A turbulence index independent framework for deriving solar wind speed and coronal electron density from radio spectral broadening

Abstract

We present a turbulence index independent framework for simultaneously deriving solar wind velocity and coronal electron density in the near-Sun region using the spectral broadening of spacecraft radio signals. The formulation accommodates arbitrary turbulence spectral indices ($p$), providing a direct analytical link between the observed Doppler spectra and underlying plasma parameters without assuming a fixed turbulence regime. This generalization extends conventional radio occultation techniques and enables consistent interpretation across multiple radio frequencies. We apply the method to X-band ($\sim$ 8.41 GHz) radio occultation measurements from JAXA's Akatsuki spacecraft during the 2016 and 2022 Venus - Earth superior conjunctions, spanning heliocentric distances of 1.4 - 10 $R_{\odot}$ and sampling both equatorial streamer regions and mid-latitude coronal holes. The retrieved electron densities exhibit systematic trends consistent with empirical coronal models and in-situ observations. By coupling the measured spectral widths with a turbulence-based frequency-scaling relation, we obtain a compact expression that links spectral broadening, solar wind speed, and electron density, applicable for any turbulence index $p$. Fast-solar-wind intervals, characterized by nearly isotropic turbulence, yield speed estimates in close agreement with expectations, while the anisotropic nature of the slow solar wind introduces small but systematic deviations. Our results refine earlier work and demonstrate that explicit consideration of near-coronal turbulence anisotropy is essential for accurate solar-wind parameter retrievals.

Figures (7)

• Figure 1: Geometry of the 2016 and 2022 radio occultation experiment shown. The points in green mark the position of the satellite during the 2016 occultation experiment, while the points in red mark the position during the 2022 occultation experiment.
• Figure 2: A comparison of the f10.7 values for the days of occultation experiments conducted in 2016 and 2022.
• Figure 3: Top left: Time series of residuals in Hz are shown in blue, and the 2nd order polynomial fit used for detrending is shown in orange. Bottom left: Time series of residuals after detrending (Doppler FF). Right: PSD of the segment done using the Welch method. The black dashed line shows the slope-fit.
• Figure 4: The PSDs of all days of experiment conducted using the probe in the year 2016. The corresponding legend marker also shows the heliocentric distance for each experiment.
• Figure 5: Same as Figure \ref{['fig:2016psd']}, but for the year 2022.