Radio-sounding observations of a coronal mass ejection during the Galileo solar conjunction in January 1997

Abstract

Frequency and amplitude fluctuations of the Galileo S-band radio signal were recorded nearly continuously during the spacecraft's solar conjunction from December 1996 to February 1997. A strong propagating disturbance associated with a CME was detected on 8 January when the radio ray path proximate point was on the east solar limb at about 32 solar radii from the Sun. Characteristics of the CME passage through the Galileo/Earth line of sight include: (a) a noticeable change of the average frequency of the radio signal, (b) a significant increase in the fluctuation level of the radio frequency and amplitude, and (c) a dual-velocity solar wind configuration with distinctly different values of the velocity (200 km/s and 360 km/s). These velocity estimates are obtained from a correlation analysis of frequency fluctuations recorded simultaneously at two widely-separated ground stations. The frequency fluctuation cross-correlation functions are relatively narrow during the passage of the disturbance through the radio ray paths, implying that the outward solar wind transport in the CME may be described as an ordered flow. The density turbulence power spectrum becomes steeper behind the CME leading front but becomes unusually flat near the trailing edge. It is shown that the effects observed in the Galileo radio-sounding data are associated with the same CME observed first (6 January) by the SOHO/LASCO coronagraphs near the Sun and later (10–11 January) in the solar wind on the WIND spacecraft near the Earth (e.g. abrupt changes in ion density, peaks in the velocity, interplanetary magnetic field enhancements).

Keywords:

• Solar corona
• Radio sounding
• Coronal mass ejection
• Galileo spacecraft

Acknowledgements

The present work was supported by the Programme ‘Plasma Processes in the Solar System’ of the Russian Academy of Sciences and by Grant 04-02-17332 of the Russian Foundation of Basic Research (RFBR). This paper presents result of research partly funded by the Deutsche Forschungsgemeinschaft (DFG) under a cooperative program between the DFG and RFBR (Grant 05-02-04002). The authors wish to express their gratitude for the continued support of the Galileo Project, the Multi-Mission Radio Science Team and the NASA Deep Space Network.