ESA Science & Technology - Density of electrons in the Sun's corona during a coronal mass ejection

This graph shows how the density of electrons in the solar corona – the hot outer atmosphere of the Sun, which consists of turbulent plasma at temperatures of millions of degrees – varies during a Coronal Mass Ejection (CME). CMEs are gigantic eruptions that release enormous amounts of matter and energy from the Sun through the corona and into space.

The measurements are based on radio sounding of the solar corona, a technique that exploits radio transmissions from planetary missions to probe the corona of the Sun. This technique can be employed 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 measurements shown in the graph were taken on 3 September 2004, from 14:05 to 17:42 UT, using the Mars Radio Science (MaRS) instrument on board ESA's Mars Express spacecraft. On this day, a CME occurred; the CME shock front first crossed the line of sight to Mars around 14:35 UT, causing a sudden increase in the noise pattern of the radio signal that lasted for several hours. Analysis of the data indicates that the density of electrons in the portion of the corona probed by the radio experienced a substantial increase as material from the CME moved across the path of the radio signals, as shown in the graph.