ESA’s Solar Orbiter offers the first high-resolution view of the entire solar disk

The Sun is the most dynamic and complex body in our Solar System. To study it more closely, the ESA’s Solar Orbiter spacecraft launched in February 2020 with six scientific instruments to explore its layers. The Max Planck Institute for Solar System Research contributed hardware for four instruments.

One of them, EUI, captures the Sun’s short-wave ultraviolet radiation from the hot outer atmosphere, the corona. Another, the PHI double telescope, observes the photosphere, the Sun’s visible surface, providing insights into the strength of the Sun’s magnetic field and the velocity of solar plasma.

Prof. Dr. Sami K. Solanki, MPS Director, and PHI Principal Investigator, said, “If you want to understand the Sun in its entirety, it is essential to look into all its layers simultaneously and with high resolution. Solar Orbiter is capable of this like no space probe before.”

“In addition to its extensive instrumentation, another of Solar Orbiter’s advantages is its unusual trajectory. It takes the spacecraft on long ellipses around the Sun – and thus repeatedly allows it to approach our star to less than a third of the distance between the Earth and the Sun. This corresponds to around 42 million kilometers.”

On 22 March of the previous year, the Solar Orbiter was about 74 million kilometers from the Sun. At this distance, the Sun was too large to fit entirely within PHI’s telescope’s high-resolution field of view. As a result, the PHI team took 25 images of different parts of the Sun over several hours. These images were later combined into a mosaic, giving researchers full-disk views of the Sun.

Only a tiny part of the captured light contains the information required for the magnetic maps of The Sun. The data collected by the Solar Orbiter is challenging to compress on board the spacecraft. Due to the vast distance from Earth and the relatively low data transfer rate, the large volumes of data generated often take months to reach Earth after the observations are made.

As the Solar Orbiter continues its journey around the Sun during measurements, each image is taken from slightly different perspectives. These variations must be carefully accounted for when assembling the mosaics. Despite this challenge, the PHI team anticipates providing similar high-resolution views of the entire solar disk more frequently in the future, approximately twice a year. These images will help deepen our understanding of how the Sun’s overall behavior is influenced by its smallest structures and processes.

The full-disk images of the photosphere released today have a resolution of about 175 kilometers per pixel, lower than those captured by powerful solar telescopes on Earth, such as the Gregor solar telescope in Tenerife, which can resolve structures as small as 50 kilometers per pixel.

However, ground-based telescopes can only capture small sections of the Sun’s surface in high resolution, and atmospheric turbulence on Earth makes it difficult to obtain clear, continuous images. Additionally, Earth’s atmosphere absorbs most of the Sun’s ultraviolet radiation, preventing simultaneous observations of the solar corona from the ground.

The new images of the Sun reveal its full complexity and beauty. In visible light, a granular pattern covers the photosphere, caused by hot plasma rising, cooling, and sinking—similar to boiling water. Sunspots, dark areas on the surface, are also visible.

PHI’s magnetic map, or magnetogram, shows that the Sun’s magnetic field is strongest in these sunspot regions, preventing hot plasma from rising. As a result, the surface in these areas is more excellent and darker. The magnetogram uses different colors to represent the strength and direction of the magnetic field, with the strongest fields shown in red (outward) and blue (inward).

The data provide a detailed view of the extreme processes inside and outside the Sun. They aim to uncover how the Sun’s magnetic field is created and why its activity peaks every eleven years. The Sun’s rotation, shown in the tachogram, causes the plasma in its interior to twist like a stirred goldfish bowl.

This rotation winds up magnetic fields, forming loops, especially above sunspots, along which solar plasma rises and sinks. When magnetic fields “short-circuit,” the Sun ejects charged plasma particles into space in chaotic phases. These particles interact with Earth’s magnetic field, creating auroras at the poles. The image depicts the Sun during such an active phase with increased sunspots.

The Sun reaches its most chaotic and active state every eleven years, marked by increased sunspots and magnetic disturbances. During the rest of its cycle, the Sun’s magnetic field is more orderly and dipole-shaped, similar to Earth’s. This stable field is influenced by the Sun’s rotation, which causes hot plasma currents to rise, fall, and circulate like a dynamo. Every 9 to 13 years, the Sun’s magnetic field completely inverts, transitioning into the more chaotic state described earlier.