Aurora: The Dancing Light
“The Valkyrior are warlike virgins, mounted upon horses and armed with helmets and spears. When they ride forth on their errand, their armour sheds a strange flickering light, which flashes up over the northern skies, making what Men call the “aurora borealis”, or “Northern Lights”.” (A notion popular to the Norse Mythology)
These mysterious yet heart capturing light performances put on by our planet for us to glimpse, in coordination with the sun’s rage is what we know are the renowned “Auroras”. For us to fully understand this statement we have to go into the physics of why the auroras happen, which we will learn in this article. We are used to seeing the plain night sky with no action, just like a blank theatre screen. But once in a while, the theatre lights up giving the best show which no technology on earth could provide, making people travel thousands of miles just to witness this beauty.
The auroras happen near the northern and southern poles. The Auroras occurring in the northern hemisphere are known as the “Aurora Borealis” and the ones in the southern pole are known as the “Aurora Australis”. The land masses which lie close to the equator or have a high light pollution have a dim chance of observing this phenomenon.
Basically the aurora is perfectly co-ordinated stage performance put on by the Earth and the sun. The earth is a huge magnet and like all magnets, has its own magnetic field. The sun is a violent hot ball of gas and radiation, spewing out matter and energy at high speeds (Solar winds). Radiation mainly consists of electromagnetic waves and the matter emitted is electrons and protons, which carry charge, and as we know moving charges induce a magnetic field around them. Therefore interaction between the solar winds (which we notice are magnetic) with earth (which is also a magnet) would a peculiar effect of either attraction or repulsion or even a more complex mutual motion. This matter and energy, on interacting with the earth’s magnetic field and the upper atmosphere produce what we see as the spectacular auroras.
Let us go step by step in understanding the above principle in a more detail manner
The Earth is a Magnet?
We know that the earth is huge sphere (almost) having three layers. The top layer is the one on which we live and is called the crust. It consists of neutral mineral oxides, sulphides and many other ores. It is mostly sand and rock and is 60km deep. Moving deeper we have the mantle which is a sea of molten rock and minerals and is 3200km deep. Next we have the core of the earth consisting of the earth’s parent metal “Iron” in molten form on the outside and solid on the inside. This layer is called the core.
Basically the solid core is a ball of iron and since the temperature is very high at the core, there exist certain ions (charge carrying particles) which keep moving around creating an electric loop. Whenever there is current in a loop a magnetic field coexists arising from the centre of the loop. Any magnet will certainly consist of a magnetic field and it really does the magnetic north at the geographic north and the magnetic south at the geographic south (although they are not aligned perfectly). The field lines flow from north to the south (magnetic poles) and the pattern of the field lines are as shown below.
Sun’s Coronal Mass Ejection
The sun’s behaviour is very complex since it produces extravagant amounts of energy. It is mainly fuelled by a process known as “nuclear fusion” of hydrogen to form helium. During this, lots of high energy photons and other subatomic particles are released at high speeds. The light halo that surrounds the sun (best visible during a solar eclipse) is called the “Corona”. It is the outermost layer of the sun and it is this layer which spews out the “solar winds” into the solar system. At certain times, there is a sudden outburst of mass and radiation mostly occurring at certain active regions on the sun’s surface (due to variable magnetic field reconnection between two active regions). This event is known as a solar flare and the projected particles travel at speeds of one million miles per hour.
When the Solar Winds Strike the Earth
When these solar winds reach the earth, they follow the magnetic lines of the earth’s magnetic field and interact with the main gases of our atmosphere (nitrogen and oxygen) present at high altitudes. The high energy particles excite the nitrogen and oxygen atoms to a high energy state and are accelerated along the magnetic field of the earth in complex paths.The electrons from the solar wind have their own electric and magnetic fields and vary as they go along the earth’s magnetic field causing it to collide with different combination of atoms. This causes the auroras to dance in the sky. The excited particles are accelerated through the earth’s magnetic field and come back where the lines become vertical i.e., the poles. The atoms cannot hold this extra energy for long and will return to their ground state throwing out the excess energy in the form of photons (which is basically light).
The colour of the light usually depends on the type of atom struck and the energy content of the solar wind and the altitude at which they collide.
- Oxygen – Green (up to 250 kilometres) or Red (above 250 kilometres)
- Nitrogen – Blue if electron is recombined or Red otherwise
The most frequent colours are the ones which require a lesser amount of energy. The frequencies of colours are
I. Green – Oxygen dominated
II. Pink – Light green + Red (Oxygen and nitrogen)
III. Pure Red – Either Oxygen or Nitrogen
IV. Yellow – Green + Red
V. Pure Blue – High energy Nitrogen.
Auroras of Different colours
The Auroras are usually oval shaped forming around the magnetic poles mostly near the arctic (aurora borealis) and Antarctic regions (aurora australis). The most famous places where people could catch the auroras are the obviously the north and south poles. But apart from these countries like Norway, Canada, Alaska, Finland, Southern parts of South America, Australia, New Zealand are a few places where the auroras are popularly witnessed. To see a live aurora, one should first get out as far as possible from the city and distinguish any close by light to get the best view. The light pollution could diminish the display of the aurora and the solar activity must be carefully monitored to know the timing of the auroras. This can be done via online astronomical information sites.