From Cluster to Smile

Cluster is a set of four spacecraft spread out around Earth to investigate the interaction between the Sun and Earth’s magnetosphere – the bubble created by Earth’s magnetic field that shields us from harmful solar radiation. Each Cluster spacecraft makes extremely precise local measurements of charged particles meeting the magnetosphere.

Cluster has helped answer an enormous number of questions, but piecing together the individual measurements to create a big picture has been very difficult.

This is where Smile comes in. Whilst Cluster is detail-oriented, Smile sees the big picture – and it’s crucial to have both of these personality types on a team. Smile will take the first ever photos and videos in X-ray and UV light showing the outer magnetosphere and the holes at Earth’s poles, called the ‘polar cusps’. Radiation from the Sun can enter through these holes, resulting in the northern and southern lights.

Smile’s photos and videos will help scientists finally understand the interactions between the outer magnetosphere of our planet and the solar wind on a large scale. For example, whilst the mission’s UV camera photographs the northern lights, its X-ray camera will photograph the magnetosphere and the polar cusps. Scientists will be able to compare the UV images with the X-ray images to find out exactly how the interaction of the solar wind with the magnetosphere causes a reaction from Earth in the form of the northern lights.

It's a bit like watching a human body react to an external stimulus – studying the stimulation and the response at the same time gives us a better understanding of cause and effect, and ultimately enables us to better predict future responses to future stimuli. For the Sun-Earth connection, we have never before been able to compare cause and effect like this; it will profoundly change our understanding of how the solar wind affects Earth.

Data collected by Cluster and other ‘in situ’ space missions were also used, together with computer models of the magnetosphere, to design Smile’s instruments and decide where they should be placed on the spacecraft. In addition, Cluster data have been used to predict where Smile’s soft X-ray instrument could be affected by energetic protons; such predictions could be used during the operation and calibration of the instrument.

Whilst Cluster is the mission that Smile will complement the strongest, there is another ESA mission that has also proved valuable for developing Smile: XMM-Newton.

Making the most of nuisance X-rays

XMM-Newton is an ESA astrophysics observatory studying extremely energetic events around the Universe, like black holes and supernova explosions. The spacecraft looks out to space from an orbit around Earth, so when it watches these events, it looks through the magnetosphere. This means that it unintentionally captures some foreground X-rays from the outer edge of the magnetosphere, where it is hit by the solar wind.

Usually, these X-rays are considered a nuisance by astrophysicists using XMM-Newton data, but they have also provided plasma physicists with the unexpected opportunity to investigate the interaction between the solar wind and the outer magnetosphere. These investigations have been valuable for preparing Smile; they have predicted a range of phenomena relevant to Smile’s observations and supported the development of a process that will be used to reconstruct the structure of the magnetopause (the boundary of the magnetosphere) from the images that Smile will take.

To summarise…

Whilst Smile is a valuable mission on its own, combining the information that it will provide with data from other missions, as well as ground-based observatories at Earth’s poles, will give us a thorough understanding of how the Sun and Earth interact. This will pave the way for future satellites that will directly monitor and forecast space weather to protect us from its harmful effects.