Description
We measure the 1-d ion velocity distribution and fluctuations associated with single-frequency ion acoustic wave scattering from a magnetic dipole immersed in an otherwise uniform singly-ionized Argon plasma that is weakly-collisional. The data are projected onto the Case-Van Kampen (CVK) spectrum for the ion acoustic wave using an integral transform. We call this transform a modified Morrison transform after the integral transform introduced to solve the initial value problem for electron plasma waves using projection onto the corresponding CVK spectrum. Here we address the boundary-value problem of ion acoustic scattering. Previous experiments on ion acoustic wave scattering from a sheath show very low levels of scattering because the plasma flow at the Bohm velocity makes a sheath somewhat like a black-hole. For an incident wave in the equatorial plane, an ion acoustic wave should mode-transform into an electrostatic ion cyclotron wave and then reflect from the cutoff where the wave frequency equals the ion cyclotron frequency. Because of the complexity of ion orbits near the magnetic dipole, which is an example of chaotic scattering, the wave scattering event should require a kinetic description.