Dynamic properties of the thermosphere inferred from Pioneer Venus Mass Spectrometer measurements

In situ measurements of neutral thermospheric composition derived from orbiter neutral mass spectrometer (ONMS) and ion mass spectrometer (OIMS) experiments on Pioneer Venus are interpreted. Observed day to night density variations with asymmetries between dawn and dusk contain pronounced signatures of various transport processes due to winds, exospheric flow, and vertical diffusion. The relative magnitudes of these processes depend significantly on the rotation rate of the thermosphere and its turbulent properties. On the basis of a theoretical threedimensional multiconstituent model describing solar diurnal tides in a rotating atmosphere, the analysis leads to the following conclusions:(1) The day-night temperature contrast on Venus is associated with wind velocities of about 200 m/s (for the lowest order harmonic) which transport 0, He, and H toward the nightside. (2) Mass exchange with the mesosphere, commensurate with an eddy diffusion coefficient of K = 3 x 10(7) is required to buffer that horizontal advection such as to reproduce the observed daytime bulge in 0 as well as the comparatively small diurnal variations in He. (3) Exospheric flow significantly affects the H distribution. (4) The observed time response and magnitude of the day-night density variations require that transport processes are only effective over time periods between 5 to 10 days, which, in comparison with the 243-day rotation rate of the solid body, implies a super rotation rate or prevailing winds in excess of 50 m/s at the equator. (5) Nonlinear mass transport results in wave steepening and contributes to amplify the density extrema in H and He; owing to the long time constant for He transport, higher order tidal modes overtake the fundamental harmonic and shift the density maximum toward dawn.