The Northern Lights or Aurora
Borealis
An aurora is
a natural light display in the sky particularly in the high latitude (Arctic
and Antarctic) regions, caused by the collision of energetic charged particles
with atoms in the high altitude atmosphere (thermosphere). The charged
particles originate in the magnetosphere and solar wind and, on Earth, are
directed by the Earth's magnetic field into the atmosphere. Most aurorae occur
in a band known as the auroral zone, which is typically 3° to 6° in latitudinal
extent and at all local times or longitudes. The auroral zone is typically 10°
to 20° from the magnetic pole defined by the axis of the Earth's magnetic
dipole. During a geomagnetic storm, the auroral zone expands to lower
latitudes.
In northern
latitudes, the effect is known as the aurora borealis (or the northern lights),
named after the Roman goddess of dawn, Aurora, and the Greek name for the north
wind, Boreas, by Pierre Gassendi in 1621.
Auroras
seen near the magnetic pole may be high overhead, but from farther away, they
illuminate the northern horizon as a greenish glow or sometimes a faint red, as
if the Sun were rising from an unusual direction. Discrete aurorae often
display magnetic field lines or curtain-like structures, and can change within
seconds or glow unchanging for hours, most often in fluorescent green.
Its
southern counterpart, the aurora australis (or the southern lights), has
features that are almost identical to the aurora borealis and changes simultaneously
with changes in the northern auroral zone. It is visible from high southern
latitudes in Antarctica, South America, New Zealand, and Australia.
Aurora timelapse:-
What is happening?
The auroras, both surrounding
the north magnetic pole (aurora borealis) and south magnetic pole (aurora
australis) occur when highly charged electrons from the solar wind interact
with elements in the earth's atmosphere. Solar winds stream away from the sun
at speeds of about 1 million miles per hour. When they reach the earth, some 40
hours after leaving the sun, they follow the lines of magnetic force generated
by the earth's core and flow through the magnetosphere, a teardrop-shaped area
of highly charged electrical and magnetic fields.
As the electrons enter the
earth's upper atmosphere, they will encounter atoms of oxygen and nitrogen at
altitudes from 20 to 200 miles above the earth's surface. The colour of the
aurora depends on which atom is struck, and the altitude of the meeting.
- Green - oxygen, up to 150
miles in altitude
- Red - oxygen, above 150
miles in altitude
- Blue - nitrogen, up to 60
miles in altitude
- Purple/violet - nitrogen,
above 60 miles in altitude
All of the magnetic and
electrical forces react with one another in constantly shifting combinations.
These shifts and flows can be seen as the auroras "dance," moving
along with the atmospheric currents that can reach 20,000,000 amperes at 50,000
volts.
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| Structure of the Magnetosphere |
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