Physics – Plasma Physics
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997jgr...102.9619c&link_type=abstract
Journal of Geophysical Research, Volume 102, Issue A5, p. 9619-9630
Physics
Plasma Physics
11
Magnetospheric Physics: Auroral Phenomena, Magnetospheric Physics: Energetic Particles, Trapped, Magnetospheric Physics: Mhd Waves And Instabilities, Space Plasma Physics: Wave/Particle Interactions
Scientific paper
Using data from 34 Pg events observed by the European incoherent scatter (EISCAT) magnetometer cross, we complete a statistical study of the horizontal polarization and amplitude variation of Pgs with latitude. The polarization statistics consistently display the same latitudinal variation, being exclusively anticlockwise equatorward of linear polarization and exclusively clockwise poleward of linear polarization. The polarization azimuth (or ellipse orientation) changes from an approximately northeast to southwest orientation equatorward of linear polarization to an approximately northwest to southeast orientation poleward of linear polarization. However, the oscillations are not polarized exactly east-west at linear polarization but some distance poleward of linear polarization. The amplitude statistics for the D component display the distinct latitudinal variation characteristic of a resonance, whilst the H component is less well defined and has a tendency to dip at the position of the D component maximum. We employ an MHD model to describe the evolution of Pg-like waves and compare the numerical results to the observational statistics. We suggest that a mechanism such as drift bounce resonance drives a radially (poloidally) polarized wave of limited radial width on L shells where the instability is operative. This mechanism favors large azimuthal wave numbers and drives dominantly Alfvénic waves. Because of their incompressible nature, these waves also possess an azimuthal (toroidal) component with a radial amplitude variation proportional to the radial gradient of the poloidal component. Hence the toroidal amplitude variation will be double peaked with a node at the position of the poloidal peak. This is in good agreement with the ground-based observations as the toroidal (poloidal) oscillations map to H (D) component oscillations on the ground due to the 90° rotation of Alfvén waves by the ionosphere. Once driven these waves evolve due to the presence of the radial plasma inhomogeneity, which causes the wave polarization to rotate from a poloidal to a toroidal configuration in time. We show how this can explain why the position of the east-west oriented polarization ellipse lies poleward of linear polarization. We conclude that Pgs are likely to be guided poloidal Alfvén waves which are being continually driven by a population of westward drifting energetic protons and also speculate about the evolution of poloidal Alfvén waves in the afternoon sector of the magnetosphere.
Chisham Gareth
Mann Ian R.
Orr Daniel
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