Our sun still puzzles researchers – and always provides surprises. One of them has now revealed images of the European space probe Solar Orbiter. From a distance of only 77 million kilometers – as close as no other probe before it – it created high-resolution images of the sun’s surface in UV light. The images reveal that our star is covered in countless miniature bursts of rays. With a diameter of 700 kilometers, these flares are millions of times smaller than the eruptions visible from Earth, but they are omnipresent. Their discovery could confirm a hypothesis that such nanoflares provide the energy for heating the solar corona.
Although our sun is the closest and best-studied star to us, many questions remain unanswered. It is still unclear why the solar corona – the sun’s outer atmosphere – is so much hotter than the sun’s surface. The mechanisms of the formation of the solar wind and the solar magnetic field have only been partially clarified. Although the NASA space probe Parker Solar Probe, launched in summer 2018, has provided initial information on the sources of the solar wind, it orbits our star on a trajectory that largely follows the solar equator. What is going on at the poles of the sun is invisible to them. That is why the European space agency ESA sent another space probe towards the sun in February 2020. Over the next few years, the Solar Orbiter will follow strongly elliptical orbits that are increasingly inclined towards the solar pole. At its closest point to the sun, it will be only 42 million kilometers away from the sun’s surface and orbit in an orbit that is inclined at 33 degrees from the equator.
“Campfire” on the sun
The Solar Orbiter has already passed the orbit of Venus since it was launched and is now roughly halfway between the Earth and the Sun. From this position – around 77 million kilometers from the sun – the space probe aimed the UV camera of its Extreme Ultraviolet Imager at the surface of our star. “The sun has never been photographed from such a close distance,” says ESA mission member David Long of University College London. “The level of detail they show is impressive.” The pictures show that the sun’s surface shows small bursts of rays everywhere. The bright spots measure barely more than 700 kilometers in diameter. “These miniature flares are millions of times smaller than the bursts of rays we can see from Earth,” says Long. “They are more like small campfires.” The researchers nicknamed this phenomenon “Campfires”.
It is not yet clear whether these mini-flares are only small versions of the known bursts of radiation or whether they are based on other mechanisms. The scientists hope to find out in the course of the solar orbiter mission through measurement data from the probe instruments. But it is already exciting what these little “campfires” could do. “These flares are completely insignificant in themselves, but if you add their effect across the entire surface of the sun, they could even be decisive for heating the solar corona,” explains Frédéric Auchère from the French Institute for Astrophysics and Space Research. Because the outer atmosphere of the sun has a temperature of more than a million degrees Celsius and is thus many times hotter than the “only” 5000 degree hot surface of the sun.
Driving force for heating the corona?
The new recordings of the “campfire” could now confirm a hypothesis. After this countless small outbreaks within the magnetic field loops provide the necessary energy. “It’s too early to be definitive, but we hope that by combining these observations with measurements from our other instruments that explore the solar wind as it passes the spacecraft, we can solve some of these puzzles,” said Yannis Zouganelis, deputy ESA project scientist for the Solar Orbiter. In addition to the Solar Orbiter’s UV camera, the spacecraft’s scientific instruments have already collected initial data. One of these is the double telescope PHI (Polarimetric and Helioseismic Imager) developed at the Max Planck Institute for Solar System Research. “The magnetic structures on the surface of the sun that PHI makes visible are the driving force behind all the processes that Solar Orbiter observes in the outer layers of the sun,” explains Sami Solanki, senior scientist at PHI. The PHI has already mapped an active region on the surface of the sun.
“We are all very happy about these first recordings – but they are just the beginning,” says Müller. “The Solar Orbiter is on a great journey through the inner solar system and will be much closer to the Sun in less than two years.”
Source: ESA, Max Planck Institute for Solar System Research