Determining the radial structure of high-m Alfvén wave by means of the “phase portrait” method

Abstract

A study is carried out of the radial (across the magnetic shells) structure of quasi-monochromatic high-m Alfvén oscillations observed by the RBSP-A satellite on October 23, 2012 at 22.00–22.30 UT. To determine the radial structure of these oscillations, the “phase portraits” method is used, based on the analysis of the phase shift between transverse components of the magnetic field of oscillations. It is shown that the observed oscillations can be explained by poloidal Alfvén waves generated on two resonance surfaces located on both sides of the local maximum in the radial distribution of the Alfvén speed. In the direction along the geomagnetic field lines, these waves are the fundamental harmonic of standing Alfvén waves. In the radial direction, they are waves running in opposite directions from two poloidal resonance surfaces towards two other (toroidal) resonance surfaces. The polarization of the waves between these resonance surfaces changes from poloidal to toroidal.