There has been no opportunity for in situ measurements of the properties of the solar wind at solar distances less than the 0.3 AU perihelia of the Helios spacecraft. All recent observations of the inner heliosphere have therefore been obtained by remote sensing. Using radio scintillation techniques, Woo and Gazis [1993] found that inside 0.3 AU, the variation in the solar wind structure is more pronounced than it is farther out, with most of the fluctuations occurring in the slow wind near the heliospheric current sheet, rather than on the leading edges of high-speed streams. In another study using IPS, Coles et al. [1991] were able to study the profiles of velocity versus solar distance between 11 and 90 solar radii (0.05 to 0.42 AU) for 16 solar wind streams. Half the streams had velocity profiles in good agreement with theoretical solar wind models that included moderate amounts of acceleration by hydromagnetic waves, but four of the streams had profiles which were too steep and four others had profiles which were too flat to be explained by such models. In a later study [ Coles and Esser, 1992], it was shown that the wave energy required to explain the acceleration of the streams with normal profiles is consistent with the upper bounds determined from other IPS observations.
Woo and Goldstein [1994] used spectral broadening of spacecraft radio
signals to probe the solar wind in the range of 3 to 8 solar radii (0.01 to 0.04
AU) under conditions of minimum solar activity. They found that the solar wind
speed was approximately 2.2 times greater at 
60 latitude than it was near
the solar equator; their result is consistent with earlier IPS data obtained at
solar distances >0.5 AU.
Other data in the inner regions of the solar wind were obtained by the Spartan 201, a spacecraft carrying a white-light coronagraph and an ultraviolet coronal spectrometer during a Space Shuttle mission in April, 1993 [ Kohl et al., 1994; Strachan et al., 1994; Fisher et al., 1994]. Although velocity, density, and temperature data were obtained out to 3.5 solar radii in both coronal streamers and coronal holes, the final results from those experiments are not yet available. Other flights of Spartan 201 are scheduled to coincide with the Ulysses passages over the solar polar regions.
Acknowledgments. This review was carried out at the Jet Propulsion Laboratory of the California Institute of Technology under a contract with the US National Aeronautics and Space Administration.