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 (∼ 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⊙ 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.