F region gravity waves in the central polar cap

This study identifies polar cap gravity waves from the perturbations that they produce in ionospheric vertical velocity. In the December 1994 to February 1995 interval, there were 15 wave groups that were defined well enough for us to study their properties. Most of the properties of these waves are similar to the gravity waves that were measured by the DE 2 satellite [Johnson et al., 1995]. Our waves had typical gravity wave speeds (75-225 m s -1 ) and produced vertical velocity fluctuations > 20 m s -1 . The typical wave period was 30 min. Source locations were mostly west of Eureka at distances ranging from 300 to > 1500 km. Source times and wave propagation directions were such that the gravity waves would not have been excluded by critical speed effects. Most of the wave groups showed large spatial resonance effects, the average vertical ionospheric displacement being 61 km pp. Spatial resonance 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 perturbations having typical magnitudes of 300 m s -1 and 20 nT (both peak to peak), respectively. The magnetic perturbations appeared to be due to E region currents driven by the electric fields associated with the convection perturbations.