Kind: captions
Language: en
From relatively minor interruptions that
create sunspots to massive surges that
drive solar storms, change is one of the
few constants on the sun's surface. So
what's behind all this variability? The
short answer is magnetism. More
precisely, it's the shifting and
twisting magnetic fields seething below
the sun's surface.
Like Earth, the sun has north and south
magnetic poles with magnetic field lines
connecting the two. At times, these
field lines follow neat vertical paths
between the northern and southern
hemispheres. But that's the exception
more than the rule. More often, the
motion of plasma inside the sun mixes up
the neatly organized field lines. This
is because plasma spins faster at the
equator than near the poles, a
difference that causes the lines to
stretch.
At the same time, swirling currents in
the convective zone twist the field
lines and cause them to kink upward.
What's more, all the stress and strain
generated by these two forces actually
strengthens the magnetic field rather
than weakening it. Imagine this spring
is a magnetic field line. The magnetic
field inside the sun is amplified, is
strengthened by the rotating motions and
the shearing motions and the churning
motions inside the sun. It wants to
expand upwards and it does until it
pokes out through the surface of the
sun. As magnetic field lines emerge,
they form loops. Where they break
through the surface, they temporarily
divert the upward flow of hot plasma and
create the relatively cool dark regions
that we know as sunspots.
But the effects can be more than
cosmetic. Often extending far above the
sun's surface, the magnetic loops
continued to be twisted by the currents
beneath. Sometimes the field lines twist
enough to cross. The resulting magnetic
reconnection, similar to a short
circuit, heats the surrounding plasma to
tens of millions of degrees. This
unleashes a solar flare, a powerful
burst of energy and particles that can
have disastrous results if directed
toward Earth. Sometimes the energy
released by these magnetic reconnections
produces another type of solar storm.
Coronal mass ejections or CMEs which can
propel huge amounts of matter away from
the sun's surface. A single CME might
blast 10 billion tons of material out
into the solar system.
While all this solar activity sounds
chaotic, it actually follows a very
regular cycle once again driven by
magnetism. About every 11 years, the
sun's magnetic field underos a dramatic
shift. Just when the twisting of the
field lines reaches its peak, which we
call the solar maximum, a realignment
occurs, flipping magnetic north and
south. Once again, the field forms neat
vertical lines and solar activity drops
to near zero. That is until movement of
the plasma inside the sun begins to act
on them and the march toward the next
solar maximum resumes.