Solar Orbiter’s polar views and observing methods

A new view of the solar poles

Solar Orbiter imaged the Sun’s south pole on Mar 16–17, 2025, from 15° below the solar equator. The mission reached 17°. Ulysses had visited high solar latitudes, without an imager. Hamish Reid of University College London’s Mullard Space Science Laboratory anticipated discoveries from the new perspective; Lucie Green expected polar flows to clarify the 11-year cycle.

PHI detected mixed magnetic polarities at the south pole around solar maximum. Team leader Sami Solanki emphasised uncertainty about the subsequent rebuilding of an orderly field. SPICE measured carbon-ion motion through Doppler shifts; Frédéric Auchère expected high-latitude measurements to improve the previous grazing view. Solar Wind Analyser principal investigator Chris Owen anticipated three-dimensional coverage. ESA’s Daniel Müller anticipated improvements with orbital tilting. North-pole images awaited download; full data were expected by October 2025. Polar vortices remained a research prospect. EUI examined coronal plasma reaching approximately 3 million °C (5.4 million °F).

Venus changes the observing angle

Venus redirected Solar Orbiter towards higher solar latitudes. ESA scheduled the Feb 18, 2025 flyby for 21:48 CET, with a closest approach of 379 km. Flight-dynamics specialist Julia Schwartz explained that a close encounter could substantially alter the orbit with little fuel. Earlier gravity assists at Earth and Venus had gradually reduced the spacecraft’s solar distance. February’s encounter changed the viewing geometry.

The team at the European Space Operations Centre in Germany planned small course corrections and simulated heating from Venus. Sam Bammens of the flight-control team expected component temperatures to remain within design limits. The spacecraft had to keep its heat shield facing the Sun, preventing its cameras from pointing at Venus. Magnetometer and Radio and Plasma Waves sensors were nevertheless scheduled to sample the planet’s environment. Venus has no global magnetic field, but its ionosphere interacts with the solar wind. These measurements supplemented the orbital manoeuvre.

A dark disc becomes a calibration target

Mercury crossed Solar Orbiter’s view of the Sun on Jan 3, 2023. PHI recorded the planet as a dark circular silhouette, distinct from the irregular sunspots elsewhere on the disc. EUI followed its movement in ultraviolet light, including the interval when Mercury had moved beyond the bright disc but remained in front of structures in the solar atmosphere. Several instruments therefore observed the same foreground object against different parts of the Sun.

For calibration, Mercury offered a particularly useful property: its silhouette should be black. Any brightness recorded within that silhouette arose from the instrument’s transmission of surrounding light, characterised by its point-spread function. Knowing that response more accurately allows scientists to correct their observations. SPICE (Spectral Imaging of the Coronal Environment) also separated emissions associated with different atmospheric temperatures: hydrogen at 10,000 K, carbon at 30,000 K, oxygen at 320,000 K and neon at 630,000 K. These temperatures concern spectral emissions, not Mercury. The transit tested imaging performance across atmospheric layers.

Building a full-disc image from smaller views

PHI (Polarimetric and Helioseismic Imager) and EUI (Extreme Ultraviolet Imager) acquired coordinated observations on Mar 22, 2023, released on Nov 20, 2024. Solar Orbiter was less than 74 million kilometres from the Sun. At that distance, each high-resolution frame covered only part of the visible disc. The spacecraft reoriented between exposures. Each instrument’s completed mosaic combined 25 images captured over more than four hours, with the resulting solar disc almost 8,000 pixels across.

The products measured:

• PHI’s visible-light image recorded the photosphere and dark sunspots.
• Its magnetogram mapped magnetic-field direction, with opposite colours representing outward and inward fields.
• Its velocity map distinguished material moving towards the spacecraft from material moving away.
• EUI recorded ultraviolet emission from the corona above the visible surface.

Sunspots are cooler than their surroundings because strong fields interfere with convective heat transport. Above active regions, coronal plasma follows magnetic structures that can connect neighbouring sunspots. The PHI team expected to produce comparable high-resolution mosaics twice annually after the initial processing.

An extra function for an ultraviolet camera

EUI acquired an additional way to observe the corona through a small modification made during construction. A protrusion weighing only a few grams was added to its safety door. With the door stopped partway open, the protrusion obscures the bright solar disc. The camera can then record ultraviolet coronal light about a million times fainter. Frédéric Auchère proposed the modification, which became known as the instrument’s occulter mode.

Tests began in 2021, and the team presented the successful results in 2023. A demonstration combined EUI’s coronal observations with a central solar-disc image from NASA’s STEREO mission, which happened to be observing from almost the same direction. The central disc in that composite was therefore supplied by a different spacecraft. Conventional coronagraphs, including Solar Orbiter’s Metis, are dedicated to suppressing the bright disc. EUI’s modification combined ordinary imaging and occultation within one instrument. It opened a region beyond the usual field of extreme-ultraviolet imagers but often hidden by traditional coronagraphs. David Berghmans of the Royal Observatory of Belgium identified that poorly observed region as an opportunity to investigate changing magnetic structures.

Matching nearby particles to distant source regions

Solar Orbiter’s instruments must reconcile two different clocks. Cameras record the Sun as it appears after light has travelled to the spacecraft. Particle sensors sample solar wind released days earlier. Stephanie Yardley’s team used observations collected from Mar 1–9, 2022, at roughly 75 million kilometres from the Sun to connect those measurements. Their work, published in May 2024, linked changes in locally measured wind to source regions observed at high resolution.

The Magnetic Connectivity Tool supported the targeting. It used observations from the six-telescope Global Oscillation Network Group to model how solar wind propagated through the Solar System. Predictions guided targeting and estimated plasma arrival. Solar Orbiter crossed streams associated with a coronal hole and an active region, recording transitions between faster and slower wind alongside changes in temperature and composition. The distinction used in this study placed fast wind above 500 km/s and slow wind below that value. The streams retained identifiable source-region characteristics. This linked images and plasma measurements, not individually tracked particles.

Short-lived jets at a coronal hole

EUI observations acquired on Mar 30, 2022 revealed small plasma jets near the south pole, a result published in August 2023. Individual jets lasted 20–100 seconds and expelled material at about 100 km/s. The instrument’s high-resolution channel observed million-degree plasma at a wavelength of 17.4 nanometres. Lakshmi Pradeep Chitta and colleagues could identify the faint events because the camera combined fine spatial detail with rapid sampling.

The jets appeared in a coronal hole, where magnetic field lines extend out into the Solar System instead of returning to the surface. Such open structures provide routes for plasma to escape. Each detected event carried around a thousandth of the energy of a nanoflare, with much of its energy going into plasma expulsion. Their prevalence suggested a substantial contribution to the wind and an intermittent flow rather than an entirely steady outpouring. Smaller events could also contribute. These remained interpretations of the observations, not a completed inventory of all wind sources. At the time, the camera still viewed the southern region at a grazing angle; higher-latitude observations offered a way to examine the same processes from a different direction.

Two spacecraft observe different stages of a flow

In March 2024, Solar Orbiter and Parker Solar Probe approached the Sun with their viewing directions approximately at right angles. ESA planned coordinated observations so that Solar Orbiter could image a source region before its outflow reached Parker a few hours later. Their distances at closest approach were approximately 45 million kilometres and 7.3 million kilometres respectively. The more distant spacecraft could therefore observe an event before the closer spacecraft encountered the emitted plasma.

The difference followed from transport speeds. Light took about 2.5 minutes to reach Solar Orbiter at that distance, whereas the wind travelled at more than a million kilometres per hour. Parker’s particle instruments could sample the outflow relatively close to its origin, before its properties changed further. Andrei Zhukov anticipated particularly useful comparisons if an observed coronal mass ejection subsequently crossed Parker. That was an observing opportunity, not a claim that such an encounter had already happened. Separately, Predictive Science planned to incorporate PHI magnetic measurements into its evolving model of the corona for the Apr 8, 2024 total eclipse, adding a viewpoint unavailable from Earth alone.

A coordinated estimate of coronal heating

Another joint experiment required Solar Orbiter to depart from its normal pointing. On Jun 1, 2022, the spacecraft rolled 45° and pointed slightly away from the Sun so that Parker Solar Probe entered Metis’s field of view. Daniele Telloni of the Italian National Institute for Astrophysics identified the opportunity. Metis suppressed the bright solar surface to observe the broader corona, while Parker measured plasma directly within the observed region. The resulting analysis was published in 2023.

The combination gave researchers an estimate of the heating rate using large-scale coronal observations alongside local plasma properties. The corona reaches around a million degrees Celsius, compared with approximately 6,000 degrees at the visible surface. Turbulence offers a mechanism for transferring energy through increasingly small scales until fluctuations interact with individual particles, mainly protons. In a magnetised plasma, both motion and magnetic fields supply stored energy. The measured rate supported turbulent energy transfer as an explanation for coronal heating. The researchers nevertheless retained an important limitation: further work was needed before the heating problem could be considered solved. A successful coordinated measurement did not establish one universal heating mechanism for every solar region.

Comparing the wind’s energy at two distances

A different alignment in February 2022 allowed both spacecraft to sample the same solar-wind stream at successive stages. Parker encountered it around 9 million kilometres from the Sun; Solar Orbiter crossed it roughly a day or two later at 89 million kilometres. Yeimy Rivera and Samuel Badman led an analysis published in August 2024 that compared the stream’s energy at the two locations. The energy accounting included the contribution stored in magnetic waves.

Near Parker, approximately 10% of the total energy was associated with the magnetic field. At Solar Orbiter, the fraction was around 1%, while the plasma had accelerated and cooled less than expected from expansion alone. Omitting the wave-energy contribution prevented the two energy estimates from agreeing. Large-amplitude Alfvén waves supplied the inferred acceleration and heating. Magnetic switchbacks—large deflections in field direction—formed part of this wave activity. The result concerned fast solar wind, defined here as exceeding 500 km/s. Analysis of slower wind remained ongoing; the result concerned fast streams.

Testing recovery without losing the protective orientation

Solar Orbiter’s close approaches also require preparations for interrupted communications. In safe mode, basic systems restart while the heat shield remains directed towards the Sun. A Sun sensor supplies the solar direction; star trackers normally determine the remaining orientation needed to aim the communications antenna at Earth. Flight controller Daniel Lakey described the additional difficulty if those trackers failed to identify suitable stars during recovery.

The backup antenna moves along only one axis. Without a known Earth direction, the spacecraft can roll while keeping its protected face sunward, sweeping a distinctive radio beacon across space. A ground station detecting that tone could establish the spacecraft’s condition and support recovery. ESA reported a successful in-flight rehearsal on Apr 2, 2024. Commands to restore normal operation had been loaded in advance in case the beacon was not detected. The team also tested communications when the heat shield partly obstructed the antenna. This was a planned recovery exercise, not an actual spacecraft emergency. Power availability is another constraint near perihelion because the arrays must tilt away from intense sunlight to avoid damage.

Radio measurements invite a different kind of inspection

In March 2025, the mission opened its first citizen-science investigation through the Solar Radio Burst Tracker project on Zooniverse. Five years of Radio and Plasma Waves measurements had been divided into six-hour segments, producing 15,000 graphs for inspection. Katerina Pesini, working at Radboud University and Paris Observatory, led the project. Volunteers were asked to identify and outline radio bursts, including faint or complicated examples that automated algorithms could miss.

Each graph was to receive eight independent inspections. The planned catalogue would record burst dates, times and intensities and support future training of automatic detection systems. The target events were Type III solar radio bursts. Electrons expelled during solar flares interact with surrounding charged particles and generate radio waves. Their occurrence varies with solar activity. The project aimed to examine those changes across the solar cycle using the instrument’s wide frequency coverage and sensitivity. Higher-latitude observations would eventually add measurements from nearer the polar directions. These were research and catalogue-building objectives; launching the volunteer project did not mean that its finished catalogue or a trained detection system already existed.

A later study follows a flare’s buildup

Results released on Jan 21, 2026 examined a flare observed on Sep 30, 2024. EUI, SPICE, STIX and PHI recorded complementary parts of its evolution. EUI resolved features a few hundred kilometres across at intervals of two seconds or less. Observations began around 23:06 UT, roughly 40 minutes before the peak. A twisted filament connected to a brightening cross-shaped magnetic structure, where smaller instabilities began to interact.

A brightening at 23:29 UT preceded the filament’s partial detachment and the main flare at approximately 23:47 UT. Chitta’s team interpreted the sequence as an avalanche of reconnection events. SPICE and STIX followed energy deposition and cooling, while PHI measured the imprint at the visible surface. Accelerated particles reached 40–50% of light speed, and downward-moving plasma continued after the main flare. The observations supported an avalanche for this event. Whether the same process explains all solar flares, or flares on other stars, remained open. Observing windows and onboard memory restrict such detailed sequences.

Composition adds another connection to the surface

On Oct 8, 2026, a further study examined a large magnetic switchback sampled when Solar Orbiter was roughly halfway between Earth and the Sun. Jesse Coburn and colleagues used the Solar Wind Analyser to measure charged oxygen and carbon within the structure. The ion mixture indicated an origin in hot magnetic loops near the solar surface. Researchers combined those measurements with solar images and magnetic-field modelling, including NASA Solar Dynamics Observatory data, to locate the likely source.

The interpretation involved interchange reconnection: a closed magnetic loop interacts with an open structure, rearranging the field and allowing previously confined plasma to escape. Wave activity and turbulence could then influence the switchback farther from the Sun. Co-author Stephanie Yardley considered these processes complementary stages rather than mutually exclusive explanations. The study connected a particular sampled structure to its solar origin through composition, modelling and images. Together with polar imaging and earlier coordinated experiments, it extended the mission’s measurements across multiple locations and physical scales. It did not remove the continuing uncertainties about how all solar-wind structures form, evolve and contribute to changing space-weather conditions.