Megan's Blog

In the news this month: discovery of antimatter in the Earth's Van Allen belts

Antimatter is often thought of as something that is only created in particle accelerators (or that only exists in science fiction movies), but it is actually present in small quantities throughout the universe.  Now, a team of researchers have detected the presence of naturally occurring antimatter right here in the neighbourhood of the Earth.  This population of antiparticles originates from cosmic ray interactions in the Earth's upper atmosphere where they are subsequently trapped in the planet's magnetosphere.

Anti-protons can be produced in a number of ways, through cosmic rays interacting with the interstellar medium, the natural decay processes of some types of particles from our own atmosphere, or in cosmic ray air showers from high energy particles impacting on the atmosphere, although most of the antiparticles would annihilate with their normal counterparts fairly quickly, especially at lower altitudes where the density of the atmosphere is higher.

The existence of anti-protons around the Earth was predicted many years ago, but predictions differ, and experiments on board both Salyut-7 and the Mir Space Station only succeeded in placing upper limits on their abundance.  The anti-protons found by the PAMELA satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) are located in the Earth's Van Allen belts, doughnut-shaped regions defined by the magnetic field of the Earth.  The magnetic fields trap charged particles, resulting in regions with a relatively high density of positively charged protons, and others with a high density of anti-protons.

Launched from the Baikonur Cosmodrome in 2006, PAMELA is designed to detect cosmic particles with energies between tens of mega electron Volts and hundreds of giga electron Volts. PAMELA's orbit takes it through the area known as the South Atlantic Anomaly, the region where the Van Allen belts pass closest to the Earth's surface.  Since it began operations in 2006, PAMELA has detected anti-protons at a rate more than 1000 times higher than that expected from Galactic sources.  The researchers say that this implies a belt of anti-protons located between two belts of ordinary matter in the Earth's Van Allen belts.

The signal detected by PAMELA is ten thousand times times stronger inside the South Atlantic Anomaly than it is outside the Earth's radiation belts, and thousands of times stronger than that expected from Galactic cosmic rays.  The likely explanation, say the researchers, is that the Earth's Van Allen belts are acting in the same way as they trap protons, trapping the anti-protons in a layer around the Earth (at least until they encounter a particle of normal matter and annihilate).

Although antimatter is pretty destructive stuff if it comes into contact with ordinary matter, luckily for orbiting spacecraft there isn't that much of it.  In 850 days of data acquisition, PAMELA's detectors collected just 28 anti-protons in the previously unknown antimatter region of the inner Van Allen belts.

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This blog post is a news story from the Jodcast, aired in the September 2011 edition.

O. Adriani, G. C. Barbarino, G. A. Bazilevskaya, R. Bellotti, M. Boezio, E. A. Bogomolov, M. Bongi, V. Bonvicini, S. Borisov, S. Bottai, A. Bruno, F. Cafagna, D. Campana, R. Carbone, P. Carlson, M. Casolino, G. Castellini, L. Consiglio, M. P. De Pascale, C. De Santis, N. De Simone, V. Di Felice, A. M. Galper, W. Gillard, L. Grishantseva, G. Jerse, A. V. Karelin, M. D. Kheymits, S. V. Koldashov, S. Y. Krutkov, A. N. Kvashnin, A. Leonov, V. Malakhov, L. Marcelli, A. G. Mayorov, W. Menn, V. V. Mikhailov, E. Mocchiutti, A. Monaco, N. Mori, N. Nikonov, G. Osteria, F. Palma, P. Papini, M. Pearce, P. Picozza, C. Pizzolotto, M. Ricci, S. B. Ricciarini, L. Rossetto, R. Sarkar, M. Simon, R. Sparvoli, P. Spillantini, Y. I. Stozhkov, A. Vacchi, E. Vannuccini, G. Vasilyev, S. A. Voronov, Y. T. Yurkin, J. Wu, G. Zampa, N. Zampa, & V. G. Zverev (2011). The discovery of geomagnetically trapped cosmic ray antiprotons ApJ, 737, L29, 2011 arXiv: 1107.4882v1