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
It is basically an electrical phenomenon.
Auroras result from emissions of photons in the Earth's
upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an
electron, and oxygen and nitrogen atoms returning from an excited state to
ground state. They are ionized or excited by the collision of solar wind (it
is basically a photon wave. Photon wave consistes of electrons and protons with
high energy.) and magnetospheric particles (this includes charged ions of
enviornmental gasses) :The Earth's magnetic field traps these particles, many
of which travel toward the poles (since poles have strongest magnetic field) where
they are accelerated toward Earth. Collisions between these ions and
atmospheric atoms and molecules cause energy releases in the form of auroras
appearing in large circles around the poles. Auroras are more frequent and
brighter during the intense phase of the solar cycle (i.e. during solar storms) when coronal mass
ejections increase the intensity of the solar wind.
Typically the aurora appears either as a diffuse glow or as
"curtains" that approximately extend in the east-west direction. At
some times, they form "quiet arcs"; at others ("active
aurora"), they evolve and change constantly. Each curtain consists of many
parallel rays, each lined up with the local direction of the magnetic field
lines, suggesting that auroras are shaped by Earth's magnetic field. Indeed,
satellites show that electrons are guided by magnetic field lines, spiraling
around them while moving toward Earth.
It is observed that large electric currents were associated
with the aurora and such currents flowing from the dayside
toward (approximately) midnight were later named "auroral
electrojets".
The colour of aurora is due to following reasons:
Oxygen emissions green or brownish-red, depending on the
amount of energy absorbed. nitrogen emissions blue or red; blue if the atom
regains an electron after it has been ionized, red if returning to ground state
from an excited state. Oxygen is unusual in terms of its return to ground
state: it can take three quarters of a second to emit green light and up to two
minutes to emit red. Collisions with other atoms or molecules absorb the
excitation energy and prevent emission. Because the very top of the atmosphere
has a higher percentage of oxygen and is sparsely distributed such collisions
are rare enough to allow time for oxygen to emit red.