Detecting the Oscillation and Propagation of the Nascent Dynamic Solar Wind Structure at 2.6 Solar Radii Using Very Long Baseline Interferometry Radio Telescopes

Abstract

Probing the solar corona is crucial to study the coronal heating and solar wind acceleration. However, the transient and inhomogeneous solar wind flows carry large-amplitude inherent Alfvén waves and turbulence, which make detection more difficult. We report the oscillation and propagation of the solar wind at 2.6 solar radii (Rs) by observation of China’s Tianwen and ESA’s Mars Express with radio telescopes. The observations were carried out on 2021 October 9, when one coronal mass ejection (CME) passed across the ray paths of the telescope beams. We obtain the frequency fluctuations (FFs) of the spacecraft signals from each individual telescope. First, we visually identify the drift of the frequency spikes at a high spatial resolution of thousands of kilometers along the projected baselines. They are used as traces to estimate the solar wind velocity. Then we perform the cross-correlation analysis on the time series of FF from different telescopes. The velocity variations of solar wind structure along radial and tangential directions during the CME passage are obtained. The oscillation of tangential velocity confirms the detection of a streamer wave. Moreover, at the tail of the CME, we detect the propagation of an accelerating fast field-aligned density structure indicating the presence of magnetohydrodynamic waves. This study confirms that the ground-station pairs are able to form particular spatial projection baselines with high resolution and sensitivity to study the detailed propagation of the nascent dynamic solar wind structure.

Application of the Tianwen-1 DOR Signals Observed by Very Long Baseline Interferometry Radio Telescopes in the Study of Solar Wind Plasma and a Coronal Mass Ejection

“…In contrast, the downlink signals generated by the onboard crystal oscillator is the incoherent signals. The stability of the ground hydrogen clock is 10 −15 s s −1 , superior to the onboard crystal oscillator of 10 −12 s s −1 (Ma et al 2022;Wang et al 2022).…”

“…A frequency spike refers to a sudden and significant increase in signal frequency that exceeds the ambient data. Ma et al (2022) shows that CMEs can cause frequency spikes that can be applied to estimate the propagation velocity of CMEs. We also find two distinctive spikes with the time lag between the arrival time at different radio telescopes in Figure 8.…”

“…We use the output of PLL with an integration time of 10 ms to calculate the velocity of the CME. As a comparison, we also use cross correlation method to calculate CME velocity (Ma et al 2021(Ma et al , 2022. The correlation coefficients between the two stations are greater than 0.9.…”

“…The projections along radial and latitudinal directions give the twodimensional velocity of the CME. This method is also effective at detecting the oscillation and propagation of the nascent dynamic solar wind structure near the Sun, e.g., at 2.6 Rs (Ma et al 2022). As a matter of fact, the density disturbance of the CME is the integration effects along the direction of the signal transmission path.…”

“…Since the radio signals of spacecraft can enter the challenging inner solar wind within 10 solar radii (Rs, 1 Rs = 695,700 km), solar conjunction observation of deep-space probes is a common research method to study the coronal structure. This technique was used by Akatsuki to show the difference in the solar-wind velocity between fast and slow winds (Chiba et al 2022), by Galileo to study the outer turbulence scale of the solar wind (Efimov et al 2002), by Venus Express to detect the strong turbulence regions in the super corona (Efimov et al 2021), by Mars Express to study the interplanetary plasma scintillation between the years 2013(Molera Calvés et al 2017Kummamuru et al 2023), by the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft to study radio Faraday rotation through a CME (Jensen et al 2018), and in Tianwen-1 (TW1) to detect the oscillation and propagation of the nascent dynamic solar wind structure (Ma et al 2022).…”