Characterising the solar-coronal electron density with BepiColombo multi-frequency range measurements

The propagation of spacecraft radio signals through the solar corona is affected by dispersive delays induced by free electrons in the plasma, particularly during close solar approaches or superior solar conjunctions, when the line of sight passes at small heliocentric distances. Models of the electron density and its variations are important both for predicting measurement noise in radiometric measurements and for assessing telecommunication channels of deep space missions across solar conjunctions. In this work, we use radiometric data of the ESA probe BepiColombo to assess and refine empirical descriptions of the average electron-density distribution in the solar corona. During superior solar conjunctions, the spacecraft radio link probes the inner corona, where solar plasma represents by far the dominant contribution to the path delay. The mission’s multi-frequency X- and Ka-band radio system allows a full separation of the plasma contribution from the non-dispersive component of the signal, enabling the exploitation of calibrated range observables for investigations of the solar corona. We analyse data acquired during five BepiColombo superior solar conjunction experiments in the rising phase of solar cycle 25, focusing on range measurements as a direct probe of the plasma-induced path delay. Unlike Doppler observables, which are sensitive to temporal variations in the integrated electron content along the propagation path, multi-frequency range measurements provide access to the absolute total electron content. We use these measurements to constrain radial electron-density profiles at heliocentric distances larger than approximately four solar radii. The results show that, for the BepiColombo data set considered here, a single-power-law radial model provides a robust representation of the average coronal electron-density behaviour, while the additional degree of freedom introduced by a two-terms model is not significantly constrained by the observations. However, the measurements show significant deviations from all tested models due to the non-stationarity of the solar activity.