A STATISTICAL STUDY OF GIANT PULSATION LATITUDINAL POLARIZATION AND AMPLITUDE VARIATION

Using data from 34 Pg events observed by the European incoherent scatt er (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 pola rization and exclusively clockwise poleward of linear polarization. Th e polarization azimuth (or ellipse orientation) changes from an approx imately northeast to southwest orientation equatorward of linear polar ization to an approximately northwest to southeast orientation polewar d 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 dis play 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 res ults to the observational statistics. We suggest that a mechanism such as drift bounce resonance drives a radially (poloidally) polarized wa ve of limited radial width on L shells where the instability is operat ive. This mechanism favors large azimuthal wave numbers and drives dom inantly Alfvenic waves. Because of their incompressible nature, these waves also possess an azimuthal (toroidal) component with a radial amp litude variation proportional to the radial gradient of the poloidal c omponent. Hence the toroidal amplitude variation will be double peaked with a node at the position of the poloidal peak. This is in good agr eement with the ground-based observations as the toroidal (poloidal) o scillations map to H (D) component oscillations on the ground due to t he 90 degrees rotation of Alfven waves by the ionosphere. Once driven these waves evolve due to the presence of the radial plasma inhomogene ity, 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 p oloidal Alfven waves which are being continually driven by a populatio n of westward drifting energetic protons and also speculate about the evolution of poloidal Alfven waves in the afternoon sector of the magn etosphere.