F-REGION GRAVITY-WAVES IN THE CENTRAL POLAR-CAP

This study identifies polar cap gravity waves from the perturbations t hat they produce in ionospheric vertical velocity. In the December 199 4 to February 1995 interval, there were 15 wave groups that were defin ed well enough for us to study their properties. Most of the propertie s of these waves are similar to the gravity waves that were measured b y the DE 2 satellite [Johnson et al., 1995]. Our waves had typical gra vity wave speeds (75-225 m s(-1)) and produced vertical velocity fluct uations > 20 m s(-1) The typical wave period was 30 min. Source locati ons were mostly west of Eureka at distances ranging from 300 to > 1500 km. Source times and wave propagation directions were such that the g ravity waves would not have been excluded by critical speed effects. M ost of the wave groups showed large spatial resonance effects, the ave rage vertical ionospheric displacement being 61 km pp. Spatial resonan ce occurs when the plasma convection velocity matches the wave speed. Our polar cap gravity waves were also associated with relatively large perturbations in ionospheric convective velocity and magnetic perturb ations having typical magnitudes of 300 m s(-1) and 20 nT (both peak t o peak), respectively. The magnetic perturbations appeared to be due t o E region currents driven by the electric fields associated with the convection perturbations.