The Van Allen Belt is a layer of charged particles that surround the earth. The particles are trapped by the earth's magnetic field along lines that run from the north magnetic pole to the south magnetic pole. The charged particles are emitted by the sun's solar corona. The corona is the upper atmosphere of the sun and is so hot that it not only creates and emits x-rays but it drives off electrons from the helium and other atoms. These electrons are highly charged particles and are emitted in all directions by the solar corona. The charged particles thus emitted are referred to as the solar wind..
These charged particles in the earth's neighborhood were first detected by rockets sent out in 1958. The rockets were sent out by a team of physicists under the direction of the American physicist, James Van Allen. The were originally referred to as Van Allen Belts but now its called the magnetosphere. The charged particles are drawn into the earth's atmosphere at the magnetic poles and interact with the molecules there to produce and array of colored lights that we know as the aurora borealis or norther lights. In the antarctic the phenomena is known as the aurora australis or southern lights.
The solar corona has no sharp outer boundary, but continues to spread out through the entire solar system. It gets thinner and thinner as it spreads and has no noticeable effect on the movements of the planets nor has it had any effect on any objects of space exploration.
The corona is only about a millionth as bright at the sun itslelf so may not be seen unless the body of the sun is obsucrred by the moon during an eclipse. At first, it was not known if the corona was from the moon. It was only after a device called the coronagraph was invented that it was determined to be of the sun. It was in the spectral lines of the corona that scientists discovered the element helium. Other special lines showed a different reading for the elements nickel, calcium and iron which was later discovered to be the elements with electrons missing and resulted in the discovlery of the charged particles as the missing electrons. It is interesting to note that helium is the only element that was discovered in a heavenly body before it was discovered on earth