Characterisation of the turbulent solar plasma in the acceleration region of the Sun using BepiColombo radiometric data

Abstract The acceleration region of the solar wind is still a little-explored region due to its harsh environment a few in situ data are available. In this work, we have studied the characteristics of the turbulent solar plasma using radio science data from BepiColombo solar conjunction experiments. In this configuration the line of sight of the radio signal passes really close to the Sun, and the main noise contribution to the data is the solar plasma. Multi-frequency radio links allow a precise calibration of Doppler and range observables by isolating the plasma contribution, which may then be used to characterise the solar corona. In this study, Doppler and range data from five BepiColombo superior solar conjunctions were analysed: this data set spans different phases of the solar cycle, allowing us to observe the variability of the solar corona due to solar activity. First, the plasma content in the data was characterised through the computation of the power spectral density. A typical regime for the solar wind is that of a well-developed turbulence, so we compared the computed spectra with the expected Kolmogorov behaviour (f-2/3 for Doppler observables and f-5/3 for range observables). Exploiting the collected open-loop recordings at high frequency (4 kHz), we fitted the intensity time series to a theoretical model of the spectrum to simultaneously estimate multiple solar wind parameters (amplitude, velocity, axial ratio, inner scale of turbulence, and power law index), characterising the turbulent regime in the vicinity of the Sun (in the range ~ 4-28 solar radii). The results show that the inner scale increases with increasing impact parameter, consistent with theoretical models, while the velocity exhibits a variable behaviour with values on average increasing up to 12 solar radii and then stabilising around 300-400 km/s.