What causes the magnetic curvature drift?

Abstract

When asked what causes the magnetic curvature drift of a charged-particle moving in a curving magnetic field, people respond that there is an `F-cross-B' motion of the `guiding center' due to the centrifugal force on the particle as it follows the magnetic field line. This and similar explanations `beg the question' by assuming that the particle follows the field line. In a curving magnetic field, however, a particle moving parallel to the field direction soon won't be. The convective rotation of the field along the particle trajectory ensures that the Lorentz force switches on, and the resulting acceleration rotates the velocity vector back into alignment periodically. The gyration is not symmetric about the field vector, and the resulting velocity offset is the curvature drift. This explanation is guided by Newton's second law of motion in vector notation. It provides a common framework for explaining the three guiding-center motions of a charged particle in a static nonuniform magnetic field: curvature drift, mirror reflection in a magnetic bottle, and gradient-B drift. The discussion aims to provide insight to instructors of electricity and magnetism or plasma physics at the intermediate- to advanced-undergraduate level.