Photographing Earth's Magnetic Field with SMILE

While human eyes can't see the solar wind interacting with Earth's magnetosphere, it is visible to SMILE, the Solar wind Magnetosphere Ionosphere Link Explorer jointly developed by China and Europe.

The first batch of SMILE's observation images and scientific data was released recently. Wang Chi, director general of the National Space Science Center (NSSC) of Chinese Academy of Sciences and Chinese principal investigator of the SMILE satellite, called it the satellite's successful demonstration of its Earth-observation imaging capability.

It can track the evolution of the magnetosphere on a global scale and acquire solar wind parameters in real time, providing a new means of observing geomagnetic storms, substorms and other space weather phenomena.

Looking for a candle in a sea of lights

The high-energy particles carried by the solar wind travel along magnetic field lines into the upper atmosphere over the polar regions, where collisions produce auroral emissions. Changes in the aurora's shape and brightness provide clues to the intensity and distribution of energy input from the solar wind into Earth's magnetosphere.

To see the aurora clearly, SMILE is loaded with an ultraviolet aurora imager, the core parts of which are a camera and an electric control case, to specifically capture the vacuum ultraviolet signals emitted by the aurora.

The challenge is that the auroral signal is seven to eight orders of magnitude fainter than the solar background. "It's like looking for a candle in a sea of lights and seeing the details of the flame," Wang Yongmei, a member of the research team and researcher at NSSC, said.

The camera must be capable of collecting, strengthening and imaging extremely weak ultraviolet aurora. Vacuum ultraviolet calibration is the key to testing and verifying whether the camera has such capability.

The lights in the ultraviolet band will be greatly absorbed in the atmosphere and are hardly discernible, so the installation, adjustment and performance test of the camera all have to be conducted in the vacuum tank.

Despite limited time, each design was subjected to whole-chain simulation and iteration, and each component was repeatedly tested in a vacuum environment.

Finally, in July 2026, the ultraviolet aurora imager successfully acquired ultraviolet aurora data in orbit.

Sensing extremely weak space magnetic field

After seeing the aurora clearly, the researchers were presented with the next challenge — to capture the magnetic field signals in space.

The magnetometer on the satellite, which undertakes this task, requires high detection sensitivity as the signal strength it is trying to catch is less than one thousandth of that of Earth's magnetic field.

The magnetometer's sensitivity is primarily limited by the sensor's magnetic core material and the coil-winding process. The researchers winded dozens of coil samples and picked the three best sensors after rigorous tests.

Then they faced an even tougher problem: suppressing magnetic interference. Many materials in a satellite are inherently magnetic, and the electrical circuits in various instruments also generate magnetic fields when powered.

If the magnetometer were mounted directly on the satellite, it would pick up substantial interference, making it impossible to distinguish genuine magnetic fields in space from signals caused by the satellite itself.

The magnetometer therefore had to be mounted at the tip of a long deployable boom, well away from the satellite body. But this posed another challenge: The boom itself could generate magnetic interference.

The team had no choice but to test every small metal part individually. After countless rounds of testing multiple batches of parts, they finally selected a batch of metal components whose magnetism was below an extremely low threshold.

Light structure for large body

The last challenge was to assemble all payloads onto the SMILE platform to make full use of their efficiency for scientific exploration.

The SMILE satellite's orbit is too high for the selected launch vehicle to reach in a single maneuver. The satellite has to use its onboard propulsion module to gradually raise its orbit through multiple orbital maneuvers, much like climbing a flight of stairs.

However, the total mass of the satellite was strictly limited to 2.25 tonnes, and most of the mass was in the propulsion fuel, leaving only over 200 kilograms for the satellite platform. The research team had to use the lightest structure to support the entire payload.

They came up with an unprecedented idea: to remove the outer shell of the propulsion module, leaving the four large fuel tanks exposed. However, during ground vibration testing, the fuel tanks shook violently and the team struggled to keep them stable.

To withstand the intense vibrations during launch, each tie rod had to be tightened to the right level of preload. Too much preload could damage the tank, while too little would fail to restrain it effectively. The team repeatedly measured assembly clearances, tightening torque and vibration response curves, making adjustment after adjustment. Finally, the test results met the required criteria.

SMILE will usher in a new era of space weather forecasting, moving from qualitative descriptions to quantitative predictions and significantly improving forecast accuracy and capability, Wang Chi said.

Source: Science and Technology Daily