A close look at solar fires: Newly analyzed data provides high-resolution views of the entire solar disk for the first time

The entire solar disk in unprecedented detail. This is illustrated by images of the visible surface of the Sun. Researchers at the Max Planck Institute for Solar System Research created it from 25 individual images taken by the ESA space probe Solar Orbiter.

When observed in March 2023, the satellite was only half the distance from the sun to Earth. Here, the solar disk was already too large to fit in one photo. If you zoom in on images from the various instruments currently available to the public, you can see where the Sun is showing its temper. The surface resembles that of boiling water.

Here, plasma is rising from inside the Sun. Dark sunspots are also spots with particularly strong magnetic fields. A wide loop of magnetic field larger than the Earth creates a racetrack for solar plasma, around which it flies at speeds of over 100,000 km/h.

No other celestial body in our solar system is as dynamic and complex as the Sun. To understand as much as possible its vagaries, ESA’s Solar Orbiter probe set out in February 2020 with a total of six instruments pointing in the direction of our star, and will explore the different layers of our star. I looked down. .

The Max Planck Institute for Solar System Research sent hardware for four instruments on the trip to the bolide. For example, EUI captures the sun’s particularly short-wave ultraviolet radiation, which comes primarily from the hot outside air, the solar corona.

The double telescope PHI focuses on the visible surface below, the photosphere. The light it emits also contains information about the strength of the sun’s magnetic field and the speed of the solar plasma. The images published today are derived from EUI and PHI data as of March 22, 2023.

as close as possible

“If we want to understand the complete picture of the Sun, it is essential to study all its layers at high resolution simultaneously,” said Professor Sami K. Solanki, MPS Director and PHI Principal Investigator. “Solar Orbiter is capable of this unlike any previous space probe.”

In addition to its extensive instrumentation, another advantage of Solar Orbiter is its unusual orbit. The spacecraft orbits the sun in a long ellipse, allowing it to repeatedly approach the star to less than a third of the distance between Earth and the sun. This equates to approximately 42 million kilometers.

Mosaic of 25 individual images

On March 22 last year, Solar Orbiter and the Sun were approximately 74 million kilometers apart. At this “close proximity”, the Sun is too large to completely fit into the field of view of PHI’s high-resolution telescope. Instead, a total of 25 images of parts of the Sun were taken over several hours, and researchers on the PHI team combined them in a mosaic to create a view of the entire disk.

“For example, the information needed for a magnetic map of the Sun is hidden in a small fraction of the captured light,” explains MPS scientist Dr. Johan Hirzberger of the PHI team that created the mosaic.

“Therefore, the data can hardly be compressed on the spacecraft. Due to the long distance to Earth and the relatively low data transfer rate, the vast amounts of data generated in this way are only available months after the actual observation. It may even reach us.”

Because Solar Orbiter continues to travel around the sun during measurements, each individual image is all taken from a slightly different perspective. These effects must be carefully considered when piecing together a mosaic.

Nevertheless, the PHI team expects that in the future it will be able to provide similar high-resolution views of the entire solar disk more quickly and regularly, about twice a year. These help us understand how our grasp of the Sun as a whole is governed by its smallest structures and processes.

The image of the entire photosphere disk released today has a resolution of about 175 kilometers per pixel. In terms of detail, it falls short of what can be obtained with the most powerful solar telescopes on Earth. For example, the Gregor Solar Telescope in Tenerife, which also includes the Max Planck Institute for Solar System Research, can depict structures just 50 kilometers wide in a pixel using a 1.5-meter mirror.

However, ground-based telescopes can only take high-resolution images of a small portion of the sun’s surface. And because of the difficult observation conditions on Earth, where turbulence always obstructs visibility, it is almost impossible to combine these “sun fragments” to form a whole. Simultaneous imaging of the corona from Earth is also impossible because Earth’s atmosphere also absorbs most of the Sun’s ultraviolet radiation.

Sunspots and complex magnetic fields

Zooming in on a new image of the Sun reveals the star’s full complexity and beauty. In visible light, a granular pattern covers the photosphere. This is a representation of hot plasma rising in the sun, cooling, and sinking again, much like water boiling on a stove. You can also see sunspots, which are dark areas on the sun’s surface.

The sun’s magnetic field is particularly strong in these regions, as shown by PHI’s magnetic map, the magnetogram. Prevents high temperature plasma from rising from deep inside. Therefore, in sunspot regions, the sun’s surface is cooler and appears darker. The different colors in the magnetogram represent the strength and direction of the magnetic field. The strongest fields are shown in red (pointing outward) and blue (pointing inward).

Every 11 years, the surface of the sun falls into chaos.

These data provide a detailed overview of extreme processes inside and outside the Sun. For example, we need to find out how magnetic fields form and why the Sun is especially active every 11 years. As shown by the red and blue tachograms in the adjacent figure, it is known that as the sun rotates, the plasma inside it rotates as well, as if a fishbowl were being violently stirred.

The magnetic field thus generated is rolled up as a result of the rotation of the plasma ball, causing the field lines to become jumbled and form loops, especially over the sunspot. Along these loops, solar plasma rises and then sinks back toward the surface. This movement can also be seen on the tachogram.

When a magnetic short circuit occurs, the Sun sends charged plasma particles into space. When these particles collide with Earth’s magnetic field, the fluorescent effects of solar particles in Earth’s atmosphere cause auroras to glow, especially in the polar regions. The image shown here shows the Sun in a chaotic period of activity, with more sunspots than usual.

However, the Sun is in this state only once every 11 years, and is otherwise inactive. The Sun’s magnetic field is actually more ordered and dipole-shaped, similar to Earth’s magnetic field. Rotation also plays a role here. The theory is that this will create a stream of hot plasma that rises and falls inside the Sun, circling like a dynamo as the Sun rotates. Every 9 to 13 years, this magnetic field completely reverses and goes through the chaotic state described above.