Solar Orbiter: Convert Images to Physics

The latest results from Solar Orbiter show that the mission is making the first direct connections between events on the solar surface and what is happening in interplanetary space around the spacecraft. It is also giving us new insights into solar “bonfires,” space weather, and disintegrating comets.

“I couldn’t be more satisfied with the performance of the Solar Orbiter and the various equipment that maintains it and its instruments running,” says Daniel Müller, ESA’s Solar Orbiter project scientist.

“It has been a real team effort under difficult circumstances this year, and now we are beginning to see that those efforts are really paying off.”

Solar Orbiter’s 10 science instruments are divided into two groups. There are six remote sensing telescopes and four instruments on site. Remote sensing instruments look at the sun and its extended atmosphere, the corona. Instruments in situ measure the particles around the spacecraft, which have been released by the sun and are known as the solar wind, along with their magnetic and electric fields. Tracing the origin of those particles and fields back to the solar surface is one of Solar Orbiter’s key goals.

During Solar Orbiter’s first close pass of the sun, which took place on June 15 and saw the spacecraft approach 77 million kilometers, both remote sensing and in-situ instruments were recording data.

Footprints of the solar wind

Data from Solar Orbiter has made it possible to calculate the region of origin of the solar wind hitting the spacecraft and to identify this ‘fingerprint’ in remote sensing images. In one example studied in June 2020, the footprint is seen at the edge of a region called a “coronal hole,” where the sun’s magnetic field reaches into space, allowing the solar wind to flow.

Although the work is preliminary, it is still beyond anything that has been possible so far.

“Never before have we been able to do such an accurate mapping,” says Tim Horbury of Imperial College London and chair of the Solar Orbiter In-Situ Task Force.

Campfire Physics

Solar Orbiter also has new information about the ‘bonfires’ of the sun that captured the world’s attention earlier this year.

The first images from the mission showed a multitude of what appeared to be small solar flares erupting on the surface of the sun. Scientists called them campfires because the exact energy associated with these events is not yet known. Without the power, it is still unclear if they are the same phenomenon as other smaller-scale eruptive events that have been seen by other missions. What makes it all so tempting is that it was long thought that small-scale ‘nano-flares’ existed in the sun, but never before have we had the means to see such small events.

“Campfires could be the nano-flares we are looking for with Solar Orbiter,” says Frédéric Auchère, Institut d’Astrophysique Spatiale, Orsay, France, and Chairman of the Solar Orbiter Remote Sensing Working Group.

This is important because nano-flares are theorized to be responsible for heating the corona, the sun’s outer atmosphere. The fact that the corona is about a million degrees Celsius while the surface is only about 5000 degrees remains one of the most puzzling problems in solar physics today. Investigating this mystery is one of Solar Orbiter’s key science goals.

To explore the idea, the researchers have been analyzing data with Solar Orbiter’s SPICE (Spectral Imaging of the Coronal Environment) instrument. SPICE is designed to reveal the velocity of gas on the solar surface. It has shown that there are indeed small-scale events where gas moves with significant velocity, but a correlation with campfires has not yet been sought.

“Right now, we only have commissioning data, taken when teams were still learning the behavior of their instruments in space, and the results are very preliminary. But clearly, we see very interesting things,” says Frédéric. “Solar Orbiter is all about discovery, and that’s very exciting.”

Surfing the tail of a kite

In addition to progress toward Solar Orbiter’s planned science goals, there has also been serendipitous science from the spacecraft.

Shortly after the launch of Solar Orbiter, it was noted that it would fly downstream of Comet ATLAS, passing through its two tails. Although the Solar Orbiter was not designed for such an encounter, and was not supposed to take scientific data at this time, mission experts worked to ensure that all instruments in situ recorded the single encounter.

But nature had one more trick to play: the comet disintegrated before the spacecraft got close. So instead of the expected strong signals from the tails, it was quite possible that the spacecraft saw nothing at all.

That was not the case. Solar Orbiter saw signatures in the comet ATLAS data, but not the kind of thing scientists would normally expect. Instead of a single strong tail crossing, the spacecraft detected numerous wave episodes in the magnetic data. It also detected dust in patches. This probably broke free from the interior of the comet when it broke up into many small pieces.

“This is the first time that we essentially traveled through the wake of a disintegrating comet,” says Tim. “There’s a lot of really interesting data there, and it’s another example of the kind of high-quality serendipitous science that we can do with the Solar Orbiter.”

Stealth space weather

Solar Orbiter has been measuring the solar wind for much of its time in space, recording a series of ejections of particles from the sun. Then, on April 19, a particularly interesting coronal mass ejection swept through the Solar Orbiter.

A coronal mass ejection, or CME, is a large space weather event, in which billions of tons of particles can be ejected from the sun’s outer atmosphere. During this particular CME, which rose from the sun on April 14, the Solar Orbiter traveled approximately twenty percent of the way from Earth to the sun.

Solar Orbiter was not the only spacecraft to observe this event. ESA’s BepiColombo Mercury mission was flying close to Earth at the time. There was also a NASA solar spacecraft called STEREO located about ninety degrees from the direct sun-Earth line, and looking directly across the area of ​​space that the CME was traveling through. He observed the impact of the CME on Solar Orbiter and then on BepiColombo and Earth. Combining the measurements from all the different spacecraft allowed the researchers to actually study how the coronal mass ejection evolved as it traveled through space.

This is known as multipoint science, and thanks to the number of spacecraft in the inner solar system now, it will become an increasingly powerful tool in our quest to understand the solar wind and space weather.

“We can look at it remotely, we can measure it in situ, and we can see how a CME changes as it travels towards Earth,” says Tim.

Perhaps as intriguing as the spacecraft that saw the event were the ones that didn’t. ESA-NASA’s SOHO spacecraft, which faces Earth and constantly watches the sun for flares like this, barely recorded it. This places the April 19 event in a rare class of space weather events, called a stealth CME. Studying these more elusive events will help us understand space weather more fully.

In the coming years, opportunities for multipoint science will increase. On December 27, Solar Orbiter will complete its first flyby of Venus. This event will use the planet’s gravity to bring the spacecraft closer to the sun, placing the Solar Orbiter in an even better position for joint measurements with NASA’s Parker solar probe, which will also complete two flybys of Venus in 2021.

As Parker makes in situ measurements from inside the solar atmosphere, the Solar Orbiter will image the same region. Together, the two spacecraft will give both the details and the big picture.

“2021 will be an exciting time for Solar Orbiter,” says Teresa Nieves-Chinchilla, a scientist for NASA’s Solar Orbiter project. “By the end of the year, all the instruments will be working together in full science mode, and we will be preparing to get even closer to the sun.”

In 2022, the Solar Orbiter will approach 48 million kilometers from the surface of the sun, more than 20 million kilometers closer than it will be in 2021.

---

---