AURORA The lights of the dark sky

Table of content Introduction Legends behind Aurora Causes of Aurora Auroral Oval Auroral mechanism How high aurora occur ? When does Aurora occur ? Place where Aurora occur ? Aurora on other planets Discovery of Aurora Types of Aurora Colours of Aurora Summary Objective

Aurora

Keywords

Solar Wind Ionosphere Auroral Oval UV image

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Introduction

The Beautiful and eerie curtains of light in night time sky is known as aurora.

Aurora is natural phenomenon of light in the upper atmosphere of earth.

The Latin word aurora means ‘sunrise’ or ‘Roman goddess of dawn’.

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In Northern hemisphere they are called as ‘Aurora Borealis’ which means ‘Dawn to north’.

In Southern hemisphere they are called as ‘Aurora Australis’ which means ‘Dawn to south’.

They can be seen above the magnetic poles of northern & southern hemisphere.

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Legends behind Aurora

The early dragon legends of China and Europe, are said to be originated from aurora.

In medieval times, the occurrence of aurora was seen as harbingers of war or famine.

The Maori of New Zealand believed that the lights were reflections from torches or campfires.

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The Menominee Indians of Wisconsin believed that the lights indicated the location of giants who were the spirits of great hunters and fisherman.

The Inuit of Alaska believed that the lights were the spirits of the animals they hunted.

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Causes of Aurora

Sun emits electrically charged particles called as ‘solar wind’.

Most of these charged particles gets deflected by earths magnetic field.

Some of the trapped particles move towards polar regions since low magnetic field.

Solar wind affecting earths magnetic field

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In polar regions they collide with gases in earths ionosphere and leads to formation of aurora.

The variation in colour of aurora depends on gas with which it undergo collision.

Solar wind entering earths magnetic field

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Auroral Oval

The electrically charged particles which stream down the magnetic field of earth reach the neutral atmosphere in the rough circle called auroral oval.

This circle is around 3000 km in diameter.

UV image of auroral oval

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Auroral mechanism Electrically charged particles

when undergo collision with gases from earths atmosphere, leads to transfer of energy.

Due to high energy, atoms undergo excitation to high energy levels.

These excited electrons later gets to normal state by release of photons (Light).

The colour of light depends on gas which gets excited and amount of energy absorbed by the gas.

Effect of charged particles on atoms

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How high Aurora occur ? Most aurora occur between 90 to 130 km above sea level, but some

extend to several hundred kilometers. The usual altitude for jet aircraft is 10 km and the ozone layer lie

between 20 and 30 km so we have to be at the height of satellite to be at same height as of aurora.

The consequence of its great height is that aurora is visible at horizontal distance of several hundred kilometers.

Satellite picture of an aurora

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When does Aurora occur ?

Mainly observed daily somewhere in the sky (but in the day time it is out-shined by the sunlight).

Due to low intensity they are not visible through out the day and hence seen during midnight.

Winter season is considered to be the best time to view aurora.

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Aurora is displayed after every 27 days in active regions. Full moon and light pollution affect the visibility of aurora. Aurora Borealis and Aurora Australis occur at same time and are

mirror image of each other.

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Place where Aurora occur

Aurora Borealis are seen in southern tip of Greenland, north Canada, Alaska, northern coast of Siberia, coasts of Troms and Finnmark.

Aurora Australis are seen only from Antarctica and surrounding ocean and may only be glimpsed from Tasmania and southern New Zealand

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Places of auroral occurrence in world map

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Aurora on other planets

Auroras have been observed on Jupiter, Saturn and Uranus but not on Mars, Venus or Mercury.

Any planet with a magnetic field and an atmosphere should likely have auroras (Mars and Venus have no global magnetosphere; Mercury has almost no atmosphere).

Conjugate auroras on Saturn

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Discovery of Aurora In 4th century B.C. Aristotle made one of the first accounts of aurora

borealis, describing “glowing clouds”.

In late 16th century, William Gilbert conducted experiments and came to conclusion that earth itself is a giant magnet with north and south pole.

Late around 20th century, Norwegian scientist Kristian Birkeland placed a spherical magnet inside the vacuum chamber and shot an electron beam at it.

It was founded that beam was guided by magnetic field to hit near the pole.

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Birkeland in his lab testing his theories on what causes aurora

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This experiment concluded that magnetic field played an important role in auroral occurrence.

Types of Aurora

Aurora are classified as Discrete Aurora and Diffuse Aurora.

Discrete aurora can be seen by naked eyes.

Diffuse aurora is a featureless glow in sky which cannot be seen by naked eyes.

Diffuse Aurora

Discrete Aurora

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Colours of Aurora The colour of aurora

depends on energy of electron and colliding gas.

High energy electron cause oxygen to emit green light and low energy electron cause red light.

Nitrogen emit blue light.

Blending of these colour also lead to purple, pink and white.

Rainbow Aurora

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Summary Aurora is bright light appearing in night sky. It is a natural phenomena which occurs only in polar regions The charged particles thrown out of suns surface, enters into earths

atmosphere and undergo collision with the gases.

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The variation in colour of aurora depends on energy of charged particles and gas with which collision takes place.

The occurrence of aurora on other planet depicts presence of atmosphere and magnetic field.

Places where aurora occurs are the tourist attraction.

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Objective To create public awareness of natural yet rare phenomena occurring

on our planet. To study scientific reason behind occurrence of auroral lights.

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Reference www.gi.alaska.edu

www.wikipedia.org

www.alaskascience.com [ Types of aurora ]

www.phy6.org [ Causes of aurora ]

www.sunearth.gsfc.nasa.gov [ Aurora on other planets, Formation of auroral oval ]

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