Summary of results on plasma compressibility in different parts of the solar wind. Plotted is the density modulation index as a function of heliocentric distances. Solid lines indicate the VLBI phase scintillation results presented in this paper and stated in Table 2. The open circle comes from the radio propagation measurements of Woo et al. (1995), and the filled circle and filled triangle represent in situ measurements of Bavassano & Bruno (1995) and Tu & Marsch (1994), respectively. The error bars convey the range of measured values reported rather than the experimental error associated with a measurement. Further description of these measurements is in the text. The measurements are generally consistent and indicate a density modulation index of 5%–15% throughout the inner solar system.