Radio waves passing through the solar corona, the outer atmosphere of the Sun, are distorted just like the air shimmers over scorching asphalt. This haze is caused by seething streams of charged particles — the solar wind. Scientists found a way to use the strength of the signal shimmer to simultaneously determine wind speed and particle density (mostly protons, hydrogen nuclei, and electrons). Previously, such measurements required knowing the type of turbulence, but the new method bypasses that: it directly captures how the spectrum of the radio waves blurs.
An unexpected twist: key data came from the Japanese Akatsuki spacecraft, which studies Venus. Its radio signals, passing near the Sun, became an ideal probe for the corona. The method works great for fast and uniform wind, but for slow wind that bursts out in clumpy streams, a correction for inhomogeneities was needed.
as written by the authors arXiv:2603.20874
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.
Although electrons are billions of times lighter than hydrogen atoms, their effect on radio waves is many times stronger.
In the movie 'Sunshine,' people fly to save the dying Sun. In real life, we listen to its breath through the twinkling of radio waves.