This is a common measurement and topic when studying the magnetosphere, magnetic mirrors, biconic cusps and polywells.
See Aurora and Ring current It is customary to discuss the direction a particle is heading by its pitch angle.
A pitch angle of 0 degrees is a particle whose parallel motion is perfectly along the local magnetic field.
The loss cone is the set of angles where the particle will strike the atmosphere and no longer be trapped in the magnetosphere while particles with pitch angles outside the loss cone will mirror and continue to be trapped.
At the point of reflection, the particle has zero parallel velocity or a pitch angle of 90 degrees.