First adiabatic invariant ($\mu =v_{\perp }^2/(2B)$)

The magnetic-moment invariant of rapid cyclotron motion in slowly varying fields. Here the per-unit-mass convention is used: the physical magnetic moment is $m\mu$. In drift kinetics, using $(\mu ,v_{\parallel })$ as coordinates removes the leading perpendicular acceleration and leaves a velocity-space measure proportional to the exact magnetic field strength.