The Sun’s activity is driven by its restless magnetic field. The powerful movements of gas we see in movies of Sun, like this one taken from NASA’s Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) instrument, are the result of invisible magnetic interactions orchestrating the activity. Unlike a simple bar magnet with its single north and south poles, the Sun’s field is highly complex. Regions of north or south polarity can appear through its surface, and they shift and change with time. This behaviour can be revealed by instruments such as SDO’s Helioseismic and Magnetic Imager (HMI), which shows these different polarities as black and white regions on the Sun. These regions show where the Sun’s magnetic field lines are reaching out into space. Some of those lines turn around and plunge back inside the solar interior. These are called closed field lines. Others stretch out into space. These are called open field lines and they form the Solar System’s interplanetary magnetic field.

A superimposition of AIA 17.1 nm images revealing the Sun’s million-degree hot corona, and the magnetic field lines derived from the HMI magnetogram, shows that the visible activity of the Sun is closely associated with the magnetic activity. Because the open field lines reach out into space, particles can flow away from the Sun creating the solar wind. These flow out past the planets of the Solar System as well as spacecraft – including ESA’s Solar Orbiter. By measuring the conditions in the solar wind, Solar Orbiter is effectively measuring the magnetic activity that has previously taken place on or near the Sun.

This powerful ‘linkage science’ is a key aspect of Solar Orbiter’s mission. It is why the instruments on the spacecraft are split into two types: in situ instruments and remote sensing. The in situ instruments record the solar wind events as they sweep past the spacecraft, whereas the remote sensing equipment looks at the Sun itself. When the in situ instruments pick up specific changes in the solar wind, data from the remote sensing instruments can be examined to see what solar event triggered those changes.