The Lights of the Sun

A "Rainbow" of Colors

The Sun emits a set of electromagnetic waves, from the far ultraviolet (FUV) like gamma rays (very high frequencies) to radio waves (very low frequencies), including X-rays, ultraviolet rays, visible light, infrared rays, and microwaves. These electromagnetic waves, whose vector is the photon, travel at a speed of ≈300,000 km/s.

With our eyes, we only see the wavelengths of the very small visible range located between 400 and 800 nm. For shorter or longer wavelengths, we need to use specialized equipment. The specialized instruments are generally ground-based or space-based telescopes equipped to observe light in different wavelengths.

The Sun sends us all the colors of light because it is filled with all the atoms present on Earth, each generating light at a certain wavelength depending on the temperature. In addition to the many different atoms (helium, hydrogen, carbon, oxygen, iron…), the Sun also contains different types of ions of each atom, with different electrical charges. Each ion can also emit light at specific wavelengths when it reaches a particular temperature.

Images of the Sun in a Chosen Variety of Wavelengths

Each observed wavelength reveals information about the different elements (atoms or ions) of the Sun's surface and atmosphere. By examining images of the Sun in a chosen variety of wavelengths, scientists can track the evolution of particles and temperatures present in the Sun's atmosphere.

This image of the Sun was constructed based on data from the SDO (Solar Dynamics Observatory) telescope of NASA. It shows a composition of different aspects of the Sun's surface or atmosphere in 10 different wavelengths chosen by scientists and invisible to the naked eye. These lights are converted into visible colors so that humans can see them. The observed object, here the Sun, thus appears in a beautiful patchwork of colors.

Since the 1900s, scientists have been cataloging the wavelengths absorbed or emitted by atoms and ions, as well as the associations between elements, wavelengths, temperatures, and colors. Telescopes make use of this precious wavelength information by embedding instruments like spectrometers that observe several wavelengths simultaneously and measure the quantity of elements present at each wavelength.

The Wavelengths of the Sun

The scientists of SDO have chosen 10 particular wavelengths to observe the atmosphere and the movements of atoms in the solar layers. The wavelengths are expressed in angstroms (symbol Å) = 0.1 nanometers, or 10^-10 meters, or one-tenth of a billionth of a meter.

1. 1700 Å in C7 and D4 (pink brown), the light at 4,500 Kelvin, emitted by the Sun's surface, the photosphere, and the chromosphere.
2. 4500 Å in A4, D7, and E4 (yellow), the light at 6,000 Kelvin, emitted by the photosphere.
3. 1600 Å in E3 and C6 (yellow green), the light emitted by carbon 4 at 10,000 Kelvin, in an area between the upper photosphere and the transition region, a region located between the chromosphere and the corona.
4. 304 Å in D3, B6, E7, and F4 (red), the light emitted by helium 2 at 50,000 Kelvin, in the transition region and the chromosphere.
5. 171 Å in D2, C4, A5, and F6 (yellow brown), the light emitted by iron 9 at 600,000 Kelvin, in the atmosphere or solar corona when it is calm.
6. 193 Å in B2, B7, F5 (bright brown), the light emitted by iron 12 at 1 million Kelvin and iron 24 at 20 million Kelvin, in the slightly hotter regions of the corona and the much hotter material of solar eruptions.
7. 211 Å in A6, B3, and F2 (purple), the light emitted by iron 14 at 2 million Kelvin, in the hotter and magnetically active regions of the corona.
8. 335 Å in C1, B5, C8, F3, F7 (blue), the light emitted by iron 16 at 2.5 million Kelvin, in the even hotter, magnetically active regions of the corona.
9. 94 Å in C3 and D5 (dark green), the light emitted by iron 18 at 6 million Kelvin, in the very hot regions of the corona during a solar eruption.
10. 131 Å in E6 (blue green), the light emitted by iron 20 and iron 23 at more than 10 million Kelvin, by the hottest elements of solar ejections.