Planet Earth has an invisible and impenetrable shield about 7,200 miles out in space that blocks killer electrons from coming in and zapping satellites and causing all sorts of other havoc in the wake of huge coronal mass ejections from the Sun. This barrier is a major discovery of the Radiation Belt Storm Probe and the Relativistic Electron Proton Telescope (REPT), launched by NASA in August of 2012.
Dr. Daniel Baker, REPT science lead at the Laboratory for Atmospheric and Space Physics at the University of Colorado, spoke about the mission and the discoveries today during his opening lecture at the 225th meeting of the American Astronomical Association in Seattle. The discoveries were detailed in an article in Nature back in November.
Baker has some great credentials for the project. He earned his Ph.D. under James Van Allen, whose gear first detected the Van Allen Belts—he preferred to call them zones—in 1957, in what many consider to be the first major scientific discovery of the space age. Baker also was an investigator on SAMPEX, a particle exploring mission that operated from 1992 until 2004.
There are twin REPT probes in a highly elliptical orbit around the Sun. One of the first major discoveries by REPT, according to Baker, was of a third Van Allen Belt, a storage belt of ultra-relativistic particles that remains constant while the other belts vary wildly because of solar events.
Interestingly enough, these particles don’t ever get in any closer than about 2.8 Earth radii. Baker says that was a strange discovery, as nature typically doesn’t like such sharp barriers, but observations so far have shown it to be an impenetrable.
“It looks, at least for the period of time, the couple of years, since the Van Allen probe launched that particles—ultrarelativistic electrons—can get in so far, they run into something almost like a glass wall, and can’t really get any further. This really was quite an interesting and fascinating puzzle.”
They looked into whether the phenomenon might be related to various actions of the Earth’s magnetic field, or even perhaps a reaction to radio waves broadcast from Earth, but those explanations fell short.
“We were left with the unsatisfying situation that very slow pitch-angle scattering and even slower radial diffusion can conspire to create this sharp gradient in the particle distributions,” Baker said. “To me, that’s not very satisfactory, but it seems to be the explanation.”
“I think its a subject now that our theoretical friends are struggling with and trying to understand and explain,” he added.
Baker says this new information is vitally important for those who may be studying x-rays from the Sun, synchrotron emissions from Jupiter, radio and x-ray emissions from distant nebulae, or extra-gallactic jets.
“All of these are visible because of energetic particles, electrons primarily, moving in strong magnetic fields,” he said. “Examining the details of how the accelerator that is so accessible to us in our own cosmic backyard can really give us much useful information about how acceleration processes work in these more removed systems.”
Baker says it is an exciting time in the field, with a great many instruments and missions collecting data.
“We have quite a golden age, in a sense, of measuring the properties of this entire magnetospheric system,” he said. “When we combine this information that we’re gathering now with the wonderful measurements of the Sun and the driving factors from the Sun, we really have the opportunity to make immense progress and to address the key questions that Van Allen and co-workers uncovered nearly 60 years ago.”
“The results from the Van Allen probes mission have in a real sense been rewriting the textbooks on many aspects of structure, acceleration, transport, and loss,” Baker concluded. “They’re giving us previously undreamt of capabilities.”