Spatio-temporal structure of a poloidal Alfvén wave detected by Cluster adjacent to the dayside plasmapause

Abstract

A case study of a poloidal ULF pulsation near the dayside plasmapause is presented based on Cluster observations of magnetic and electric fields. The pulsation is detected close to the magnetic equatorial plane at L shells L=[4.4, 4.6] and oscillates with a frequency of f=23 mHz. Investigating the wave energy flux reveals the standing wave nature of the observed pulsation. An estimation of the azimuthal wave number exposes a narrow azimuthal structure of the wave field with m≈160. Spatial and temporal characteristics of the pulsation are analyzed in detail by representing data in a field line related coordinate system and a range-time-intensity representation. This allows an estimation of both the spatial extension of the wave field in the radial direction and its temporal decay rate. The analysis furthermore indicates that the same field lines are excited to a standing wave oscillation twice. Furthermore an accurate identification of a phase jump of the wave field across L shells is possible. Comparing the radial localization of the detected wave with theoretically expected field line eigenfrequencies reveals that the wave field is confined in the Alfvén resonator at the outer edge of the plasmapause.