The newest photographs from the Solar Orbiter reveal unprecedented detail of the whole Sun. They were taken on March 7, 2022, when the spacecraft passed between the Earth and the Sun.
The Extreme Ultraviolet Imager (EUI) took one of the photographs, which is the highest resolution image of the Sun’s complete disc and outer atmosphere, the corona, ever taken.
Another picture, captured by the Spectral Imaging of the Coronal Environment (SPICE) instrument, is the first full-Sun image in 50 years and is by far the best one. It was shot at the Lyman-beta wavelength of ultraviolet light released by hydrogen gas.
The photographs were collected while Solar Orbiter was halfway between our planet and its parent star, at a distance of around 75 million kilometers. EUI’s high-resolution telescope captures photos with such high spatial resolution that a mosaic of 25 separate photographs is required to cover the full Sun at such a close distance. Because each tile takes around 10 minutes, including the time for the spacecraft to point from one section to the next, the whole picture was obtained over a period of more than four hours, taken one after the other.
In total, the final image contains more than 83 million pixels in a 9148 x 9112 pixel grid. For comparison, this image has a resolution that is ten times better than what a 4K TV screen can display.
EUI images the Sun at a wavelength of 17 nanometers, in the extreme ultraviolet region of the electromagnetic spectrum. This reveals the Sun’s upper atmosphere, the corona, which has a temperature of around a million degrees Celsius.
At the 2 o’clock (near the image of the Earth for scale) and 8 o’clock positions on the edges of the Sun, dark filaments can be seen projecting away from the surface. These ‘prominences’ are prone to erupt, throwing huge quantities of coronal gas into space and creating ‘space weather’ storms.
In addition to EUI, the SPICE instrument was also recording data during the crossing. These too needed to be pieced together as a mosaic.
SPICE is designed to trace the layers in the Sun’s atmosphere from the corona, down to a layer known as the chromosphere, getting closer to the surface. The instrument does this by looking at the different wavelengths of extreme ultraviolet light that come from different atoms.
In the SPICE sequence of images purple corresponds to hydrogen gas at a temperature of 10,000°C, blue to carbon at 32,000°C, green to oxygen at 320,000°C, yellow to neon at 630,000°C.
This will allow solar physicists to trace the extraordinarily powerful eruptions that take place in the corona down through the lower atmospheric layers. It will also allow them to study one of the most puzzling observations about the Sun: how the temperature is rising through the ascending atmospheric layers.
Usually the temperature drops as you move away from a hot object. But above the Sun, the corona reaches a million degrees Celsius whereas the surface is only about 5000°C. Investigating this mystery is one of the key scientific objectives of Solar Orbiter.
The images were taken on 7 March, precisely when Solar Orbiter crossed the Sun-Earth line, so the images can be compared with Earth-bound solar instruments and cross-calibrated. This will make it easier to compare results from different instruments and observatories in future.
On March 26, Solar Orbiter reaches another mission milestone: its first close perihelion. The spacecraft is now inside the orbit of Mercury, the inner planet, taking the highest resolution images of the Sun it can take. It is also recording data on the solar wind of particles that flows outwards from the Sun.
And this is just the start, over the coming years the spacecraft will repeatedly fly this close to the Sun. It will also gradually raise its orientation to view the Sun’s previously unobserved polar regions.
Solar Orbiter is a space mission of international collaboration between ESA and NASA.