The Earth is constantly bombarded by high energy particles called cosmic rays. These are generated by the Sun and by other sources further afield. (The source of the highest energy cosmic rays is still a mystery).

The particles are generally protons, electrons and helium nuclei and when they collide with nuclei in the Earth’s upper atmosphere they can produce showers of daughter particles. These showers can be so extensive that they can easily be observed from the ground.

Astronomers long ago realised that these collisions must produce antiprotons, just as they do in particle accelerators on Earth. But this raises an interesting question: what happens to the antiprotons after they are created?

Clearly, many of these antiparticles must be annihilated when they meet particles of ordinary matter. But some astronomers always suspected that the remaining antiprotons must become trapped by the Earth’s magnetic field, forming an antiproton radiation belt.

Now astrophysicists say they’ve finally discovered this long-fabled belt of antiprotons.

In 2006, these guys launched a spacecraft called PAMELA into low Earth orbit, specifically to look for antiprotons in cosmic rays.

But, like most spacecraft in low Earth orbit, PAMELA must pass daily through the South Atlantic Anomaly, a region where the Van Allen Radiation Belts come closest to the Earth’s surface. It’s here that energetic particles tend to become trapped. So if any antiprotons are caught up in the mix, that’s where PAMELA ought to find them.

Now the PAMELA team has analysed the 850 days of data, looking only at the times when the spacecraft was in the South Atlantic Anomaly (about 1.7 per cent of this time).

Lo and behold, these guys found 28 antiprotons. That’s about three orders of magnitude more than you’d expect to find in the solar wind, proving that the particles really are trapped and stored in this belt.

This constitutes “the most abundant source of antiprotons near the Earth”, say the PAMELA team.

The South Atlantic Anomaly is well known as a thorough nuisance. Because of the high energy particles here, the Hubble Space Telescope must be switched off when it passes through several times a day; and the International Space Station has extra shielding to protect astronauts from its effects.

The discovery of an additional belt of antiprotons won’t have much impact on the danger it represents–the number of antiprotons is tiny compared to the electrons and protons trapped there.

But it’s always interesting to have theoretical predictions confirmed. That’s good science at work.

Ref: arxiv.org/abs/1107.4882: The Discovery Of Geomagnetically Trapped Cosmic Ray Antiprotons