Turbulence of the inner solar wind at solar maximum: Coronal radio sounding with Galileo in 1999/2000

Coronal sounding experiments were carried out using the S-band signals of the Galileo spacecraft during the period of high solar activity in 1999-2000. The parameter studied in this work is the frequency of the downlink carrier signal, which was recorded at the widely spaced ground stations of the NASA Deep Space Network. The measurements apply to both the initial thermal acceleration region of the solar wind at heliocentric distances R in the range from 5 to 10R⊙ (R⊙ = solar radius) and the extended acceleration region (10R⊙ < R < 50R⊙). Temporal spectra of the frequency measurements yield values for the intensity of the frequency fluctuations σf, and the spectral index αf of the temporal spectra. It is found that the spectral index tends to be low (αf ∼ 0.2-0.3) at heliocentric distances less than 10R⊙. The spectra steepen (αf ∼ 0.5-0.7) at distances beyond 20R⊙. Furthermore, temporal spectra at solar distances R ≲ 10R⊙ sometimes display a sharp decrease in spectral density at fluctuation frequencies ν ∼ 0.04-0.06 Hz. The change in the plasma turbulence regime in the region of 10-20R⊙ evidently occurs not only during the solar activity minimum, as detected in earlier experiments, but also during times of high solar activity.