Time delays in the solar wind flow past Venus: Galileo‐Pioneer Venus correlations

The Galileo and Pioneer Venus Orbiter (PVO) spacecraft obtained simultaneous measurements at Venus during the Galileo flyby on February 9 and 10, 1990. The magnetic field data sets from the two spacecraft show corresponding convected features, even when Pioneer Venus is near periapsis on the nightside of the planet. Comparison of the observation times of these corresponding features reveals substantial delays at PVO associated with the deceleration of the flow carrying the draped magnetic field past Venus. Convection lags of up to 10 min are identified near PVO periapsis at an altitude of 1/4 Venus radii (1500 km) above the planet surface. A three‐dimensional convected field gas dynamic model is used to calculate expected lag times by tracing field lines from PVO to the open solar wind. Expected lags from the model are relatively insensitive to changes in field orientation and Mach number. This simple gas dynamic model with the standard ionopause obstacle size gives convection lags too small to explain the Galileo‐PVO observations. A likely explanation is that mass loading of the solar wind by the Venus atmosphere contributes to the deceleration of the flow near the planet. The observation of similar magnetic structures in the solar wind and deep in the Venus nightside ionosphere suggests that the impenetrable obstacle model is inappropriate in this region and the solar wind magnetic field is convected into the nightside ionosphere on timescales of 10 min or more.