Smile's X-ray vision

The Sun continuously throws out a steady stream of charged particles known as the solar wind, and occasionally huge eruptions known as coronal mass ejections.

Since the 1950s, we’ve known that Earth has been defending itself against these dangers with a giant magnetic shield called the magnetosphere. We can think of the magnetosphere as a huge bubble full of magnetic field lines that are generated mostly from deep within our planet. But despite decades of research, there’s still a lot we don’t know about this shield. It is invisible, enormous, and we’re right inside it, making it really difficult to actually look at.

So far, we’ve being researching the magnetosphere using spacecraft that measure the electric and magnetic fields around them to make ‘point measurements’ – peeks at what is going on in one specific location at one specific moment. By combining all these point measurements, we’ve built up a rough idea of what the magnetosphere looks like and how it behaves.

For the first time, Smile’s X-ray camera will monitor the entire Sun-facing edge of the magnetosphere (the magnetopause) continually for several years, letting us really see how it moves and changes shape in response to the Sun’s attack.

Think of it like this: imagine that our understanding of Earth’s ocean has so far been based on what individual people around Earth feel when they go for a swim. Perhaps in some places they feel colder, or that the water is saltier. They might see a starfish, a whale, or no life at all. Now imagine how our understanding changes if we take a step back and analyse the entire ocean from a distance, recording how it changes as the minutes go by.

This is what SXI is doing for Earth’s magnetosphere: the images and data that it collects will give us a completely different – and much bigger – picture of Earth’s defence against the Sun.

The infographic below explains how SXI’s observations will be combined with those from Smile’s ultraviolet camera to give us our first-ever simultaneous view of how the interaction of the solar wind with Earth’s magnetosphere causes a reaction in the form of the auroras.

Why X-rays?

Smile is putting to the test a novel method that involves detecting the X-ray light emitted when charged solar wind particles interact with neutral particles in Earth’s upper atmosphere.

We first discovered this process 30 years ago, somewhat by accident. In 1996, scientists using an X-ray mission called ROSAT proposed to look at Comet Hyakutake. They were surprised to discover that when charged solar wind particles hit neutral particles around the comet, they emitted X-rays. They named this process ‘solar wind charge exchange’.

The discovery led them to question whether the same process happens around Earth, when the solar wind hits particles in our upper atmosphere. Looking at images from X-ray telescopes like ROSAT and ESA’s XMM-Newton, which look out towards the distant Universe from inside Earth’s magnetosphere, they indeed saw nearby X-ray emission.

With SXI, Smile will detect when and where this low-energy, or ‘soft’, X-ray emission is emitted, to study how exactly Earth’s magnetic bubble responds to the solar wind.

How exactly does SXI work?

SXI has a wide field of view. From an orbit that goes high above the North Pole, it will record X-ray emission from the Sun-facing edge of Earth’s magnetic field – the shield-shaped magnetopause – as well as the holes in the magnetic field at Earth’s poles. The GIF below shows SXI in action.

The camera uses an X-ray focusing technique called ‘micro pore optics’. Inspired by the workings of a lobster’s eyes, micro pore optics uses square tubes to funnel X-ray light down onto two detectors. It is a lightweight, well-established and cost-effective technique for making X-ray observations.

The detectors that collect the light are charged-coupled devices (CCDs) – thin wafers of silicon that are sensitive to light. Lots of space missions carry CCDs, but at 8.1 x 8.1 cm, Smile’s are the largest ever to leave planet Earth. This gives them a wide field of view and as high resolution as possible. Smile's CCDs will be cooled to a chilly -120 °C for maximum sensitivity. Shutters will protect the CCDs when Smile passes through the dangerous Van Allen radiation belts surrounding Earth.

European endeavour

SXI is the European Space Agency (ESA)-led instrument on Smile. On behalf of ESA, the University of Leicester developed and built the instrument, in collaboration with Mullard Space Science Laboratory and the Open University (all based in the UK), working with several other institutions across Europe. SXI uses solely European parts for all systems.