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

TL;DR

This paper introduces a unified, frequency-independent method to estimate solar wind speeds and electron densities in the near-Sun corona using spectral broadening, validated with spacecraft data across multiple bands.

Contribution

It generalizes spectral broadening analysis for Kolmogorov turbulence, enabling consistent solar wind and density measurements across different radio frequencies.

Findings

Measured solar wind speeds of 100-150 km/s with S-band data.

Observed speeds of 150-400 km/s in coronal-hole regions with X-band data.

Provided a frequency-scaled relation linking spectral width to plasma parameters.

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.