VLBI data processing on coronal radio-sounding experiments of Mars express

TL;DR

This paper presents VLBI data processing methods applied to Mars Express solar corona experiments, revealing plasma turbulence characteristics and solar wind speeds at different heliocentric distances through radio signal analysis.

Contribution

It introduces novel VLBI analysis techniques to measure plasma turbulence and solar wind speeds during Mars Express solar conjunctions, enhancing understanding of solar wind dynamics.

Findings

Power-law spectrum of frequency fluctuations close to Kolmogorov turbulence.

Electron column density fluctuations vary with solar distance.

Fast solar wind flows >700 km/s detected at high heliolatitudes.

Abstract

The ESA's Mars Express solar corona experiments were performed at two solar conjunctions in the years 2015 and 2017 by a number of radio telescopes in the European VLBI Network. This paper presents the methods to measure the frequency and phase fluctuations of the spacecraft radio signal, and the applications to study the characteristics of the plasma turbulence effects on the signal at a single station and at multiple stations via cross-correlation. The power spectra of the frequency fluctuations observed between 4.9 and 76.3 $\rm R_{s}$ have a power-law shape close to a Kolmogorov spectrum over the frequency interval $ \nu_{lo}< \nu <\nu_{up}$, where the nominal value of $\nu_{lo}$ is set to 3 mHz and $\nu_{up}$ is in the range of 0.03 $\sim$ 0.15 Hz. The RMS of the frequency fluctuations is presented as a function of the heliocentric distance. Furthermore, we analyse the variations of the electron column density fluctuations at solar offsets 4.9 $\rm{R_{s}}$ and 9.9 $\rm{R_{s}}$ and the cross-correlation products between the VLBI stations. The power density of the differential fluctuations between different stations decreases at $\nu < 0.01$ Hz. Finally, the fast flow speeds of solar wind $>700$ $\rm{km~s^{-1}}$ are derived from the cross-correlation of frequency fluctuations at $\nu < 0.01$ Hz. The fast flow speeds of solar wind correspond to the high heliolatitude of the coronal region that the radio rays passed. The VLBI observations and analysis methods can be used to study the electron column density fluctuations and the turbulence at multiple spatial points in the inner solar wind by providing multiple lines of sight between the Earth and the spacecraft.