Campfires on the Sun

Scientists studying the first images returned from a new European Space Agency spacecraft have found a remarkable array of never-before-seen solar features, including a large number of mini-flares they have dubbed “campfires”.

“It’s amazing how much stuff is going on,” says David Berghmans of the Royal Observatory of Belgium, principal investigator for the instrument used to take the images. “We started giving it crazy names like campfires and dark fibrils and ghosts.”

Of these, the campfires are presently the most interesting because they may help solve one of the Sun’s greatest mysteries: why the Sun’s corona is over one million degrees Celsius, while its surface is a relatively cool 5500 degrees.

“It’s counterintuitive,” says Daniel Müller, the mission’s project scientist, with ESA. “It’s as if you were to light a fire and as you move away it doesn’t get cooler [but] really starts to burn you.”

There are multiple theories for this, he says, but most are related to releases of energy from the solar magnetic field, possibly from a myriad of tiny flares that in combination release a great deal of magnetic energy.

It’s too soon to know whether the campfires are indeed flares, he says, but “because of their multitude, they could contribute to heating the solar corona.”

Not that anything on the Sun is truly tiny. Even the smallest campfires currently visible to Berghmans’s instrument measure 400 kilometres across, “about the size of a European country,” he says. Though, he adds, “I’m expecting that as we make higher-resolution [images] we will see yet smaller ones”.

Nor does anyone know how hot the campfires are or how much energy they are releasing. “They are very bright and very dynamic, which means there is a lot of energy in there,” Berghmans says, “but to determine the temperature and energy we need [other] instruments. The next phase is to combine data.”

The images were part of the first download of data from the ESA’s Solar Orbiter mission, which was launched on 10 February on a 10-year mission in which it will gradually move closer to the Sun. Eventually it will come as close to it as 42 million kilometres – slightly more than one-quarter the distance from the Sun to the Earth.

The present images were taken when it made its first close approach, at 123 million kilometres, about halfway to its ultimate target. But already they are the closest images ever taken of the Sun.

NASA’s Parker Solar Probe, launched in August 2018, is designed to get even closer – to within seven million kilometres. “But the environment that close is very harsh,” says Holly Gilbert, Director of the Heliophysics Science Division at NASA Goddard Space Flight Centre and a project scientist on Solar Orbiter (which is in part a collaborative effort between ESA and NASA).

Harsh enough, she says, that the Parker’s camera doesn’t even point at the Sun, but instead points away from it to collect images of what is going on behind the spacecraft. “Solar Orbiter is the limit where cameras can take photos of the Sun itself,” she says.

Not that taking photos is the only thing Solar Orbiter is designed to do. All told, it carries a suite of 10 instruments, including a magnetograph to map the Sun’s magnetic field by looking at its effect on spectral lines in the Sun’s visible light, plus four instruments designed to measure the solar wind as it speeds by the spacecraft. (The solar wind is a stream of charged particles emitted by the Sun).

Already, the magnetograph has allowed the scientists to view the magnetic field from two angles simultaneously: one with Earth-based telescopes, and the other from Solar Orbiter’s point of view.

“It’s the first magnetograph to look at the Sun from the side,” says the instrument’s principal investigator, Sami Solanki, director of the Max Planck Institute for Solar System Research in Göttingen, Germany. “We know the magnetic field is wholistic, and connects very different parts of the Sun. We are now starting to see the whole beast.”

Ultimately, Solar Orbiter will swing into an orbit that will carry it above the plane of the Solar System so that it can peer at the Sun’s poles—something that has never been done before.

“The poles are terra incognita,” says Solanki. “It’s like the Earth, 150 years ago. Nobody had been at the poles. There will be a lot of things to learn there.”

But that won’t happen until 2025.

Meanwhile, the spacecraft will continue to work its way closer to the Sun and look for opportunities when it lines up with two other solar-orbiting spacecraft capable of measuring the solar wind: NASA’s Parker Solar Probe, and BepiColombo, a joint mission between ESA and the Japanese Aerospace Exploration Agency (JAXA), currently en route to Mercury.

This, Gilbert says, will allow Solar Orbiter and whichever of the others is in the right position at the right time to simultaneously measure the solar wind in the same direction from the Sun at two different distances. Solar Orbiter will also peer directly at the Sun, hoping to unravel how the effects of activity on or near its surface propagate outward from the Sun, and ultimately to Earth.

“We will take advantage of these alignments when they occur,” says Christopher Owen, of University College London and Principal investigator of the Solar Orbiter’s solar wind analyser. With a small fleet of spacecraft now out there, he adds, “it’s a rich time for this kind of science.”

Ultimately, the primary goal in all of this research will be learning enough about the workings of solar flares and the Sun’s atmosphere to understand dangerous solar storms well enough to be able to provide advance warnings so that people here on Earth have time to take precautions if a threatening burst of particles is soon to be heading our way.