Smile's ultraviolet vision

The solar wind is a steady stream of charged particles continuously sent out by the Sun. Earth defends itself against this stream with a giant magnetic shield called the magnetosphere. As the magnetosphere reacts to the charged particles, brief disturbances called magnetic storms ripple through it.

These disturbances – either the near-daily small ‘substorms’ or the rarer and more violent full ‘geomagnetic storms’ – send bursts of particles along Earth’s magnetic field lines, towards the North and South Poles. There, they can cause the sky to light up with the northern and southern lights.

Looking down from above, Smile will use its ultraviolet cameras to see the full glowing circle around the North Pole. It will reveal where exactly the northern lights are and how they are moving.

Geomagnetic storms happen when the Sun sends a huge blast of charged particles, known as a coronal mass ejection, straight in our direction. They cause especially bright auroras that can be seen much further from the poles than usual.

Following the May 2024 and November 2025 solar storms, for example, geomagnetic storms caused northern lights to be visible across much of Europe, as far south as Portugal and Spain.

These geomagnetic storms usually last two to three days. Previous spacecraft have only been able to observe the resulting auroras for up to about 15 hours at a time. By recording for 45 hours at a time with UVI, Smile will give us a much better idea of how geomagnetic storms lead to the beautiful lights that have mesmerised humans for thousands of years.

Our Sun’s outbursts also have the potential to disrupt and damage satellites as well as vital infrastructure on the ground like power grids and radio communications. By comparing the data Smile collects to reports of damage to infrastructure, we can hopefully better understand the consequences of solar activity on planet Earth.

The infographic below shows how UVI’s observations will be combined with those from Smile’s X-ray camera to give us our first-ever simultaneous view of how the interaction of the solar wind with Earth’s magnetic field causes a reaction in the form of the auroras.

Why ultraviolet light?

Our eyes see only visible wavelengths of light from the auroras, but in fact they emit mostly in the ultraviolet (UV) part of the spectrum. This means that UV cameras can see them much better than ‘normal’ visible light cameras can.

What’s more, a spacecraft using a visible light camera would only see auroras on the night side of Earth – just like we can only see them when it’s dark, even though they are there throughout the day. Using a UV camera, it will be much easier for Smile see the full ring of light around the North Pole. It only has to filter out the faint ‘ultraviolet dayglow’ emitted by Earth’s upper atmosphere.

How exactly does UVI work?

Smile will watch the northern lights without interruption for 45 hours at a time thanks to its special orbit around Earth. The spacecraft will travel out far above the North Pole with its instruments switched on, before coming very close to the South Pole to deliver its data every two days.

UVI uses four mirrors coated with thin film to guide ultraviolet light onto a semiconductor-based CCD sensor. UVI also makes use of an image intensifier to increase the inherently low signal from the auroras.

UVI will be able to take an image up to every 60 seconds, with a resolution of 30–90 km, depending on where Smile is in its orbit.

International endeavour

UVI was developed by the Chinese National Space Science Center, in collaboration with the Belgian Centre Spatial de Liège. It includes significant contributions from the University of Calgary (Canada), the European Space Agency (ESA) and the Chinese Polar Research Institute.