The
Sun-Earth Connection
The Sun and Earth are intimately connected. Other than the light and heat
that we receive from the sun, there are many other things that we also experience
in this relationship. First of all, there is more than just empty space beyond
Earth's atmosphere; there is an environment of rich, active, electrically charged
gases, called plasmas, that the Earth resides in.
The origin of these plasmas is the sun, more specifically the solar
wind. The most noticeable traces of this plasma environment are the Northern
and Southern Lights, or the aurora.
The activity of the sun is also thought to affect our weather and climate,
both long and short term, as solar behavior can last over centuries to mere
minutes. Of particular note, the activity of the aurora are closely tied to
the activity of dark spots found on the suns surface, called sunspots. In
fact,
the "Little Ice Age", a period of abnormally low temperatures during the 15th-18th
centuries, was also a period when there were very few sunspots. Solar flares
are thought to slightly change the density of our atmosphere and may cause
blackouts, communications disruptions, and magnetic storms.
Solar
Physics is the study of the sun and its dynamics. The sun is, of course,
the center of our solar system, containing some 98% of our solar system mass
(Jupiter makes up most of the rest). A huge nuclear furnace, the sun's core
is an astounding 15,000,000° C and pressures 340 billion times that of
air at sea level. The sun creates its light and heat energy through a process
called fusion. Four hydrogen nuclei fuse
to form a helium nucleus. About 700 million tons of hydrogen are converted into
helium every second, producing 5 million tons of pure energy!
The visible surface of the sun is called the photosphere (each square centimeter
of the photosphere emits the same light as a 6000 Watt lamp) and is 6,000°
C. The next layer is the mostly nonvisible chromosphere, and is the layer where
flaculae, prominences and solar flares arise. Solar flares occur because of
magnetic reconnection, when the magnetic field lines of the sun tear and rejoin.
The final outer layer is the corona, which is visible on earth only during an
eclipse. At this layer, prominences, very large structures of hot gas, develop.
It is from here that the energy is finally radiated from the sun at speeds of
1.5 million kilometers to 3 million kilometers per hour.
At the
University of New Mexico we study fusion
physics, as well as magnetic reconnection
in the laboratory, both relevant to solar
dynamics.
The sun's corona seen during a solar eclipse.
The solar wind is a low density (1022 times less dense
than air at sea level) plasma moving anywhere about
400 km/s (1.5 million km/h) to 800 km/s (3 million km/h). It is ejected from
the corona of the sun basically as the hot gas boils off the surface. The solar
wind carries the magnetic field of the sun with it to the far reaches of the
solar system. This solar wind constantly buffets the earth and other planets.
Fortunately, the earth's magnetic field protects it from most effects of the
solar wind.
Sketch of the earth's magnetosphere.
Most planets, the sun, and even some galaxies have a magnetic field associated
with them. Essentially, the earth is a huge magnet with field lines extending
far into space. Were it not for the solar wind, the shape of the magnetic field
would be a dipole, looking much like the familiar iron fillings around a bar
magnet. However, because the solar wind blows by our magnetic field, the side
nearest the sun is compressed, and the side farthest the sun is stretched out.
It looks much like a boat or aircraft wake. This region around the earth is
called the magnetosphere.
On the nearside, the wind runs into Earth's magnetic field forming a bowshock.
It is here that the incoming particles are heated and slowed, either to be detoured
around the earth or bouncing back into the wind in a turbulent region called
the foreshock. The detoured particles travel around the earth in a shape known
as the magnetosheath, creating a long tail known as the magnetotail. Along the
center of the magnetotail is a region called the neutral sheet where the earth's
magnetic field changes direction. Some of the solar wind particles do get trapped
in our magnetosphere. It is also known that some of these particles travel down
to the Earth's poles to a region called the polar cusp. These particles energize
the atmosphere causing the aurora.
As the solar wind buffets the magnetosphere, magnetic field lines can tear
and reconnect. This reconnection can also take place far down in the magnetotail
as it "flaps" in the wind. When this happens, bursts of energetic particles
may enter the earth's aurora, leading to a spectacular display of the Northern
Lights. Waves, known as Alfvén waves, also are excited which travel
up and down the magnetic field lines line waves on a string. This reconnection
process is being studied at UNM using computational
models. It is also being studied in the laboratory.
The magnetosphere as a whole is studied from satellite
observations.
An
aurora is basically a plasma located at the polar
caps caused by an influx of energetic particles, possibly from the solar wind.
As these particles move toward the cap regions they run into the Earth's atmosphere
and ionize some of the gasses. The visual spectacle displayed is an aurora:
huge, moving, ethereal, colorful displays of light in the night sky! They are
also called the Northern and Southern Lights. The movement of the aurora is
thought to be by the same process that creates the images on this display screen:
a thin electron beam (representing the energetic particles), used to light this
phosphorescent screen (representing the upper atmosphere which is ionized),
is altered by changing electric and/or magnetic fields. So, electric fields
high above Earth, the changing magnetic fields of the solar wind, and Earth's
own changing magnetic fields move the solar wind which forms the dynamic aurora.
When a particular elemental gas, such as oxygen or nitrogen, is ionized, it
gives off a characteristic color. This is how we get different colored "neon"
lights. Oxygen, as it exists in the upper atmosphere, emits either green light
or rarely even red, nitrogen glows blue or red, helium emits light blue and
pink. The various manifestations of the aurora are shown below. Small traces
of other colors can also be seen from the other gasses in the atmosphere in
these photos.
At UNM we study the aurora through laboratory
studies
simulating processes important in forming aurora.
Images of the earth's aurora.
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