Van Allen belt

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two toroidal zones containing high-energy ionized particles trapped in Earth's magnetic field and surrounding the planet, beginning at an altitude of about 800 kilometers (500 miles) and extending tens of thousands of kilometers into space.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Either of two torus-shaped areas of high-energy charged particles which partly surround Earth, trapped by its magnetic field. The areas are characterised by intense radiation. When the belts "overload", particles strike the upper atmosphere and fluoresce, causing the polar aurora.
• noun Any of similar areas around another planet.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a belt of charged particles (resulting from cosmic rays) above the Earth trapped by the Earth's magnetic field