Van Allen Probes show how to accelerate electrons

NASA's Van Allen Probes orbit through two giant radiation belts surrounding Earth. 

Their observations help explain how particles in the belts can be sped up to nearly the speed of light. 

Credit: NASA

One of the great, unanswered questions for space weather scientists is just what creates two gigantic donuts of radiation surrounding Earth, called the Van Allen radiation belts.

Recent data from the Van Allen Probes, two nearly identical spacecraft that launched in 2012, address this question.

The inner Van Allen radiation belt is fairly stable, but the outer one changes shape, size and composition in ways that scientists don't yet perfectly understand.

Some of the particles within this belt zoom along at close to light speed, but just what accelerates these particles up to such velocities?

Recent data from the Van Allen Probes suggests that it is a two-fold process: One mechanism gives the particles an initial boost and then a kind of electromagnetic wave called Whistlers does the final job to kick them up to such intense speeds.

"It is important to understand how this process happens," said Forrest Mozer, a space scientist at the University of California in Berkeley and the first author of the paper on these results that appeared online in Physical Review Letters on July 15, 2014, in conjunction with the July 18 print edition.

"Not only do we think a similar process happens on the sun and around other planets, but these fast particles can damage the electronics in spacecraft and affect astronauts in space."

Over the last few decades, numerous theories about where these extremely energetic particles come from have been developed. They have largely fallen into two different possibilities.

The first theory is that the particles drift in from much further out, some 400,000 miles or more, gathering energy along the way.

The second theory is that some mechanism speeds up particles already inhabiting that area of space. After two years in space, the Van Allen Probes data has largely pointed to the latter.

Additionally, it has been shown that once particles attain reasonably large energies of 100 keV, they are moving at speeds in synch with giant electromagnetic waves that can speed the particles up even more, the same way a well-timed push on a swing can keep it moving higher and higher.

"This paper incorporates the Whistler waves theory previously embraced," said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But it provides a new explanation for how the particles get their initial push of energy."

This first mechanism is based on something called time domain structures, which Mozer and his colleagues have identified previously in the belts.

They are very short duration pulses of electric field that run parallel to the magnetic fields that thread through the radiation belts.

These magnetic field lines guide the movement of all the charged particles in the belts: The particles move along and gyrate around the lines as if they were tracing out the shape of a spring.

During this early phase, the electric pulses push the particles faster forward in the direction parallel to the magnetic fields.

This mechanism can increase the energies somewhat, though not as high as traditionally thought to be needed for the Whistler waves to have any effect.

However, Mozer and his team showed, through both data from the Van Allen Probes and from simulations, that Whistlers can indeed affect particles at these lower energies.

More Information: Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by Nonlinear Whistlers Phys. Rev. Lett. 113, 035001 – Published 14 July 2014 - F. S. Mozer, O. Agapitov, V. Krasnoselskikh, S. Lejosne, G. D. Reeves, and I. Roth

Van Allen Probes: 'Zebra stripe' structure in Earth's inner radiation belt

The Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) is a time-of-flight versus energy spectrometer, the most prominent feature of which is the sensor known as the “puck” (because of its resemblance to a hockey puck). 

RBSPICE measures medium energy protons, electrons, and ions (H+, He+, and O+) as functions of energy and angle, and is capable of measuring the full range of expected ring current energies, intensities, and ion compositions from quiet conditions to extreme events, with a factor of ten margin against intensity saturation. 

Credit: NASA/JHUAPL

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

Most surprisingly, this structure is produced by the slow rotation of Earth, previously considered incapable of affecting the motion of radiation belt particles, which have velocities approaching the speed of light.

Two giant belts of radiation surround Earth. The inner belt is dominated by electrons and the outer one by protons. 

Credit: NASA

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

Scientists had previously believed that increased solar wind activity was the primary force behind any structures in our planet's radiation belts.

These zebra stripes were shown to be visible even during low solar wind activity, which prompted a search for a new physical mechanism of their generation.

That quest led to the surprising discovery that the stripes are caused by rotation of Earth. The findings are reported in the March 20 issue of the journal Nature.

"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) in Laurel, Md., 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."

More information: Nature paper dx.doi.org/10.1038/nature13046

NASA Van Allen Probes observations helping to improve space weather models

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. 

Their observations help improve computer simulations of events in the belts that can affect technology in space. 

Credit: John Hopkins University Applied Physics Laboratory /NASA

Using data from NASA's Van Allen Probes, researchers have tested and improved a model to help forecast what's happening in the radiation environment of near-Earth space, a place seething with fast-moving particles and a space weather system that varies in response to incoming energy and particles from the sun.

When events in the two giant doughnuts of radiation around Earth, called the Van Allen radiation belts, cause the belts to swell and electrons to accelerate to 99 percent the speed of light, nearby satellites can feel the effects.

Scientists ultimately want to be able to predict these changes, which requires understanding of what causes them.

Now, two sets of related research published in the Geophysical Research Letters improve on these goals.

By combining new data from the Van Allen Probes with a high-powered computer model, the new research provides a robust way to simulate events in the Van Allen radiation belts.

Geoff Reeves

"The Van Allen Probes are gathering great measurements, but they can't tell you what is happening everywhere at the same time," said Geoff Reeves, a space scientist at Los Alamos National Laboratory (LANL), in Los Alamos, N.M., a co-author on both of the recent papers.

"We need models to provide a context, to describe the whole system, based on the Van Allen Probe observations."

Prior to the launch of the Van Allen Probes in August 2012, there were no operating spacecraft designed to collect real-time information in the radiation belts.

Understanding of what might be happening in any locale was forced to rely mainly on interpreting historical data, particularly those from the early 1990s gathered by the Combined Release and Radiation Effects Satellite (CRRES).

Imagine if meteorologists wanted to predict the temperature on March 5, 2014, in Washington, D.C. but the only information available was from a handful of measurements made in March over the last seven years up and down the East Coast.

That's not exactly enough information to decide whether or not you need to wear your hat and gloves on any given day in the nation's capital.

Artist's rendition of the Van Allen Probes in orbit. Credit: NASA

Thankfully, we have much more historical information, models that help us predict the weather and, of course, innumerable thermometers in any given city to measure temperature in real time.

The Van Allen Probes are one step toward gathering more information about space weather in the radiation belts, but they do not have the ability to observe events everywhere at once.

So scientists use the data they now have available to build computer simulations that fill in the gaps.

The recent work centers around using Van Allen Probes data to improve a three-dimensional model created by scientists at LANL.

The project was called DREAM3D, the Dynamic Radiation Environment Assimilation Model in 3 Dimensions. Until now the model relied heavily on the averaged data from the CRRES mission.

The Dynamic Radiation Environment Assimilation Model (DREAM) was developed at LANL to understand and to predict hazards from the natural space environment and artificial radiation belts produced by high altitude nuclear explosions.

DREAM was initially developed as a basic research activity to understand and predict the dynamics of the Earth's radiation belts. 

It uses Kalman filter mathematical techniques to assimilate data from space environment instruments with a physics-based model of the radiation belts.

DREAM can assimilate data from a variety of types of instruments and data with various levels of resolution and fidelity by assigning appropriate uncertainties to the observations.

Data from any spacecraft orbit can be assimilated but DREAM was originally designed to work with input from the LANL space environment instruments on geosynchronous and GPS platforms.

With those inputs, DREAM can be used to specify the energetic electron environment at any satellite in the outer electron belt whether space environment data are available in those orbits or not.

Even with very limited data input and relatively simple physics models, DREAM specifies the space environment in the radiation belts to a high level of accuracy.

DREAM is currently being tested and evaluated as we transition from research to operations.

NASA Van Allen Probes: Mysteries of Earth's radiation belts uncovered

The twin Van Allen Probes were launched on August 30, 2012 into elliptical, near-equatorial orbits around the Earth. 

Remarkably, rather than seeing just the well-known two-belt structure, the mission found almost immediate evidence of the clear three-belt structure portrayed in green in this diagram. 

Image courtesy of Andy Kale, University of Alberta.

Just over a year since launch, NASA's Van Allen Probes mission continues to unravel longstanding mysteries of Earth's high-energy radiation belts that encircle our planet and pose hazards to orbiting satellites and astronauts.

Derived from measurements taken by a University of New Hampshire-led instrument on board the twin spacecraft, the latest discovery reveals that the high-energy particles populating the radiation belts can be accelerated to nearly the speed of light in conjunction with ultra-low frequency electromagnetic waves operating on a planetary scale.

This mode of action, as detailed in a paper recently published in the journal Nature Communications, is analogous to that of a cyclical particle accelerator like the Large Hadron Collider (LHC).

However, in this case, the Earth's vast magnetic field, or magnetosphere, which contains the Van Allen belts, revs up drifting electrons to ever-higher speeds as they circle the planet from west to east.

The recent finding comes on the heels of a related discovery—also made by the UNH-led Energetic Particle, Composition, and Thermal Plasma (ECT) instrument suite—showing similar particle acceleration but on a microscopic rather than a planetary scale.

Harlan Spence

"The acceleration we first reported operates on the scale size of an electron's gyromotion—it is a really local process, maybe only a few hundred meters in size," notes Harlan Spence, director of the UNH Institute for the Study of Earth, Oceans, and Space, principal scientist for the ECT, and coauthor on the Nature Communications paper.

"Now we're seeing this large-scale, global motion involving ultra low-frequency waves pulsing through Earth's magnetosphere and operating across vast distances up to hundreds of thousands of kilometers."

And, Spence adds, "in all likelihood both processes are occurring simultaneously to accelerate particles to relativistic speeds."

Understanding the complex dynamics of the particle acceleration will help scientists make better predictions of space weather conditions and, thus, offer better protections to orbiting satellites crucial to modern-day society.

Having twin spacecraft making simultaneous measurements in different regions of nearby space is a key part of the mission as it allows the scientists to look at data separated in both space and time.

"With the Van Allen Probes, I like to think there's no place for these particles to hide because each spacecraft is spinning and 'glimpses' the entire sky with its detector 'eyes', so we're essentially getting a 360-degree view in terms of direction, position, energy, and time," Spence says.

Van Allen Probes: Giant Electric Fields Supercharge Particles In Radiation Belts

Huge electric fields in the radiation belts around Earth may help explain how electrons surrounding the planet can be accelerated to speeds near that of light, researchers have found in a new study.

These findings, detailed Dec. 2 in the journal Physical Review Letters, could help shed light on the radiation belts of planets such as Jupiter, Saturn, Uranus and Neptune, as well as the behaviour of the sun during flares and of bodies beyond the solar system, such as stellar nurseries, neutron stars and incredibly energetic black holes known as quasars.

After humanity began exploring space, the first major find made there were the Van Allen radiation belts, zones of magnetically trapped, highly energetic charged particles discovered in 1958.

The Van Allen Probes aka Radiation Belt Storm Probes (RBSP) Mission, part of NASA's Living With a Star program, will provide unprecedented insight into the physical dynamics of the radiation belts. 

Credit: NASA

These belts generally consist of two rings: 

• an inner zone with both high-energy electrons and very energetic positive ions that remains stable in intensity over the course of years to decades; and
• an outer zone made up mostly of high-energy electrons whose intensity swings over the course of hours to days, primarily depending on the influence from the solar wind, the deluge of radiation streaming from the sun. 

Earlier this year, scientists also detected a third radiation belt temporarily surrounding Earth.

The gigantic amounts of radiation the Van Allen belts generate can pose serious risks for satellites. To learn more about them, NASA launched twin spacecraft, the Van Allen probes, in the summer of 2012.

The satellites are armed with a host of sensors to comprehensively analyze the plasma, energetic particles, magnetic fields and plasma waves in these belts with unprecedented sensitivity and resolution.

Two giant swaths of radiation, known as the Van Allen Belts, surrounding Earth were discovered in 1958. In 2012, observations from the Van Allen Probes showed that a third belt can sometimes appear. 

The radiation is shown here in yellow, with green representing the spaces between the belts. 

Credit: NASA/Van Allen Probes/Goddard Space Flight Center.

Now, using the Van Allen probes, scientists have detected structures that pop in and out of existence in the outer belt that could help explain the high-energy electrons seen in that zone.

The structures in question are known as "double layers." They are each made up of a pair of parallel layers of particles with opposite electrical charge that move along Earth's magnetic field.

The probes saw huge numbers of double layers in the outer belt —7,000 in the course of a minute, each lasting on the order of seconds.

These double layers were discovered "sort of by accident as the Van Allen probes passed through this region of space, and only captured a snapshot," said study lead author Forrest Mozer, a physicist at the University of California, Berkeley.

"The spacecraft will get back to this region maybe eight months, maybe 10 months from now, and we're setting up our instruments to do what we now know they should — to collect data at that site then with a continuous view, to really get definitive information on what is going on there."