New evidence for the origin of giant pulsations

Abstract

We report on an extended interval of giant pulsation activity in the magnetosphere observed at multiple ground stations and in geosynchronous spacecraft particle data. The pulsations display wave properties typical of previously reported giant pulsations, including reversal of phase across the latitude of maximum amplitude as expected for field line resonances, comparable H (geomagnetic northward) and D (geomagnetic eastward) component amplitudes, and strong localization in latitude to within a few degrees. The recording of pulsation activity at numerous ground sites provides an opportunity to examine the local time dependence of the frequency of giant pulsations. We find that throughout a long-lasting event, the frequencies of giant pulsations are fixed at particular local times but decrease systematically as local time increases between midnight and noon. Geosynchronous proton fluxes between 100 and 150 keV oscillated at the same frequency as pulsations observed on the ground in the same local time sector. Nearly conjugate observations indicate the observed giant pulsations have odd spatial symmetry in the magnetosphere. Calculations of the azimuthal wave number using three Northern Hemisphere stations and one Southern Hemisphere station also suggest the observed giant pulsations have odd symmetry. Numerical calculations of resonant particle energies and the combined ground and spacecraft observations presented support the view that giant pulsations have odd spatial symmetry and are driven by a population of particles in drift resonance. Copyright 2001 by the American Geophysical Union.