A generalized method for estimating solar wind speeds and densities using spectral broadening for a Kolmogorov turbulence spectrum

Abstract

We present a unified method to derive both solar wind velocities and coronal electron densities in the near-Sun corona using Doppler spectral broadening of spacecraft radio signals. The method is generalized to be frequency independent under the assumption that electron density fluctuations follow a Kolmogorov spectrum. We validate the approach using S-band data from India's Mars Orbiter Mission during the October 2021 superior conjunction at 5-8 R$_\odot$, and X-band data from Japan's Akatsuki during June 2016 and October 2022 conjunctions spanning 1.4-10 R$_\odot$. From S-band we obtained wind speeds of 100-150 km s$^{-1}$ and electron densities of order $10^{10}$ m$^{-3}$. X-band results show speeds ranging from $\sim$150 km s$^{-1}$ near the equator to $\sim$400 km s$^{-1}$ in coronal-hole regions, with consistent radial trends in density. We provide a compact, frequency-scaled relation that maps Doppler spectral width to both $v$ and $N_e$. The formulation enables consistent application across telecommunication bands and complements in-situ probes for coronal plasma studies.

Figures (4)

• Figure 1: Radial profiles of the electron density and solar wind velocity derived from radio occultation experiments conducted with Akatsuki (2016, 2022) and the Mars Orbiter Mission (MOM, 2021). Left panel: Electron density variation ($N_e$) as a function of heliocentric distance, showing consistency across different epochs and spacecraft geometries. Right panel: Solar wind velocity profiles obtained simultaneously from the same set of occultations, demonstrating the comparative acceleration trends and variability with solar offset.
• Figure 2: Electron density estimates ($N_e$) derived using our unified Doppler spectral broadening technique for both the Mars Orbiter Mission (MOM) and Akatsuki radio occultation (RO) experiments are shown as a function of solar offset ($R_\odot$) compared against other studies in the literature.
• Figure 3: Solar wind speed estimates using the above set of equations for both MOM and Akatsuki RO experiments compared against measurements and models in the literature spanning the past half century.
• Figure 4: The daily F10.7 values during the period when occultation experiments were conducted in the years 2016, 2021, and 2022 are shown in blue, orange, and red, respectively. The hollow circles on the curves indicate the days on which RO experiments were conducted.