The longitudinal invariant of a particle trapped in a magnetic mirror, J = ∫ ∫ a b p ∥ ∥ d s , where the integral is between the two turning points, is also an adiabatic invariant. This guarantees, for example, that a particle in the magnetosphere moving around the Earth always returns to the same line of force. The adiabatic condition is violated in transit-time magnetic pumping, where the length of a magnetic mirror is oscillated at the bounce frequency, resulting in net heating.