A nonuniform magnetic field can be used to trap a charged particle in a region of space. Figure shows a schematic view of the operation of such a magnetic mirror. The charged particles tend to move in circles about the field direction. Suppose they also are drifting laterally, say, to the right in given figure. The motion is therefore that of a helix, like a coiled spring. The field increases near the ends of the “magnetic bottle,” and the force has a small component pointing toward the centre of the region, which reverse the direction of the motion of the particles and causes them to spiral in the opposite direction, until they are eventually reflected from the opposite end. The particles continue to travel back and forth, confined to the space between the two high-field regions. Such an arrangement is used to confine the hot ionized gases (called plasmas) that are used in research into controlled thermonuclear fusion.  

Figure A:- A charged particle spiraling in a nonuniform magnetic field.The field is reater at the left and right ends of the region than it is at the centre, particles can be trapped, spiraling back and forth between the strong field gegions at the ends.
Note that the magnetic force vectors at each end of this "magnetic bottle" have components that point toward the centre, it is these force components that serve to confine the particles

Figure:- the Earth's mangetic field, showing protons and electron trapped in the Vain Allen radiation belts.
 








A similar phenomenon occurs in the Earth’s magnetic field, as shown in figure 2 electrons and protons are trapped in different regions of the Earth’s field and spiral back and forth between the high field regions near the poles in a time of a few seconds. These fast particles are responsible for the so called Van Allen radiation belts that surround the Earth.