Probing the solar corona with Doppler and range measurements of the spacecraft BepiColombo

Abstract

The Mercury Orbiter Radio-science Experiment (MORE), onboard the ESA-JAXA mission BepiColombo, is dedicated to the study of Mercury’s interior structure and rotational state, and to fundamental physics tests. The MORE radiotracking system relies on a multi-frequency radio link: the onboard Deep Space Transponder receives an uplink in X-band and retransmits it back coherently in X- and Ka-band, the Ka-band transponder allows to establish a two-way link in Ka-band.During the cruise phase, BepiColombo experienced four superior solar conjunctions. These periods, spanning for 15 days centered about the minimum impact parameter of the radio path between the spacecraft and the Earth, are exploited for tests of relativistic delay and Doppler shift. Thanks to the multi-frequency link the signal due to the solar corona plasma can be isolated through precise calibrations, exploiting the dispersive nature of the plasma1,2. This allows MORE to remove the plasma noise from the Doppler and range data in order to perform the fundamental physics test, but also to characterize the inner solar corona.In this work we focus on the analysis of the data from the first (10th-24th March 2021) and second (29th January - 12th February 2022) solar conjunction experiments to characterize the properties of the solar wind.For each radiotracking pass the power spectral density of Doppler measurements is compared with the expected power spectral index of a Kolmogorov turbulence (f-2/3)3.Solar corona plasma data are also used to localize the plasma structures of the solar corona along the line of sight by means of cross-correlations between uplink and downlink time series of the plasma content obtained from Doppler data. This allows us to analyze in more detail large solar phenomena, such as coronal mass ejections.Exploiting the collected open-loop recordings at high frequency (4 kHz), the solar wind velocity can be estimated assuming a theoretical model for the intensity spectrum4. The intensity timeseries are used to fit theoretical spectrum parameters (amplitude, velocity, axial ratio, inner scale of turbulence and power law index), characterizing the solar wind in the vicinity of the Sun.   Finally, the range data set allows us to retrieve the total electron content along the radio path. This absolute measurement is used to adjust models of the solar wind density beyond four solar radii. 1 Bertotti et al, “Doppler tracking of spacecraft with multi-frequency links”,Astronomy and Astrophysics 269, 608-616, 19932 Bertotti et al, “A test of general relativity using radio links with the Cassini spacecraft”, Nature 425, 374-376, 20033 R. Woo and J.W. Armstrong, “Spacecraft Radio Scattering Observations of the Power Spectrum of Electron Density Fluctuations in the Solar Wind”, Journal of Geophysical Research 84, no. Al2, 19794 S. L. Scott, W. A. Coles and G. Bourgois, “Solar wind observations near the sun using interplanetary scintillation”, Astronomy and Astrophysics 123, 207-215, 1983