Velocity-space measure
= Velocity-space measure
{title2=$d^3v=2\pi B\,d\mu\,dv_\parallel$}
For a gyrophase-independent <phase-space distribution function>, $d^3v=2\pi v_\perp\,dv_\perp\,dv_\parallel$. With <first adiabatic invariant> $\mu=v_\perp^2/(2B)$, this gives the measure in the title. When the <magnetic field> changes, both the measure and any explicit factor $\mu B$ in a <moment> must be perturbed.