Charged particles moving in the Earth's magnetic field travel in spiral paths around the geomagnetic field lines. Their helical trajectories result from the fact that their motion is both parallel and perpendicular to the magnetic field, which exerts a force on the perpendicular component of their velocity that causes them to move around the field lines ("cyclotron" motion) as well as along them. The angle between the direction of the magnetic field and a particle's spiral trajectory is referred to as the "pitch angle," which in a non-uniform magnetic field changes as the ratio between the perpendicular and parallel components of the particle velocity changes. Pitch angle is important because it is a key factor in determining whether a charged particle will be lost to the Earth's atmosphere or not.
As particles spiraling along geomagnetic field lines get closer to the Earth, the strength of the magnetic field increases, which causes the parallel component of the particles' velocity to decrease (with a corresponding increase in the perpendicular component). As the parallel component goes to zero, the pitch angle increases to 90 degrees. If this happens at an altitude where the atmosphere is sufficiently tenuous (above ~100 km) that the particles are unlikely to interact with atmospheric neutrals and ions, they reverse direction and travel back up the field lines. They continue spiraling along the field lines until they reach a point in the opposite hemisphere where the magnetic field strength is sufficient to cause them to reverse direction again. The points at which the pitch angle goes to 90 degrees and the particles reverse direction are known as "mirror points." Charged particles that are reflected back and forth along geomagnetic field lines between mirror points--such as those that constitute the ring current and the radiation belts--are considered "trapped" and their repeated reflection between mirror points is known as their "bounce" motion.
Not all charged particles gyrating along geomagnetic field lines become or remain trapped, however. If the mirror point occurs at an altitude where the atmosphere is dense enough for a charged particle to collide with atmospheric particles (that is, below ~100 km), the particle will soon be absorbed by the atmosphere instead of being continuously reflected by the magnetic mirror force. Particles lost in this way must have pitch angles (in the equatorial plane, where the magnetic field is weakest) that fall within a solid angle known as the atmospheric "loss cone." The size of the loss cone varies with the radial distance of the field line from the Earth: the greater the distance, the smaller the angle of the loss cone. At L = 8, for example, the loss cone angle is about 2 degrees. Thus charged particles with equatorial pitch angles of 2 degrees or less will, after a few bounces, be lost to the atmosphere as result of collisions with atmospheric neutrals and ions. Those with pitch angles greater than the loss cone angle will continue to bounce between mirror points until an interaction with a plasma wave reduces their pitch angle and "scatters" them into the loss cone (or they are lost as a result of charge exchange with geocoronal atomic hydrogenx). "Pitch angle scattering" (also known as "pitch angle diffusion") is one of the two main processes by which magnetospheric charged particles (both ions and electrons) are lost to the atmosphere. Charge exchange--which affects only ions--is the other principal loss process.