Antimatter has received a lot of research attention here on Earth, but satellite data recently confirmed that antimatter particles, specifically antiprotons, are being held in orbit by the Earth’s magnetic field.

Antiprotons, the antimatter version of protons, are formed naturally in interstellar space when cosmic rays collide with other atoms, generating a proton and antimatter partner. (Here on Earth, antiprotons are created in a similar manner: by accelerating protons to near light speeds then colliding them into some static target.) Cosmic rays that slam into the Earth's upper atmosphere can create antiprotons through a similar process. But a second pathway that involves a few more steps produces the majority of these particles in our planet's vicinity.

Small numbers of neutrons (neutrally charged particles) escape the upper atmosphere, where they first decay into protons that are captured by the Earth’s magnetic field. Following collisions with cosmic rays, these protons produce antineutrons (in pairs with neutrons), that then decay into antiprotons. These antiprotons will remain held in orbit until they collide with normal matter and are annihilated; they typically travel distances of several Earth radii before this happens.

Researchers predicted a few years ago that these antiprotons would be held in orbit by the Earth’s magnetic field. Other charged particles (primarily protons and electrons from cosmic rays and the solar wind) are held in the inner and outer Van Allen radiation belts, torus-shaped regions about 100-10,000 km (inner) and 13,000-60,000 km (outer) above the Earth’s surface.

The PAMELA satellite (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), launched in 2006, was used to study an area known as the South Atlantic Anomaly (SAA), where the inner Van Allen belt comes closest to the Earth’s surface. It's normally a region satellites and spacecraft avoid due to the high flux of potentially damaging energetic particles, but it was the target of this study for precisely that reason.

It took over 850 days of data acquisition, but PAMELA spotted 28 trapped antiprotons, in the kinetic energy range 60-750 MeV (mega-electron volts). This may seem like a small amount, but this only represents the few that were reliably detected. With all these data, the researchers determined that the flux of antiprotons in this region is three order of magnitude higher than in interstellar space. The actual antiproton-to-proton ratio in the SAA region is about an order of magnitude higher than the galactic ratio, and well matches predictions made in 2007.

This is the first time antiprotons have been discovered trapped by the Earth’s magnetic field. Even more impressively, they confirm past theoretical predictions of their existence. It isn’t clear if there are any practical applications for this discovery (unless we can somehow mine this source of antimatter), but hopefully these results will help further our understanding of antimatter in the Universe.

Astrophysical Journal Letters, 2011. DOI: 10.1088/2041-8205/737/2/L29 (About DOIs)