On Magnetic Compression in Gyrokinetic Field Theory

ABSTRACT The role of magnetic compression in gyrokinetic theory is addressed. The steps are the infinitesimal transform of the Lagrangian for electrodynamics, the field equations and the gyrocentre drifts from the functional derivatives of the resulting gyrokinetic Lagrangian. The consistency of energy and momentum follows from the canonical form. Then the low‐beta compressional cancellation of the change to the magnetic field strength in a self‐consistent equilibrium is shown. This generalises a much earlier result from linear theory. The main point herein is to couch it within gyrokinetic field theory so that it remains valid under full self‐consistent nonlinearity.