Fluctuations in plasma electron density may play a role in solar coronal energy transport and dissipation of wave energy. Transcoronal spacecraft radio sounding observations reveal frequency fluctuations (FF) that encode the electron number density disturbances, allowing exploration of coronal compressive wave and advected inhomogeneity models. Primary FF observations from MESSENGER 2009 and published FF residuals from HELIOS 1975-1976 superior conjunctions were combined to produce a composite view of equatorial region FF near solar minimum over solar o↵set range 1.4-25R. Methods to estimate the electron number density fluctuation variance from the observed FF were developed. We created a simple stacked flux tube model that incorporated both propagating slow density waves and advected spatial density variations to explain the observed FF. Slow density waves accounted for most of the FF at low solar o↵set, while spatial density inhomogeneities advected at solar wind speed dominated above the sonic point at 6R. Corresponding spatial scales ranged 1-38 Mm, with scales above 10 Mm contributing most to FF variance. Flux-tube structuring of the model introduced radial elongation anistropy at lower solar o↵sets, but geometric conditions for isotropy were achieved as the the flux tube widths increased further out in the corona. The model produced agreement with the FF observations up to 12R. FF analysis provides information on electron density fluctuations in the solar corona, and should take into account the background compressive slow waves and solar wind-related advection of quasi-static spatial density variations.

Trans-coronal radio observations were taken during the 2011 observing campaign of the Akatsuki spacecraft through superior conjunction. The observed X-band (8.4 GHz) signals exhibit frequency fluctuations (FF) that are produced by temporal variations in electron density along the radio ray path. A two-component model for interpretation of the FF is proposed: FF scales largely with acoustic wave amplitude through the inner coronal regions where the sound speed dwarfs the solar wind outflow speed, while FF in the region of solar wind acceleration is dominated by the increased density oscillation frequency on the sensing path that results from bulk advection of the plasma inhomogeneities. An estimate of fractional electron density fluctuation is obtained from the mid-corona. A radial profile of slow solar wind speed is determined in the extended corona using mass-flux continuity principles. The coronal sonic point for slow solar wind is estimated to range from 4 to 5 solar radii from the heliocenter.

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