ESA Science & Technology - The morphology of a coronal mass ejection as revealed by radio-sounding experiments

This illustration shows the morphology of a Coronal Mass Ejection (CME) – a gigantic eruption that releases enormous amounts of matter and energy from the Sun through the corona and into space – as revealed by radio-sounding experiments.

Radio sounding of the solar corona is a technique that exploits radio transmissions from planetary missions to probe the corona of the Sun. This technique can be performed when a spacecraft is located at superior solar conjunction – meaning that Earth, Sun and the spacecraft lie on the same line, with the spacecraft located on the opposite side of the Sun with respect to our planet. In this configuration, or more precisely just before and after it, radio signals sent out by the spacecraft pass through the solar corona – the hot outer atmosphere of the Sun, which consists of turbulent plasma at temperatures of millions of degrees – as they travel towards Earth. Electrons in the coronal plasma interact with the radio signals, causing a frequency shift that can be measured on Earth and analysed to infer the electron density in the corona.

The upper part of the illustration shows the limb of the Sun (on the right), a CME moving away from the Sun (in the centre) and the path travelled by radio waves sent out by a spacecraft on their way to Earth (on the left); all components are shown as viewed from 'above', perpendicularly to the ecliptic plane. The lower part of the illustration shows a graph depicting how the density of electrons varies in time as a CME moves across the path of a radio signal that is traveling from the spacecraft to Earth.

Based on data collected during four CMEs in 2004 using ESA's Mars Express spacecraft, scientists have been able to probe the morphology of a CME in great detail. According to the data, when the path of the radio signal is traversed by a CME, the electron density first undergoes a gentle rise, followed by a steeper increase and, eventually, by a smooth decline, as shown in the graph. This suggests that the proper, dense structure of a CME is preceded by a shock front and a series of smaller fronts that consist of less dense material. The smaller fronts build up as the CME itself propagates outward through the corona, pushing material ahead of it and piling it up in a similar way to a bulldozer. In contrast, material immediately behind the CME has extremely low density, as indicated by the eventual density decrease. These results have been presented by Pätzold et al., 2012.