What causes the magnetic curvature drift?

TL;DR

This paper clarifies the physical origin of magnetic curvature drift, emphasizing the role of Lorentz force and particle motion dynamics, and offers a unified explanation for guiding-center motions.

Contribution

It presents a Newton's second law-based explanation for magnetic curvature drift, unifying it with mirror reflection and gradient-B drift in static nonuniform magnetic fields.

Findings

Provides a clear, physics-based explanation of curvature drift.

Unifies understanding of guiding-center motions in magnetic fields.

Aims to improve teaching of plasma physics concepts.

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.