VEGA Balloon meteorological measurements

Abstract

The VEGA Balloons obtained in-situ measurements of pressure, temperature, vertical winds, cloud density, ambient illumination, and the frequency of lightning during their 48 hour flights in the Venus middle cloud layer (50 to 55 km altitude). In addition, the VLBI tracking experiment provided measurements of balloon positions and horizontal winds along their trajectories. We have used these measurements to develop a comprehensive description of the meteorology of the Venus middle cloud layer. The static stability is usually positive, with values ranging from 0 to 2.0 K/km. There is a 6.5 K offset between the VEGA-1 and VEGA-2 temperature profiles. This large horizontal temperature gradient is probably associated with an east-west temperature disturbance that drifts with the prevailing winds. Vertical winds are large (1–3 m/s) and variable, with turbulent episodes lasting about one hour. This turbulence is associated with upward heat fluxes that range from 0 to 350 W/m 2. Cloud density decreases with altitude. No completely cloud-free regions were observed. No lightning was detected. VLBI tracking results indicate zonal wind speeds of 69.4 and 66.0 m/s for VEGA-1 and VEGA-2, respectively. VEGA-1 observed little meridional transport, but VEGA-2 measured 2.5 m s −1 northward winds, which pushed it almost 500 km toward the equator during its flight.