VHF ECHOES FROM THE HIGH-LATITUDE MESOSPHERE AND LOWER THERMOSPHERE: OBSERVATIONS AND INTERPRETATIONS.

Abstract

Recently published data describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow ( plus or minus 2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km plus or minus 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.