Artist's impression of cosmic rays striking Earth (Simon Swordy/University of Chicago, NASA)
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When a moderate-sized M-class flare erupted from the Sun on May 17, it sent out a barrage of high-energy solar particles that belied its initial intensity. These particles traveled at nearly the speed of light, crossing the 93 million miles between the Sun and Earth in a mere 20 minutes and impacting our atmosphere, causing cascades of neutrons to reach the ground — a rare event known as a ground level enhancement, or GLE.
The first such event since 2006, the GLE was recorded by a joint Russian/Italian spacecraft called PAMELA and is an indicator that the peak of solar maximum is on the way.
The PAMELA spacecraft — which stands for Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics — is designed to detect high-energy cosmic rays streaming in from intergalactic space. But on May 17, scientists from NASA’s Goddard Space Flight Center convinced the Russian team in charge of PAMELA to grab data from the solar event occurring much closer to home.
This graph shows the neutrons detected by a neutron detector at the University of Oulu in Finland from May 16 through May 18, 2012. (University of Oulu/NASA's Integrated Space Weather Analysis System)
The result: the first observations from space of the solar particles that trigger the neutron storms that make up a GLE. Scientists hope to use the data to learn more about how GLEs are created, and why the May 17 “moderate” solar flare ended up making one.
“Usually we would expect this kind of ground level enhancement from a giant coronal mass ejection or a big X-class flare,” said Georgia de Nolfo, a space scientist at NASA’s Goddard Space Flight Center. “So not only are we really excited that we were able to observe these particularly high energy particles from space, but we also have a scientific puzzle to solve.”
Fewer than 100 GLEs have been recorded in the last 70 years, with the most powerful having occurred on February 23, 1956. Like most energetic solar outbursts, GLEs can have disruptive effects on sensitive electronics in orbit as well as on the ground, and based on recent studies may even have adverse effects on cellular systems and development.
The M-class flare from AR 1476 on May 17, 2012 (at right) Courtesy NASA/SDO and the AIA science team.
The Fermi Gamma-ray Space Telescope (formerly called GLAST). Credit: NASA
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A couple of years ago, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, PAMELA, sent us back some curious information… an overload of anti-matter in the Milky Way. Why does this member of the cosmic ray spectrum have interesting implications to the scientific community? It could mean the proof needed to confirm the existence of dark matter.
By employing the Fermi Large Area Telescope, researchers with the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University were able to verify the results of PAMELA’s findings. What’s more, by being in the high energy end of the spectrum, these abundances seem to verify current thinking on dark matter behavior and how it might produce positrons.
“There are various theories, but the basic idea is that if a dark matter particle were to meet its anti-particle, both would be annihilated. And that process of annihilation would generate new particles, including positrons.” says Stephan Funk, an assistant professor at Stanford and member of KIPAC. “When the PAMELA experiment looked at the spectrum of positrons, which means sampling positrons across a range of energy levels, it found more than would be expected from already understood astrophysics processes. The reason PAMELA generated such excitement is that it’s at least possible the excess positrons are coming from annihilation of dark matter particles.”
But there has been a glitch in what might have been a smooth solution. Current thinking has the positron signal dropping off when it reaches a specific level – a finding which wasn’t verified and led the researchers to feel the results were inconclusive. But the research just didn’t end there. The team consisting of Funk, Justin Vandenbroucke, a postdoc and Kavli Fellow and avli-supported graduate student Warit Mitthumsiri, came up with some creative solutions. While the Fermi Gamma-ray Space Telescope can’t distinguish between negatively charged electrons and positively charged positrons without a magnet – the group came up with their needs just a few hundred miles away.
Earth’s own magnetic field…
This illustration shows how the electron-positron sky appears to the Large Area Telescope. The purple region contains positrons while electrons are blocked by the Earth's bulk, the orange region contains electrons but is inaccessible to positrons, and the green region is completely out of the Earth's shadow for both positrons and electrons. Image courtesy Justin Vandenbroucke, Fermi-LAT collaboration.That’s right. Our very own planet is capable of bending the paths of these highly charged particles. Now it was time for the research team to start a study on geophysics maps and figure out precisely how the Earth was sifting out the previously detected particles. It was a new way of filtering findings, but could it work?
“The thing that was most fun about this analysis for me is its interdisciplinary nature. We absolutely could not have made the measurement without this detailed map of the Earth’s magnetic field, which was provided by an international team of geophysicists. So to make this measurement, we had to understand the Earth’s magnetic field, which meant poring over work published for entirely different reasons by scientists in another discipline altogether.” said Vandenbroucke. “The big takeaway here is how valuable it is to measure and understand the world around us in as many ways as possible. Once you have this basic scientific knowledge, it’s often surprising how that knowledge can be useful.”
Oddly enough, they still came up with more than the expected amount of antimatter positrons as previously reported in Nature. But again, the findings didn’t show the theoretical drop-off that was to be expected if dark matter were involved. Despite these inconclusive results, it’s still a unique way of looking at difficult studies and making the most of what’s at hand.
“I find it to be fascinating to try to get the most out of an astrophysical instrument and I think we did that with this measurement. It was very satisfying that our approach, novel as it was, seemed to work so well. Also, you really have to go where the science takes you.” says Funk. “Our motivation was to confirm the PAMELA results because they are so exciting and unexpected. And as far as understanding what the Universe is actually trying to tell us here, I think it was important that PAMELA results were confirmed by a completely different instrument and technique.”
PAMELA data show clear deviations from a single power law model between protons and helium nuclei. Credit: Adriani, et. al, Science.
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High energy particles called cosmic rays are constantly bombarding Earth from all directions, and have been thought to come from the blast waves of supernova remnants. But new observations from the PAMELA cosmic ray detector show an unexpected difference in the speeds of protons and helium nuclei, the most abundant components of cosmic rays. The difference is extremely small, but if they were accelerated from the same event, the speeds should be the same.
The PAMELA instrument. Image courtesy of Piergiorgio Picozza
PAMELA, the Payload for Anti-Matter Exploration and light-Nuclei Astrophysics, is on board the Earth-orbiting Russian Resurs-DK1 satellite. It uses a permanent magnet spectrometer along with a variety of specialized detectors to measure the abundance and energy spectra of cosmic rays electrons, positrons, antiprotons and light nuclei over a very large range of energy from 50 MeV to hundreds of GeV.
Just as astronomers use light to view the Universe, scientists use galactic cosmic rays to learn more about the composition and structure of our galaxy, as well as to find out how things like how nuclei can accelerate to nearly the speed of light.
Oscar Adriani and his colleagues using the PAMELA instrument say their new findings are a challenge to our current understanding of how cosmic rays are accelerated and propagated. “We find that the spectral shapes of these two species are different and cannot be well described by a single power law,” the team writes in their paper. “These data challenge the current paradigm of cosmic-ray acceleration in supernova remnants followed by diffusive propagation in the Galaxy.”
Instead, the team concludes, the acceleration and propagation of cosmic rays may be controlled by now unknown and more complex processes.
Supernova remnants are expanding clouds of gas and magnetic fields and can last for thousands of years. Within this cloud, particles are accelerated by bouncing back and forth in the magnetic field of the remnant, and some of the particles gain energy, and eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy as cosmic rays.
One key question that scientists hope to answer with PAMELA data is whether the cosmic rays are continuously accelerated over their entire lifetime, whether the acceleration occurs just once, or if there is any deceleration.
Scientists say that determining the fluxes in the proton and helium nuclei will give information about the early Universe as well as the origin and evolution of material in our galaxy.
Adriani and his team hope to uncover more information with PAMELA to help better understand the origins of cosmic rays. They say possible contributions could be from additional galactic sources, such as pulsars or dark matter.
