Observations of Scattered Ion Orbits in a Dipole Magnetic Field and Their Collective Modes

Ion orbits in a dipole magnetic field are known to exhibit chaotic behavior [1,2]. Measurements of f_0,i (v) in a low-temperature Ar plasma have been shown to agree well with a simple Monte Carlo (MC) simulation of the unperturbed ion orbits [3]. Knowledge of f_0,i (v) and methods of transient chaos are used here to predict transport and electrostatic modes associated with ion orbits. First, density measurements are made in tandem with plasma source modulation to accentuate the effect of predicted “sticky” (long-lived) ion orbits. A second experiment uses an antenna to produce perturbations in the ion acoustic range of frequencies, synchronized with Laser Induced Fluorescence (LIF) measurements, to obtain f_1,i (v) profiles at multiple spatial positions and frequencies. The measured {f_1,i (v)} are then fed into an MC simulation which uses integration along unperturbed orbits to estimate perturbed electrostatic mode amplitudes over a large spatial extent.

[1] Dragt, Alex J., and John M. Finn. "Insolubility of trapped particle motion in a magnetic dipole field." Journal of Geophysical Research 81, no. 13 (1976): 2327-2340.

[2] Jung, Christof, and H-J. Scholz. "Chaotic scattering off the magnetic dipole." Journal of Physics A: Mathematical and General 21, no. 10 (1988): 2301.

[3] McLaughlin, Jacob W., Daniel V. Pette, and Fred N. Skiff. "Spatially resolved measurements of plasma ion velocity distributions in a dipole magnetic field." Physics of Plasmas 32, no. 3 (2025).