The Earth has a strong electric current at its core, which produces an intense magnetic field. This magnetic field holds a magnetosphere in place which contains the Van Allen belts. The magnetosphere consists of regions (belts) surrounding the Earth where charged particles are trapped by the magnetic field. The Earth's magnetic field and the magnetosphere it produces protect us from dangerous radiation from solar flares.
Charged naturally become trapped in a magnetic field; however the density of the Earth's magnetosphere is the highest among the planets of the solar system. In fact the only other rocky planet with a magnetic field is Mercury with field strength 100 times smaller than the Earth's.
The strength of Earth's magnetic field is decreasing and since the magnetosphere is generated by the magnetic field, it is likely decreasing also, although this has not been directly confirmed. If Earth's magnetic field were to decay completely, the magnetosphere would also disappear since there would be nothing to confine the charged particles. Now the planetary dynamo model says that the field will reverse and rebuild but it has to go through a period of collapse before it can reverse.