What do we think of first when we hear the word CORONA?



Nice, light, Mexican beer.



Royal crown or crown of the United Kingdom.



One type of this virus has afflicted us all quite a bit. About 7 million people have died recently from COVID-19. More information can be found on the respective wikipedia pages(https://en.wikipedia.org/wiki/Coronavirus).

D. SOLAR (or generally stellar) CROWN or CORONA

Solar crown (corona) is the outer atmosphere of the Sun and all the stars. It extends up to millions of kilometers above the surface of the Sun. It is very low density (about 10-12 times thinner than the Earth’s atmosphere). Due to the low optical emissivity relative to the Sun’s surface, the corona is only visible during a solar eclipse (https://en.wikipedia.org/wiki/Stellar_corona). During the eclipse, the disk of the moon passes in front of the disk of the sun. So we are not blinded by the light of the Sun and can comfortably see its crown. This is exactly what is seen in the photo above that the solar disk has been covered up, allowing the solar corona to show through.

But, something very mysterious happens in the Sun: the temperature of the corona can reach 1,000,000 degrees Kelvin*, much higher than the temperature of the previous layer of the star. Experience tells us that if the center of the Sun is very hot, as we move away from it, its temperature should decrease. We observe this decrease in temperature in a fire: as we move away from the fire, its temperature decreases. Unfortunately, the observational data for the Sun show exactly the opposite!

The next chart is from the University of Florida in America (https://pressbooks.online.ucf.edu/astronomybc/chapter/15-1-the-structure andcomposition-of-the-sun/) shows that regions up to about 2,000 kilometers from the center of the Sun are around 10,000 degrees Kelvin. Suddenly, in Corona the temperature exceeds 100,000 degrees. How does the solar corona heat up to these enormous temperatures? The heating of the corona is still today (2023) a puzzle for astrophysics and solar physics.

The approximate temperature in the solar atmosphere plotted against the height

The temperature of the Sun (Temperature (K)) at various distances from its center (Height in Solar Atmosphere (km)). For example, we read from the graph that at a distance of about 2,000 km from the center of the Sun its temperature is a little less than 10,000 K.

To answer the question of how the solar corona heats up, in 2018 NASA launched the Parker satellite, which will go very close to the Sun. It will reach about 10 solar radii (R sun≈ 700,000 km) from its center. No other mission to the Sun has come this close. Its purpose is to observe the outer corona of the Sun. It is designed to withstand temperatures of 1,370 °C (Wikipedia).

NASA’s Parker Solar Probe (artist’s impression) is on a mission to ‘touch the sun’. China hopes to harness the sun’s power by putting a power station into orbit and beaming the energy back to Earth.

Parker will be the fastest human-made object: in 2024 it will come very close to the center of the Sun and orbit at a speed of 700,000 kilometers per hour. At that speed you could go from New York to Tokyo in less than a minute!

Unfortunately, Parker will not live beyond 2025. In total it will only have 7 years to live and we believe that we will learn a lot about the corona heating problem.

A very nice and informative article and video, but unfortunately in English, is the following: https://www.nasa.gov/science-research/heliophysics/nasas-parker-solar%20probe-and-the-curious-case-of-the-hot-corona/

And at the end of his life, how will he die?

* 273 Kelvin degrees = -0.15 ˚C