|
Light From the Sun
The Sun may be described as a giant
ball of incandescent gas. As such, it emits the whole spectrum
of electromagnetic waves. It emits visible light, of course,
but also infrared and ultraviolet "light"as well
as radio waves, x-rays, and gamma rays. All of these forms
of energy are related to what we know of as "light"—only
the wavelength is different. We are unable to detect wavelengths
other than visible light with our eyes, though we can "sense"
infrared waves from the Sun with our skin—it feels warm.
We can also sense ultraviolet with our skin when it gets burned.
Microwaves at high intensity can be sensed as warmth as well,
since like food, our skin can be heated by microwaves. However,
the Sun gives off far too few radio waves in the microwave
region to be sensed as heat. X-ray, and gamma rays from the
Sun are thankfully blocked by the atmosphere but may pose
a hazard to space travelers unprotected by Earth's layer of
air.
All of the light and waves (properly called
electromagnetic waves) emitted by the Sun travel at the speed
of light, 300,000 kilometers per second, or 186,000 miles-per
second. With this incredibly fast speed, you might suppose
that sunlight reaches us almost instantaneously. This is not
true. Because of the large distance from the Sun to the Earth,
it actually takes light 8 minutes to travel from the surface
of the Sun to the Earth. If a dramatic event takes place on
the Sun, light (and information about the event) reaches the
Earth 8 minutes later.
Outer Layers of the Sun
Nothing could be more obvious than
the edge of the Sun. When we watch a sunset or sunrise it
is apparent that the Sun is well defined in its shape and
size. However, this is really not true. Most of the yellowish-white
light we see comes from the photosphere of the Sun, a thin
layer of the Sun that is about 300 km thick. The photosphere
is efficient at emitting light, but in reality the photosphere
has little material in it and is very diffuse. The density
of the photosphere is less than that of the most perfect vacuums
that scientists can set up on Earth. This photosphere is a
shell, a thin layer representing about 5/100 of one percent
of the radius of the Sun. There are other layers of the Sun
beyond the photosphere. The chromosphere is above the photosphere
and was discovered during a total solar eclipse and named
for its red color. Above the chromosphere is the corona (crown!),
which extends outwards into the solar system. This hot, diffuse
material from the outermost layer of the Sun's atmosphere
expands continuously into space and may extend halfway to
the nearest star. The material in the corona and solar wind
is very rarefied. For example, the electrons in the corona
are so widely spaced apart that a million cubic kilometers
of the corona would weigh only about 10 grams. This material
is also very hot—millions of degrees, even though it
is farthest from the center of the Sun.
The corona is dim compared to the rest
of the Sun, and is only visible by eye during a total solar
eclipse, with a brightness of about the full Moon. The shape
of the corona is not constant and varies with the number of
sunspots and solar activity. Using a special telescope that
creates an artificial eclipse, the corona can be seen and
studied regularly.
The Solar Wind
The Sun is always in the process
of boiling off the outer layer of its atmosphere. There is
a constant flow of electrified matter moving away from the
Sun. This solar wind represents the expansion of the Sun's
corona into the space between the planets. The solar wind
is very different from a wind on Earth. The moving matter
is an electrified stream of charged particles, such as protons
and electrons. Although the charged particles are small, being
parts of atoms, there is a large amount of matter being blown
off the Sun. It is estimated that a million tons of matter
is blown off the Sun every second, which is still small compared
to the mass of the Sun.
The existence of the solar wind was first inferred
from the directions of comet tails, which point away from
the Sun instead of trailing behind the comet along its orbital
path. The space between the planets is not empty but filled
with the solar wind. The solar wind has been measured by instruments
on spacecraft traveling to observe Venus, Mars, and Jupiter.
The solar wind can be gusty, with normal speed
of 300-700 kilometers per second and gusts of twice that amount.
At a speed of 400 kilometers per second, the solar wind would
take 2-4 days to travel the Sun-Earth distance, versus only
8 minutes for light.
Plasma
The solar wind is now understood
to be a plasma, a mixture of protons and electrons that moves
away from the Sun at supersonic speeds. Plasma is a very hot
state of matter where atoms have been ripped apart into their
smaller parts, protons and electrons. Since plasmas are charged,
and affected by magnetic fields, it is very important to understand
how the solar wind plasma is affected by the magnetic fields
of the planets it encounters. For example, the solar wind
has a very different effect on the Earth, which has a strong
magnetic field, than it has on Mars, which has a much weaker
magnetic field.
Energetic Particles
In addition to the solar wind particles,
occasionally the Sun produces bursts of more energetic (faster)
particles. Some of these are produced in solar flares, which
show themselves as intense brightening of a small patch of
the photosphere—also giving off blasts of x-rays, gamma
rays, and radio waves. Flares produce a beam of energetic
particles in space that can reach us at nearly the speed of
light if we are in its path.
The Sun also has a second way of producing
such energetic particle storms. Sometimes large loops of coronal
material erupt into the solar wind in what is called a coronal
mass ejection or CME. The CME can produce a shock wave in
the solar wind as it travels outward, and the shock wave itself
makes a broad source of energetic particles. Because the CME
shock wave source is much wider than the flare beam, CMEs
are thought to be the primary cause of energetic particle
events detected near the Earth. The CME disturbance in the
solar wind, including the shock wave ahead of it, reaches
Earth in 2-3 days, faster than most solar wind. It is this
disturbance that causes geomagnetic storms on the Earth. The
auroras that occur with these storms are a result of particles
in the magnetosphere reacting to the passing disturbance.
|
