Transversely localized shear Alfven waves in resonance cones

Abstract

Using fully electromagnetic 2.5-D particle-in-cell (2DPIC) simulations, we study the scattering of shear Alfven waves (SAWs) having long perpendicular wavelengths from multiple localized density cavities. We show that the scattered fields are radiated in conical wave structures, which are the inertial Alfven wave (IAW) cones with apexes at the cavities and half-cone angle approximately determined by the cold plasma theory. The waves within the cones obey the basic dispersion relation of IAWs. The scattered fields have relatively short transverse length scale lengths and are confined within the cones, which are embedded in the large-scale incident SAW. The resulting wave structures and their Fourier spectral behaviour found in the simulations are compared with observations on transversely localized IAWs from Freja, FAST and Polar. We highlight the fine structures in the parallel electric fields within the cones and in the associated parallel and transverse currents. The current closure consisting of the parallel and transverse currents forming loops inside the cone is seen. We demonstrate parallel acceleration of the electrons forming energetic tails and bulk transverse heating of ions in the scattered fields. The scattering mechanism discussed here provides a linear mechanism for converting large-scale SAW into the solitary structures of small-scale IAWs.