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The rotational transform associated with the magnetic surfaces of a toroidal magnetic field with a nonplanar axis is an example of the angle anholonomy which recently has been much discussed in quantum and classical dynamics (the Berry phase and Hannay angle). The same anholonomic angle appears in the phase of a charged particle spiraling around its guiding center in a strong magnetic field. This accounts for a contribution to the longitudinal invariant, associated with the guiding-center motion, which is different for guiding-center orbits that circulate in opposite directions and is absent for orbits that are reflected between mirrors.
Bhattacharjee et al. (Tue,) studied this question.