Lesson Objectives:- The cause of solar activity and sunspots
- Solar prominences and solar flares
- The sunspot cycle
Solar weather, or solar activity, is caused by strong magnetic fields which are constantly forming and changing due to the convecting plasma in the outer layers of the Sun.
The most striking form of solar activity -- sunspots -- appear dark in photographs because they are cooler than the surrounding photosphere. Sunspots are about 4000 Kelvin while the photosphere has an average temperature of 5,800 Kelvin.
How are they so cool relative to their surroundings? The answer is that sunspots are regions that are isolated from the plasma around them by strong magnetic fields. These tight magnetic fields suppress convection within the sunspot and prevent surrounding plasma from entering the sunspot.
During the few weeks that an individual sunspot lasts, the plasma in that sunspot cools. Eventually, the magnetic field weakens or changes, hotter plasma flows in, and the sunspot disappears.
Solar prominences occur where two sunspots are connected by a loop of magnetic field lines. Gas in the Sun's chromosphere and corona becomes trapped in these loops, sometimes rising more than 100,000 kilometers above the Sun's surface.
Some scientists believe that these magnetic field lines sometimes become twisted and eventually snap, and that is what causes intense storms on the surface of the Sun. When these lines snap, they release energy that heats the nearby plasma to 100 million Kelvin, generating the intense radiation that we see from solar flares.
As we have learned before, the Corona is nearly 1 million Kelvin and the Chromosphere is roughly 10,000 Kelvin. Both are much hotter than the photosphere, which is 5800 Kelvin. This happens because the same magnetic fields that keep sunspots cool also carry energy upwards, depositing heat in the upper layers of the Sun's atmosphere.
Coronal mass ejections, which occur when solar flares eject highly energetic charged particles from the Sun's corona, can reach Earth if they happen to be aimed in our direction. They have strong magnetic fields and can disrupt electricity and radio communications, and damage orbiting satellites.
The sunspot cycle is a notable pattern in sunspot activity. At the time of highest average solar activity, called the solar maximum, there are dozens of Sunspots on the Sun, which results in many prominences, flares, and coronal mass ejections. At solar minimum, sunspots are rare and there is little solar activity.
The sunspot cycle has an average period of 11 years, which means usually about eleven years pass from one solar maximum to the next. Scientists tie the sunspot cycle and other solar activity to the Sun's magnetic field which is constantly changing. Its magnetic field is constantly changing due to convection in its interior and the Sun's rotation.
The Sun's pattern of rotation is different from the planets -- it rotates faster near its equator than at its poles, causing magnetic field lines to twist. It may be that sunspots form as these magnetic field lines get more and more twisted with time, until they eventually snap back and reset.