Lesson 40: Sun-Earth Connection
In this lesson, we will learn about the Sun"s weather, or solar activity, and
see what causes it. We will look at specific types of solar activity, including
sunspots, prominences, and flares. Finally, we will look at a pattern that
exists in solar activity known as 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.