Variability of equatorial mesospheric echoes

Abstract

Selected results from recent mesospheric campaigns with the 50-MHz Jicamarca radar in August 1998, December 1999 and January 2000 are reported. The data were acquired with 300 and 450 m rangemresolution and processed with 1 minute time resolution. The focus of this study is on signal-to-noise ratio and its variability on short and long time scales. System changes such as transmitter power were monitored using the signal level near the tropopause. We found that the daily mesopheric signal and its variability increase with solar activity monitored by the sunspot number. Several short-term enhancements of signal power could be linked to solar x-ray flares. Both observations agree with the basic assumption that high electron densities and strong density gradients enhance the illumination of mesospheric turbulence. Daily recurring and descending layers in August is evidence for strong tidal modulation of the dynamic processes that cause the echoes. The good range and time resolution allows to identify many complex structures within the echo layers, including braids and cat's eyes that may be evidence for dynamical instabilities in the mesosphere.