Solar Orbiter, from the European Space Agency (ESA), delivers four images of the Sun which were assembled from observations made by two instruments on board the probe. These never-before-seen images are complete views of the visible surface of the Sun taken at the highest resolution to date.
Solar Orbiter was less than 74 million kilometers from the Sun when these images were captured on March 22, 2023: they are composed of a mosaic of photographs (read box).
No object in our Solar System is as dynamic and multifaceted as the Sun. The ESA probe scrutinizes it with no less than six instruments which allow it to dissect its different strata and understand it better.
For these four portraits, two instruments were used: the Polarimetric and Helioseismic Imager (PHI) works in visible light and traces maps of the disordered magnetic field of the Sun and the movements on its surface. Information which can be compared with the images produced by the Extreme Ultraviolet Imager (EUI) which reveals the incandescent outer atmosphere of the Sun, also called the solar corona, in the ultraviolet wavelength.
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The surface of the Soleil is made up of a plasma – a charged gas – incandescent and hot, in perpetual movement. Almost all solar radiation is emitted by this layer, whose temperature is between 4500 and 6000°C. Below, hot, dense plasma is churned in what scientists call the Sun’s convection zone, much like magma in the Earth’s mantle. This movement gives the surface of the Sun a grainy appearance.
The most striking features in the images are sunspots that look, in visible light, like dark marks or holes in an otherwise smooth surface. Sunspots are cooler than their surroundings and therefore emit less light.
Magnetogram and tachogram
The magnetic map, or magnetogram, drawn by PHI shows that the Sun’s magnetic field is concentrated in the sunspot regions: it is oriented either outwards (red) or inwards (blue), where find these spots. The intense magnetic field explains why the plasma inside sunspots is colder. Normally, convection moves heat from the Sun’s interior to its surface, but this phenomenon is disrupted by charged particles that are forced to follow the dense lines of the magnetic field in and around sunspots. “The Sun’s magnetic field is essential for understanding the dynamic nature of our star, from the smallest to the largest scale,” recalls Daniel Müller, Solar Orbiter project scientist, in an ESA press release.
The speed and direction of the movement of matter on the surface of the Sun can be observed on the speed map produced by PHI, also called a tachogram. Blue indicates movement toward Solar Orbiter, while red indicates movement away from the probe. This map indicates that while the plasma on the Sun’s surface ordinarily rotates in the same direction as the Sun’s general rotation, around its axis, it is pushed outward around sunspots.
As for the image of the solar corona taken by the EUI, it depicts what is happening beyond the photosphere. Glowing plasma is spewed above active sunspot regions. With a temperature of a million degrees, this plasma follows magnetic field lines that exit the Sun and often connect neighboring sunspots.
Stéphanie Jaquet
