The orbiting PAMELA charged-particle spectrometer has revealed a significant population of antiprotons magnetically trapped in Earth’s inner Van Allen Belt. That trapped population, attributed to the decay of antineutrons produced in collisions of high-energy cosmic rays in the atmosphere below, had been predicted and looked for, but never found. Of course, cosmic-ray collisions in the atmosphere also produce antiprotons directly. But they are much less likely to escape annihilation in the atmosphere and reach the relative vacuum of the Van Allen Belt. Like a free neutron, an antineutron beta-decays with a half-life of 10 minutes—plenty of time to climb out of the atmosphere. The trapped decay antiprotons, with kinetic energies up to a few GeV, spiral around geomagnetic flux lines and drift along them, bouncing back toward equatorial latitudes from mirror points where flux lines converge toward Earth’s magnetic poles. The flux of trapped antiprotons is a thousand times the flux of direct cosmic-ray antiprotons from beyond Earth’s magnetosphere. That makes the trapped antiprotons the largest accessible concentration of antimatter. But its estimated total mass over the entire magnetosphere is probably less than a microgram. With a maximum orbital altitude of 600 km, PAMELA can access the trapped antiprotons only when it’s traversing the South Atlantic Anomaly, indicated in the figure, where the inner Van Allen Belt comes closest to Earth’s surface. (O. Adriani et al., PAMELA collaboration, Astrophys. J. Lett. 737, L29, 2011.)—Bertram Schwarzschild
