Look into the depths of the Sun with ultraviolet eyes

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Images recently transmitted to Earth by the European Space Agency’s Solar Orbiter has to be seen to be believed. The largest of them exposes the impressive chaos of the Sun’s surface. Above a soup of glowing whorls, tendrils of plasma millions of degrees bow and dance in the space. Zooming in reveals the fine details, the strands of the arc structure and the chttps://bigthink.com/https://bigthink.com/https://bigthink.com/https://bigthink.com /https://bigthink.com/https://bigthink.com/https://bigthink.com/https://bigthink.com/urls terrifying whirlwinds. These breathtaking views are all invisible to our eyes. But by capturing and imaging ultraviolet (UV) light, the probe’s cameras can slice through layers of the Sun’s atmosphere to reveal the mysterious physics beneath.

Before diving, a little reminder: SOL 101, if you will. The innermost visible layer of the Sun is the photosphere. Its opaque surface produces most of the sunlight we see on Earth’s surface and hosts sunspots. The temperature there is about 5,800 degrees Kelvin (about 10,000 degrees Fahrenheit), cold enough for the Sun. Above the photosphere is the chromospherewhich owes its name to its wild red color and is observable during solar eclipses. It has a very low density and a very variable temperature. A thin, misunderstood transitional layer then leads to crown. The corona is the outermost layer of the Sun’s atmosphere. This is where incredibly hot plasma roars, solar flares erupt and giant shards of matter are ejected through space at a million miles an hour.

Imaging these layers requires cameras sensitive to different colors, or wavelengths, of light. Each color is produced by processes within a particular solar layer and passes through the upper layers to reach space where we capture it. This concept is beautifully illustrated with an image from NASA.

Each of the wavelengths observed by NASA’s Solar Dynamics Observatory, or SDO, was chosen to highlight a specific aspect of the Sun’s surface or atmosphere. This image shows images from both the Advanced Imaging Assembly (AIA), which helps scientists observe how solar matter moves through the Sun’s atmosphere, and the Helioseismic and Magnetic Imager (HMI), which focuses on the motion and magnetic properties of the Sun’s surface. (Credit: NASA/SDO/GSFC)

On Earth, the human eye is sensitive to see light from the photosphere. It is the orange-yellow ball with dark sunspots, visible on the top row, third from the left, and on the middle row on the far left. In the upper layers, powerful energetic processes heat and ionize – push electrons away – gas molecules to form a hot plasma. This process emits light of very certain colors. It’s the ultraviolet colors that we can’t see, but our cameras can.

Like most astronomy images, black and white prints of UV light are false color so they can be analyzed and appreciated. This process is a bit like a medical x-ray: you can’t see the x-rays themselves, but you can see the image of the relative intensity of their flux, which remains on a piece of x-ray film. Two special instruments on board the solar orbiter recently relayed UV portraits of the Sun.

The first is the Extreme Ultraviolet Imager (EUI) camera system. (For the curious, here is a scientific discussion of every detail.) This instrument took the whopping 9148×9112 pixels picture of the entire Sun at a wavelength of 174 Angstroms (Å). The Angstrom is a scientific unit for the peak-to-peak distance of the light wave, and 174 of them is about one thousandth the width of an average cell in the human body. The smallest light wave our eyes can see is close to 3800 Å – 20 times larger than the wavelength used for the EUI image.

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The smaller the wavelength of a light wave, the higher its energy. 174 Å is so small and so energetic that it sits at the top of the UV spectrum, with almost the energy to transition to X-rays. Luckily for us, Earth’s atmosphere blocks this high-energy light in the Extreme UV spectrum. Low-energy UV light, with a longer wavelength, penetrates the atmosphere enough to cause sunburn.

The EUI image captures the transition layer and the coldest parts of the corona layer in extreme detail. Another instrument, Spectral Coronal Environment Imaging (SPICE), takes images that see multiple sub-layers of the corona layer. Here is the complete scientific description. Images from the SPICE instrument illustrate photography at several invisible wavelengths. Its UV-sensitive camera sits behind a diffraction grating that splits light into beams of different colors, like a prism. This allows it to take pictures in a range of selected wavelengths, approximately 705-790 Å and 970-1050 Å.

You can see a set of four images – made up of the entire sun, at four unique wavelengths – here. These wavelengths are chosen for a specific reason. They correspond to the atomic emission of plasma in a range of temperatures between approximately 10,000 and more than 1 million K; about double those numbers for degrees Fahrenheit. Scientific analysis of these variable-wavelength SPICE images will shed light on several solar processes.

The distribution and the evolution of the UV light follow the evolution of the plasma in the crown. This makes it possible to analyze the density, flow rate and elemental composition of the plasma. For example, the redshift or blueshift of UV light can reveal the speed of plasma particles in the filament. The behavior of filaments, their temperature, and the overall temperature gradient across the solar atmosphere are areas beyond our current scientific understanding. These data should facilitate better explanations.

The UV cameras aren’t the Solar Orbiter’s only fascinating technology. Flying close to the Sun places it in an incessant, scorching sunbath, requiring specialized designs for the craft and its instruments. One side always faces the Sun. This side is covered in reflective foil and heat shielding, dotted with small doors that can be opened briefly to gather light for the on-board instruments.

The solar flux can be so intense that the solar panels must rotate to prevent overheating, and the camera doors must be very wide so that light can still pass through when they expand from heating. On the other side, the instruments themselves huddle together in the shade away from the heat. SPICE is designed so that infrared light that is not imaged passes completely through the craft and back into space without depositing heat.

Since its launch aboard an Ariane 5 rocket in February 2020, solar orbiter followed a series of elliptical orbits which send the craft plunging toward the Sun, then lifting its gravity to well or near Earth distance. Along this rotating path, it will continue to transmit images of the Sun for several years. Astronomers and solar physicists hope these images will help peel back the layers of the mysterious glowing sphere that supports our tiny grain of life.

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