Sunday, June 16, 2024

Solar Orbiter delivers spectacular images of the Sun

Powerful flares, breathtaking views across the solar poles, and a curious solar “hedgehog” are among the haul of spectacular images, movies, and data returned by Solar Orbiter from its first close approach to the Sun. Although the analysis of the new dataset has only just started, it is clear that the ESA-led mission is providing the most extraordinary insights into the sun’s magnetic behaviour and the way this shapes space weather.

Solar Orbiter’s closest approach to the Sun, known as perihelion, took place on March 26. The spacecraft was inside the orbit of Mercury, at about one-third the distance from the Sun to the Earth, and its heatshield was reaching around 500C. But it dissipated that heat with its innovative technology to keep the spacecraft safe and functioning.

Solar Orbiter carries 10 science instruments – nine are led by ESA member states and one by NASA – all working together in close collaboration to provide unprecedented insight into how our local star “works.” Some are remote-sensing instruments that look at the Sun, while others are in-situ instruments that monitor the conditions around the spacecraft, enabling scientists to “join the dots” from what they see happening at the Sun, to what Solar Orbiter “feels” at its location in the solar wind millions of kilometres away.

When it comes to perihelion, clearly the closer the spacecraft gets to the Sun, the finer the details the remote sensing instrument can see. As luck would have it, the spacecraft also soaked up several solar flares and even an Earth-directed coronal mass ejection, providing a taste of real-time space weather forecasting, an endeavour that is becoming increasingly important because of the threat space weather poses to technology and astronauts.

Introducing the solar hedgehog

“The images are really breathtaking,” says David Berghmans, Royal Observatory of Belgium, and the principal investigator (PI) of the Extreme Ultraviolet Imager (EUI) instrument, which takes high-resolution images of the lower layers of the Sun’s atmosphere, known as the solar corona. This region is where most of the solar activity that drives space weather takes place.

The task now for the EUI team is to understand what they are seeing. This is no easy task because Solar Orbiter is revealing so much activity on the Sun at the small scale. Having spotted a feature or an event that they cannot immediately recognize, they must then dig through past solar observations by other space missions to see if anything similar has been seen.

“Even if Solar Obiter stopped taking data tomorrow, I would be busy for years trying to figure all this stuff out,” says Berghmans.

One particularly eye-catching feature was seen during this perihelion. For now, it has been nicknamed “the hedgehog.” It stretches 25,000 kilometres across the Sun and has a multitude of spikes of hot and colder gas that reach out in all directions.

Joining the dots

Solar Orbiter’s main science goal is to explore the connection between the Sun and the heliosphere. The heliosphere is the large “bubble” of space that extends beyond the planets of our solar system. It is filled with electrically charged particles, most of which have been expelled by the sun to form the solar wind. It is the movement of these particles and the associated solar magnetic fields that create space weather.

To chart the Sun’s effects on the heliosphere, the results from the in-situ instruments, which record the particles and magnetic fields that sweep across the spacecraft, must be traced back to events on or near the visible surface of the sun, which are recorded by the remote sensing instruments.

This is not an easy task as the magnetic environment around the sun is highly complex, but the closer the spacecraft can get to the Sun, the less complicated it is to trace particle events back to the Sun along the “highways” of magnetic field lines. The first perihelion was a key test of this, and the results so far look very promising.

On March 21, a few days before perihelion, a cloud of energetic particles swept across the spacecraft. It was detected by the Energetic Particle Detector (EPD). The most energetic of them arrived first, followed by those of lower and lower energies.

“This suggests that the particles are not produced close to the spacecraft,” says Javier Rodríguez-Pacheco, University of Alcalá, Spain, and EPD’s PI. Instead, they were produced in the solar atmosphere, nearer the Sun’s surface. While crossing space, the faster particles pulled ahead of the slower ones, like runners in a sprint.

On the same day, the Radio and Plasma Waves (RPW) experiment saw them coming, picking up the strong characteristic sweep of radio frequencies produced when accelerated particles – mostly electrons – spiral outwards along the sun’s magnetic field lines. RPW then detected oscillations known as Langmuir waves. “These are a sign that the energetic electrons have arrived at the spacecraft,” says Milan Maksimovic, LESIA, Observatoire de Paris, France, and RPW PI.

Of the remote sensing instruments, both EUI and the X-ray Spectrometer/Telescope (STIX) saw events on the Sun that could have been responsible for the release of the particles. While the particles that stream outwards into space are the ones that EPD and RPW detected, it is important to remember that other particles can travel downwards from the event, striking the lower levels of the Sun’s atmosphere. This is where STIX comes in.

While EUI sees the ultraviolet light released from the site of the flare in the atmosphere of the Sun, STIX sees the X-rays that are produced when electrons accelerated by the flare interact with atomic nuclei in the lower levels of the Sun’s atmosphere.

Exactly how these observations are all linked is now a matter for the teams to investigate. There is some indication from the composition of the particles detected by EPD that they were likely accelerated by a coronal shock in a more gradual event rather than impulsively from a flare.

“It could be that you have multiple acceleration sites,” says Samuel Krucker, FHNW, Switzerland, and PI for STIX.

Adding another twist to this situation is that the Magnetometer instrument (MAG) did not register anything substantial at the time. However, this is not unusual. The initial eruption of particles, known as a Coronal Mass Ejection (CME), carries a strong magnetic field that MAG can easily register, but energetic particles from the event travel much faster than the CME and can rapidly fill large volumes of space, and therefore be detected by Solar Orbiter. “But if the CME misses the spacecraft, then MAG will not see a signature,” says Tim Horbury, Imperial College, U.K., and MAG PI.

When it comes to the magnetic field, it all begins at the Sun’s visible surface, known as the photosphere. This is where the internally generated magnetic field bursts into space. To know what this looks like, Solar Orbiter carries the Polarimetric and Helioseismic Imager (PHI) instrument. This can see the north and south magnetic polarity on the photosphere, as well as the rippling of the sun’s surface due to seismic waves travelling through its interior.

Coming soon

The perihelion was a huge success and has generated a vast quality of extraordinary data. And it’s just a taste of what is to come. Already the spacecraft is racing through space to line itself up for its next – and slightly closer – perihelion pass on Oct. 13 at 0.29 times the Earth-Sun distance. On Sept. 4, it will make its third flyby of Venus.

Solar Orbiter has already taken its first pictures of the Sun’s largely unexplored polar regions, but much more is to come.

On Feb. 18, 2025, Solar Orbiter is scheduled to encounter Venus for a fourth time. This will increase the inclination of the spacecraft’s orbit to around 17 degrees. The fifth Venus flyby on Dec. 24, 2026, will increase this still further to 24 degrees, and will mark the start of the “high-latitude” mission.

In this phase, Solar Orbiter will see the Sun’s polar regions more directly than ever. Such line-of-sight observations are key to disentangling the complex magnetic environment at the poles, which may hold the secret to the Sun’s 11-year cycle of waxing and waning activity.

“We are so thrilled with the quality of the data from our first perihelion,” says Daniel Müller, ESA Project Scientist for Solar Orbiter. “It’s almost hard to believe that this is just the start of the mission. We are going to be very busy, indeed.”

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