Aurora Borealis and Aurora Australis are glows that
are sometimes visible in the Northern and Southern Hemispheres,
respectively. They are informally known as the northern
lights and the southern lights. The glows are strongest
near the poles and originate in the Van Allen radiation belts,
which are regions where high-energy charged particles of the
solar wind that travel outward from the Sun are captured by
the Earth’s magnetic field. The outer Van Allen radiation belt
consists mainly of protons, whereas the inner Van Allen belt
consists mainly of electrons. At times, electrons spiral down
toward Earth near the poles along magnetic field lines and
collide with ions in the thermosphere, emitting light in the
process. Light in the aurora is emitted between a base level of
about 50–65 miles (80–105 km), and an upper level of about
125 miles (200 km) above the Earth’s surface.
The solar wind originates when violent collisions
between gases in the Sun emit electrons and protons, which
escape the gravitational pull of the Sun and travel through
space at about 250 miles per second (more than 1 million
km/hr) as a plasma known as the solar wind. When these
charged particles move close to Earth, they interact with the
magnetic field, deforming it in the process. The natural undis
turbed state of the Earth’s magnetic field is broadly similar to
a bar magnet, with magnetic flux lines (of equal magnetic
intensity and direction) coming out of the south polar region
and returning back into the north magnetic pole. The solar
wind deforms this ideal state into a teardrop-shaped configuration
known as the magnetosphere. The magnetosphere has
a rounded compressed side facing the Sun, and a long tail
(magnetotail) on the opposite side that stretches past the
orbit of the moon. The magnetosphere shields the Earth from
many of the charged particles from the Sun by deflecting
them around the edge of the magnetosphere, causing them to
flow harmlessly into the outer solar system.
The Sun periodically experiences periods of high activity
when many solar flares and sunspots form. During these
periods the solar wind is emitted with increased intensity,
and the plasma is emitted with greater velocity, in greater
density, and with more energy than in its normal state. During
these periods of high solar activity the extra energy of
the solar wind distorts the magnetosphere and causes more
electrons to enter the Van Allen belts, causing increased
auroral activity.
When the electrons from the magnetosphere are injected
into the upper atmosphere, they collide with atoms and
molecules of gases there. The process involves the transfer of
energy from the high-energy particle from the magnetosphere
to the gas molecule from the atmosphere, which becomes
excited and temporarily jumps to a higher energy level. When
the gas molecule returns to its normal, regular energy level, it
releases radiation energy in the process. Some of this radiation
is in the visible spectrum, forming the Aurora Borealis in
the Northern Hemisphere and the Aurora Australis in the
Southern Hemisphere.
Auroras typically form waving sheets, streaks, and glows
of different colors in polar latitudes. The colors originate
because different gases in the atmosphere emit different characteristic
colors when excited by charged particles from the
magnetosphere, and the flickering and draperies are caused
by variations in the magnetic field and incoming charged particles.
The auroras often form rings around the magnetic
poles, being most intense where the magnetic field lines enter
and exit the Earth at 60–70° latitude.
Aurora Australis See AURORA.
Aurora Borealis See AURORA.
avalanche See MASS WASTING.
are sometimes visible in the Northern and Southern Hemispheres,
respectively. They are informally known as the northern
lights and the southern lights. The glows are strongest
near the poles and originate in the Van Allen radiation belts,
which are regions where high-energy charged particles of the
solar wind that travel outward from the Sun are captured by
the Earth’s magnetic field. The outer Van Allen radiation belt
consists mainly of protons, whereas the inner Van Allen belt
consists mainly of electrons. At times, electrons spiral down
toward Earth near the poles along magnetic field lines and
collide with ions in the thermosphere, emitting light in the
process. Light in the aurora is emitted between a base level of
about 50–65 miles (80–105 km), and an upper level of about
125 miles (200 km) above the Earth’s surface.
The solar wind originates when violent collisions
between gases in the Sun emit electrons and protons, which
escape the gravitational pull of the Sun and travel through
space at about 250 miles per second (more than 1 million
km/hr) as a plasma known as the solar wind. When these
charged particles move close to Earth, they interact with the
magnetic field, deforming it in the process. The natural undis
turbed state of the Earth’s magnetic field is broadly similar to
a bar magnet, with magnetic flux lines (of equal magnetic
intensity and direction) coming out of the south polar region
and returning back into the north magnetic pole. The solar
wind deforms this ideal state into a teardrop-shaped configuration
known as the magnetosphere. The magnetosphere has
a rounded compressed side facing the Sun, and a long tail
(magnetotail) on the opposite side that stretches past the
orbit of the moon. The magnetosphere shields the Earth from
many of the charged particles from the Sun by deflecting
them around the edge of the magnetosphere, causing them to
flow harmlessly into the outer solar system.
The Sun periodically experiences periods of high activity
when many solar flares and sunspots form. During these
periods the solar wind is emitted with increased intensity,
and the plasma is emitted with greater velocity, in greater
density, and with more energy than in its normal state. During
these periods of high solar activity the extra energy of
the solar wind distorts the magnetosphere and causes more
electrons to enter the Van Allen belts, causing increased
auroral activity.
When the electrons from the magnetosphere are injected
into the upper atmosphere, they collide with atoms and
molecules of gases there. The process involves the transfer of
energy from the high-energy particle from the magnetosphere
to the gas molecule from the atmosphere, which becomes
excited and temporarily jumps to a higher energy level. When
the gas molecule returns to its normal, regular energy level, it
releases radiation energy in the process. Some of this radiation
is in the visible spectrum, forming the Aurora Borealis in
the Northern Hemisphere and the Aurora Australis in the
Southern Hemisphere.
Auroras typically form waving sheets, streaks, and glows
of different colors in polar latitudes. The colors originate
because different gases in the atmosphere emit different characteristic
colors when excited by charged particles from the
magnetosphere, and the flickering and draperies are caused
by variations in the magnetic field and incoming charged particles.
The auroras often form rings around the magnetic
poles, being most intense where the magnetic field lines enter
and exit the Earth at 60–70° latitude.
Aurora Australis See AURORA.
Aurora Borealis See AURORA.
avalanche See MASS WASTING.
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