What are solar prominences? Solar prominence is a large, bright structure of plasma that follows magnetic fields and loops out from the surface of the sun.
You may have seen images of solar prominence when you look at pictures of the sun. They are long twisting swirls that loop out from the sun and usually fall back in again. They don’t appear to be very large because we have a difficult time understanding how big the sun actually is, but they are often tens of thousands to hundreds of thousands of kilometers long. The longest solar prominence on record was 800,000 km long. That is more than twice the distance from Earth to the moon. They can travel at speeds of between 10 km/s and 1,000 km/s.
There are a few things we need to break down for this question. We need to look at what a plasma is and why it exists on the sun. We need to look at why the plasma is ejected from the sun, and we need to look at what happens to the prominence.
A plasma is the fourth state of matter. The first three are solids, liquids, and gases. A plasma in physics is a superheated state of matter. The sun is made of plasma because of its heat. At the surface it is about 5,500 ℃ and that rises to about 15 million ℃ at its core. Normally, atoms have a positively charged nucleus and negatively charged electrons. At the temperatures you get in the sun, the electrons are torn away from the nucleus. That leaves positively charged hydrogen nuclei, positively charged helium nuclei, and negatively charged free electrons that are mixed together in a churning plasma. Because the Sun is made of electrically charged plasma, it generates an enormous magnetic field.
The plasma is ejected from the sun because of the way its magnetic field moves. Because the sun is a dynamo, it has a very powerful magnetic field. However, the magnetic field is not stable because of the way the sun rotates. The magnetic field is dragged round as the plasma rotates, but the sun has differential rotation. The equator rotates every 25 days, but the poles take about 35 days to rotate. That means the magnetic field gets twisted and tangled. Imagine if you had an elastic band and you twisted it in the middle faster than you twisted it at the ends, very quickly it would become tangled. It would form large loops, and the loops in the sun’s magnetic field trap the plasma.
Just like with an elastic band, as the magnetic field twists, it builds up energy. Sometimes, the magnetic field will suddenly rearrange itself and this releases an enormous amount of energy. This energy can lift huge loops of plasma high above the Sun. Sometimes the plasma falls back to the surface, but sometimes part of it escapes into space as part of a coronal mass ejection. One solar prominence can contain billions of tons of plasma.
Many prominences are stable for days or even weeks because the magnetic field holds them above the surface. Eventually the magnetic field changes. Some of the plasma then falls back to the Sun, while some erupts into space.
The point where the prominence comes from can often be seen on the surface of the sun because it is both brighter and darker. The process that leads to a prominence often starts with a sunspot. These are relatively cool (about 4,000 ℃) regions where powerful magnetic fields emerge from inside the sun. They look dark because they are cooler than the surrounding surface. The magnetic field often loops from one sunspot to another. Then, around the sunspot is an active region where the magnetic field is strongest and most tangled. These regions often appear much brighter because the gas there is hotter and more energetic. Then, the solar prominence itself glows red because it is thinner and mostly made up of hydrogen.
The first time they were described was in the 14th century during a solar eclipse. With modern telescopes and space telescopes, we can see the sun quite clearly, but in the 14th century it was impossible to see the sun because it is too bright. The only way to look at it was when there was a full solar eclipse. The bright disk of the sun is covered by the moon and the prominences are visible around the edge. And this is what I learned today.
Sources
https://science.nasa.gov/learn/heat/big-ideas/big-idea-3-1
https://www.sidc.be/article/prominences-rise-glory
https://science.nasa.gov/sun/solar-storms-and-flares
https://scied.ucar.edu/learning-zone/sun-space-weather/solar-prominence
https://en.wikipedia.org/wiki/Solar_prominence
https://en.wikipedia.org/wiki/Plasma_(physics)
Photo by Bruno Abdiel: https://www.pexels.com/photo/a-bright-sun-setting-in-the-sky-5892396/