Measurements of Ion Dynamics and Acoustic Wave Reflections in Dipole Magnetic Fields

Kinetic plasma waves are well understood in environments with straight or helical unperturbed particle orbits. Many environments, such as astrophysical plasmas, have strongly non-uniform fields, and the unperturbed orbits may be chaotic. Dipole magnetic fields provide a relatively simple magnetic field geometry with more general particle orbits, which range from periodic to hyper-chaotic motion. We employ measurements of wave reflections from dipole-confined plasma, together with direct measurements of the perturbed ion-velocity distribution function, to explore plasma waves in environments with complex orbits. Conventional laser-induced fluorescence (LIF) techniques provide equilibrium Ion Velocity Distribution Functions (IVDFs) which characterize ion dynamics, while the measurement of the coherent plasma ion response to an applied perturbation provides insight into wave behavior which is also measured using electric probes. The incident wave begins as an Ion Acoustic Wave (IAWs) in a plasma with low magnetic field and mode-transforms into an electrostatic ion cyclotron wave as it approaches a dipole magnet in the equatorial plane. Depending on wave frequency, there may be spatial layers with trapped ion Bernstein waves. We present Ar-II IVDFs in the equatorial plane of a dipole magnet in three dimensions, using the commonly employed 668 nm scheme and a novel 394 nm scheme. We study IAW reflections from a dipole magnet as a function of radial distance and wave frequency.