Whistler modes in highly nonuniform magnetic fields. I. Propagation in two-dimensions

Abstract

The propagation of whistler wave packets is studied in a large laboratory plasma with nonuniform ambient magnetic fields. The wave packets are excited by magnetic loop antennas and, for uniform fields, have the topology of helicon modes in unbounded plasmas. The waves propagate across the separatrix of X-type nulls and across circular field lines of magnetic islands. Classical wave refraction is not observed when the magnetic gradient scale length becomes shorter than the wavelength. Since the ambient magnetic field is generated by a line current, it is two dimensional and the wave magnetic field is measured and displayed in transverse planes. Using wave bursts, the space-time evolution of wave packets is observed. The group velocity has been measured. Waves can cross a separatrix and refract into nearly parallel whistler modes. The phase rotation of helicons is lost on a strongly curved field. In highly nonuniform magnetic fields, the phase velocity is determined from multipoint magnetic hodograms, the group velocity from the Poynting vector fields. The energy flow is highly field aligned while the phase flow can be highly oblique to the ambient magnetic field without encountering cyclotron resonance absorption. In the limiting case of circular field lines, counter propagating waves create azimuthal standing waves while still propagating radially. Standard concepts of plane wave refraction do not apply to whistler modes in highly nonuniform magnetic fields. Implications of these observations on research in space plasmas and helicon devices will be pointed out.