Alamos National Laboratory

In the intervening half-century, various spacecraft, including Explorers 2 and 3 carrying investigations by Van Allen, took measurements in the radiation belts on their way towards more distant reaches of space. And several missions – like the 1990 Combined Release and Radiation Effects Satellite (a joint NASA/Department of Defense mission) and the low-altitude Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite – have greatly increased our knowledge of the region. SAMPEX, launched in 1992, in particular has revealed the belts to be far more dynamic and complex than originally thought.

But to date, the dots haven’t been fully connected between the energetic particles that populate the belts – some of which move at nearly the speed of light – and the electric and magnetic waves that propel the particles hither and yon. Given the right circumstances, these particles can have profoundly negative effects on the hundreds of commercial and operational satellites that orbit in the magnetosphere and can pose serious radiation hazards to astronauts and people in aircraft flying over Earth’s poles.

So the time was ripe for an investigation dedicated solely to the region – the Radiation Belt Storm Probes mission. Slated for launch in 2012, RBSP is part of NASA’s Living With a Star Geospace program to explore fundamental processes that operate throughout the solar system, in particular those that generate hazardous “space weather” (radiation) effects near the Earth and phenomena that could affect solar system exploration. Scientists and engineers from UNH’s Space Science Center are involved in the design and construction of instruments on two of the five RBSP experiments on each spacecraft.

rbsp flyby illustration
Artist's rendering showing the two RBSP spacecraft.
Courtesy of Johns Hopkins University Applied Physics Laboraory


“The mission’s goal is to study radiation belt particles,” says Harlan Spence, principal investigator of the Energetic Particle, Composition, and Thermal Plasma (ECT) instrument suite on the mission. Spence, director of the UNH Institute for the Study of Earth, Oceans, and Space, adds, “But we know we can’t adequately study the particles in the absence of the things that are causing them to change – wave phenomena and magnetic and electric fields – so with two spacecraft working in tandem we’ll be making comprehensive, quantitative measurements in the radiation belts as never before.”

Collectively, the ECT suite will provide five of the six charged particle measurements required to achieve the mission’s goals (see sidebar story on the ECT instrument suite). Another instrument – the Radiation Belt Storm Probes Ion Composition Experiment being built by a team headed up by Louis Lanzerotti of the New Jersey Institute of Technology – will obtain the sixth critical measurement. (The mission will also include the Relativistic Proton Spectrometer, principal investigator David Byers of the National Reconnaissance Office, which will make particle measurements of the inner radiation belt.)

As it happens, Lanzerotti, along with Michael Schulz, is author of the definitive work on the Van Allen belts, “Particle diffusion in the radiation belts,” published in 1974. Thirty-six years later, the RBSP mission will allow him to rewrite the book

Says Lanzerotti, “This mission takes me back to where my career began in the very earliest days of the nation’s space program. I look forward to learning all that we didn’t know when we wrote our book about four decades ago.”

Prior to SAMPEX the radiation belts were thought of as very simple, static, persistent, donut-shaped regions. Says Spence, “We now know that sometimes the belts disappear almost completely and then come back in timescales of days. The magnetosphere opens up and the particles drain away, they move around, they go away in space. They don’t vanish but there are wild variations, orders of magnitude in terms of flux.” Not stuff you’ll find in the textbook.

However, because SAMPEX is in a low-altitude orbit skimming through a tip-of-the-iceberg region of the belts “It’s not where the action is occurring,” Spence notes. “The two RBSP satellites will be right in the heart of it measuring all the waves and particle interactions and analyzing the differences of two regions in time and space.”

The mission, in other words, should provide the bigger picture of the Van Allen belts. Working in tandem with ECT is the Electric and Magnetic Field Instrument Suite and Integrated Science or EMFISIS experiment for which Craig Kletzing of the University of Iowa is the principal investigator. Kletzing, a former UNH research associate professor, is collaborating with mission co-investigator Roy Torbert, director of the UNH Space Science Center, who directs a team building a critical component of the EMFISIS instrument suite.

UNH engineers are designing and building the computer that will be tasked with coordinating the timing of the onboard field and wave experiments and “packaging” the data for transmission back to Earth. (See the Winter 2009 Spheres story Braving the Storm.) The Electric Field and Waves Suite experiment, with principal investigator John Wygant of the University of Minnesota, will round out the field measurements.

Both Kletzing and Spence note that the mission’s primary goal is to investigate the underlying physics of what’s occurring in the radiation belts for basic scientific understanding (i.e., to rewrite the textbook). This will lead to more accurate models that will, in turn, allow better prediction of space weather and help protect both machine and man from intense radiation events. (All the RBSP science instruments will be surrounded by material 10 times the normal wall thickness of a spacecraft in order to shield the instruments from the full force of their quarry.)

  ect on rbsp
  Artist's rendering of RBSP satellite with locations of ECT instrument suite.

“Here we are more than 50 years after Explorer 1 still trying to decipher the problem, which is not an easy one to solve,” Kletzing says adding, “we want to understand how the entire system works and it involves complicated physics.”

At its heart, the mission is aimed at understanding the universal and ubiquitous process of particle acceleration or, more specifically, the acceleration, transport, and loss of particles in magnetic fields. Happily, scientists have a “local laboratory” – Earth’s magnetosphere – to explore how the process works throughout the cosmos. And the energy range of particles in our planet’s radiation belts does not represent a cosmic sandbox; the particles go from essentially zero energy to those traveling a good fraction of the speed of light.

Says Spence, “We know almost without question that these particles are accelerated by electric fields, but electric fields can come in many forms and so we would like to be able to develop our understanding to the point of predictability. Right now we can’t predict whether the belts are going to go up or down in intensity or move around this way or that way because we really don’t understand all the possible mechanisms that can accelerate or decelerate the particles at any given time. With RBSP, each instrument team will be working together to understand the physical processes responsible for what we’re measuring.”

James Van Allen did not live to see the mission dedicated to the study of “his” radiation belts. He died on August 9, 2006 at the age of 91, and Kletzing recalls that the tireless, legendary space physicist worked regularly in his Department of Physics and Astronomy office-laboratory up until a month before he passed away.

For detailed information, visit the RBSP mission and instrument pages.

by David Sims, Science Writer, Institute for the Study of Earth, Oceans, and Space. Published in Summer 2010 issue of EOS Spheres.