Aurora- Light of Mystery.

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Aurora - Light of Mystery

What is aurora? 

Auroras, or polar lights, are the luminous phenomenon of the upper atmosphere occurs in high latitudes of both hemispheres. Auroras in the northern hemisphere are called aurora borealis and those in the south hemisphere are called aurora australis. Aurora (Latin for ‘dawn’) is beautiful and amazing lights which are visible in the dark sky in the poles. It can appear as many different forms, but usually it is a greenish quivering glow near the horizon. In 1621 the term ‘aurora’ was coined by the French astronomer. More and more observations were done and a concrete description was archived soon afterwards. Many theories were developed this phenomenon. Some suggested that it was the reflection of sunlight of artic light and some believed it was the firelight at the edge of the world; however both hypotheses are rejected because it was found that aurora was found 100-400km above the earth surface which is well beyond the atmosphere.  Around the 17th century it has been discovered that it is caused by the interaction between energetic plasma particles from outside atmosphere with atoms of higher atmosphere. Till now, not all the questions about aurora have been answered, but with the escalating astronautic technology, we have a much better understanding on this puzzling phenomenon.

How does aurora form?

At every moment the sun is giving out charged particles in solar wind. Some of these particles are captured by the earth magnetic field and the bombardment of the solar wind with the atmospheric particles in the poles will then gives out energy as light. However this is just the basic principle of the formation of aurora, there are many more mechanisms going on at the same time which when they add up together will give rise to this stunning phenomenon.

Earth and its outer space

It is commonly believed that earth lies in an empty space in the universe which is vacuum. This is not absolutely true because vacuum is a space with nothing, no even air, but there are actually particles existing in the interstellar space. At sea level the density of air molecules is 1 quintillion per cm3; at 100km roughly where aurora occurs, density is 1 trillion per cm3; in the interstellar space the density is merely 1 per cm3. Therefore as a result, most of outer space is filled with electrically conductive plasma. Besides of particles, there are magnetic field, electric current, electromagnetic force, gravitational forces… It is these invisible, but important factors which give rise to the interaction between the sun and the earth.

Sun and solar wind

The sun is like a burning furnace which as a temperature as high as one million degrees. The light and heat energy comes from the interior nuclear fusion. At this temperature molecules like hydrogen on the sun surface will form plasma particles. Plasma is the fourth state of matter which occurs in extremely high temperature which is on the order of 10000°C. The temperature of the sun surface is about 5500°C and the temperature of the inner core is approximately 15000000°C, therefore all matter exists as plasma in the sun. In this state particles disintegrate into its constituent atoms and atoms may further decompose into proton and neutron. These charged particles are emitted continuously from the sun in all directions as the particles are too hot and the sun’s gravity can no longer hold it down. This forms the solar wind.  As the solar wind leaves the sun, it picks up the local magnetic field which contributes from the sun spots and the sun’s magnetic poles which forms the Interplanetary Magnetic Field which is perpendicular to the direction of solar wind. The wind together with the weak magnetic field travels in a speed of 400km/s so they can make the trip to the earth in only a few days. The output of the plasma particles is affected by the coronal mass emission, sun spot and solar flares so its intensity varies from time to time. As the number of sun spots rises and drops in an irregular cycle of 11 years. At each peak the violent energy is given out from the sun spots and the aurora activities on earth will be much stronger than usual.

Earth’s magnetic field

The earth has a bipolar magnetic field which means the field has two poles, north and south. The lines of magnetic force emerge from the South Pole and reenter in the North Pole, forming a symmetrical pattern just like a bar magnet. The more concentrated the field lines are, the stronger the magnetic force. Despite the existence of the magnetic field, it has little influence to our everyday environment because most of the materials we encounter in everyday life are electrically neutral. In fact this field has been protecting the earth’s atmosphere by stopping the solar wind in a magnetic shock wave. Without this magnetic field, the atmosphere will be exposed to the high-energy solar wind and the solar particles will strip the air of the earth.

Solar wind, magnetosphere and magnetic reconnection

When the solar wind approaches the Earth’s magnetic field, it will compresses the magnetic field lines facing it on the day side of the Earth and confines those lines into loops; it stretched those field lines on the night side and turn them into a long cylinder which is called the magnetotail. The magnetic fields are divided into two parts, the close and open loops. This shaped region of magnetic field is called the magnetosphere. In the areas where two regions join are the cusps over the north and south poles. Cusps are parts of magnetosphere which are directly opened to the solar wind.

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        The shape of the magnetosphere is contributed by the interaction between the Interplanetary Magnetic Field (IMF) and the earth’s magnetic field. As the solar wind approaches the Earth, it will try to drag the earth’s magnetic field just as it does to the Sun’s. However as the earth’s magnetic field is much stronger than the earth so the solar wind can only morph into a compact dayside section and a stretched night side tail. The magnetotail extends away the sun into the outer space and it has been observed that it extends at least 20 times the radius of the ...

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