LARGE-SCALE SOLAR-WIND STRUCTURE NEAR THE SUN DETECTED BY DOPPLER SCINTILLATION

STUDIES of the solar wind in the inner heliosphere (between the solar-wind source surface at approximately 0.01 AU and 0.3 AU are hampered by the lack of in situ spacecraft measurements. Radio propagation measurements-using both natural1-7 and spacecraft8-14 radio signals-have provided many insights, but information on large-scale solar-wind structure inside 0.3 AU that can be related to coronal features or direct spacecraft measurements at larger distances has nevertheless remained elusive. Here we report the detection of solar-wind structure between 0.08 and 0.53 AU, based on the response of the 13-cm radio signals from the Pioneer Venus Orbiter to electron-density fluctuations and solar-wind speed within this region. These Doppler scintillation measurements were made during the late declining phase of the most recent cycle of solar activity, when the solar wind exhibited recurrent high-speed streams. Near 0.5 AU, we find narrow regions of enhanced scintillation which appear to be associated with compressed plasma at the leading edges of these streams, consistent with previous scintillation measurements15,16. Inside 0.2 AU, however, scintillation enhancements are conspicuously absent from the fast streams, and instead occur in regions where the average solar wind is slow. They exhibit high variability, and appear to be the interplanetary manifestation of coronal mass ejections. The plasma structures giving rise to these enhanced scintillations apparently undergo significant evolution inside 0.3 AU.