Aurora

Magnetosphere of the earth

The region around a planet dominated by its magnetic field

is called the magnetosphere. We all know that our earth has

a magnetic field. There is a convection motion of charged

molten ions far below the surface at the earth's asthenosphere

(see layers of earth), and it is the cause of the earth's magnetic

field. The earth's magnetic field is strongest among the other

rocky planets in the solar system and it plays a crucial role in

our planet's habitability.

Aurora

Aurora is a natural light visible in the night sky of the arctic or

antarctic circle during the winter months. It typically appears as

green, pink, red, yellow, blue and violet light. The lights are

dancing in the sky and shifting gently. In the northern hemisphere

this phenomena is called aurora borealis or northern light and in

the southern hemisphere, it is called aurora australis or southern

light. It is visible in a band known as the "auroral zone", which is

3° to 6° wide in latitude and between 10° to 20° from the

geographical poles of the earth. It is visible clearly at dark cloud

free nights. In the northern hemisphere, the aurora is visible in

Norway, Finland, Sweden, Iceland, Alaska, Canada, Northern part

of Russia etc. In the southern hemisphere it is visible in Antarctica,

Argentina, Chile. But sometimes it can be visible in Australia and

New zealand. The region currently displaying an aurora is called

"auroral oval".

Aurora is also distinctly seen in Saturn and Jupiter.

Solar wind and Solar Storm

Aurora is created due to the activity of the Sun. Sun is a giant ball of

hot gases. At very high temperatures the gases stay in plasma state,

that is the gases are ionised (electrically charged). These ions

continuously stream from the sun's surface, it is the solar wind. 

We all know that our earth has a magnetic field. The magnetic field

lines converge towards the pole. The magnetic field protects us from

the harmful solar wind, otherwise it strips away the ozone layer (see

atmosphere of earth). But during solar wind some charged particles

also trap in our atmosphere. Sun doesn't always send the same

amount of charged particles or energy. Sometimes the sun has a

huge outblast and sends more particles than usual solar wind, this

event is called a solar storm. During one kind of solar storm the

sun burps and ejects a huge mass of charged particles to space,

this event is called coronal mass ejection.

When the solar storm reaches earth, some charged particles seep

through the pole region and enter earth's atmosphere. They interact

with the gas particles of the atmosphere causing the aurora lights.

Colours of aurora lights: 

Red: At the higher altitude  (200-300 km above the earth's surface),

when the charged solar particles interact with the low concentration

of oxygen atoms, then red (wavelength 630 nm) colour light is visible

in the sky. But its intensity is very low and visible only due to high

solar activity.

Green: At low altitude (100-300 km above the earth's surface),when

the solar particles collide with the high concentration of oxygen atoms,

the green (557.7 nm) light is visible. Nitrogen in the atmosphere plays

an important role here. Nitrogen (N2) is stable as a molecular form,

so atomic nitrogen is very rare. When the nitrogen molecule collides

with an oxygen atom, it transfers the energy of the oxygen and as a

result the green colour fades away. The green colour is also mixed

with the red and as a result yellow or pink hue is visible.

Blue: At lower altitude (100-200 km above the earth's surface), the

oxygen atoms are not common. The solar particles collide with the

nitrogen molecules and as a result blue and purple colour curtains

are visible. Very high solar activity can cause auroral light at this low

altitude. 

Yellow aurora arises when the red colour is mixed with green.

Pink or magenta colour arises when the atmosphere consists of

both oxygen and nitrogen. When the red light from excited oxygen

is  mixed with the blue light of the nitrogen, produces the pink

colour aurora.  

Ultra violet aurora:

Ultra violet aurora can't be visible with naked eyes, it needs a special

instrument to be detected. When the charged particles from the sun

collide with the gases of the atmosphere, it produces various

wavelengths of light including ultraviolet.

Sub storm of aurora:

Due to the disturbance of the magnetosphere the energy released

from the tail of the magnetosphere to the high latitude ionosphere.

Substorm is characterised as a sudden brightness and increase in

auroral activity. It is observable mainly in the polar region.

Cause: When the solar wind reaches the Earth's magnetosphere,

it creates a disturbance. This disturbance causes the magnetic lines

to come into close proximity, they break and reconnect in a new

configuration. This process is called magnetosphere reconnection.

This process involves transformation of magnetic energy stored in

Earth's magnetotail into kinetic energy, thermal energy and particle

acceleration. These accelerated charged particles cause the aurora

in the sky.

Auroral Noise:

Auroral noise occurs when the charged particles from the sun hit

the inversion layer (temperature rises with altitude,see the

atmosphere of the Earth) of the earth. It is detected as a static or

crackling sound in a radio receiver.