Auroras: The Earth's Very Own Light Show #nature

Auroras and the Earth's Magnetic Field

Aurora Borealis- Wish I could see them with my own eyes someday!
Image credit: pbs.org

1. Introduction
Auroras (or Aurora Polaris) are bright displays of shimmering light that appear at night over the Arctic and Antarctic Circles, close to Earth’s poles. The Auroras that appear at the geographic North pole of the Earth are called Aurora Borealis, also known as the Northern Lights. The Auroras that appear on the geographic South pole on the other hand are called Aurora Australis, or the Southern Lights.

2. Why do Auroras come in different colors and shapes?

Auroras are usually caused by streams of ionized, charged plasma particles (electrons and protons) from the Sun, known as the solar wind, interacting with the magnetosphere of the Earth. Most of the charged particles are deflected by the Earth’s magnetosphere.

    i.    Reasons for varying colors:

When the charged particles are funneled down into the Earth’s atmosphere through the magnetic north and south pole, they precipitate into the Earth’s ionosphere. They collide with atoms in the atmosphere, mostly oxygen and nitrogen, transferring some of their energy to the atoms, which then release it as light. The atoms within our atmosphere dictate the colors of the aurora. Different atoms have different energy-levels as they absorb specific amounts of energy from the electrons to produce specific wavelengths or colors of light. Bigger solar storms that deposit more energy in the magnetosphere result in more red and purplish-pink colors which is lower energy light than the blue end of the spectrum, due to the density of the atmosphere and the photoelectric effect.

 

Colors	Explanation
Green	The common green is given off by oxygen at altitudes 100–150km up.
Red	Oxygen has an excited state which releases red light, at altitudes around 200–250km up where the atmosphere is much less dense, during big solar events.
Blue, violet, pink and turquoise-green	The blue, violet and pink colors are released by nitrogen molecules, and the nitrogen atoms emit a turquoise-green often obscured by the bright oxygen green.
Crimson	Hydrogen gives out a pink-hued crimson. These atoms are generally lower in the atmosphere and so in more energetic displays these colors can be seen at the bottom of a green band of aurora.
Infrared and ultraviolet (I would have colored them, but trust me, you won't see it)	Infrared light, or heat is produced by the charged particles funneling down, and the ultraviolet or high energy light is often produced but is invisible to humans.
Yellow and pink	The three primary colors, red, green, and blue, can combine to form different colors of light, such as yellow and pink.

    ii.        Reasons for different shapes:

Scientists are still trying to answer this question. The shape of the aurora seems to depend on-

a)    where in the magnetosphere the charged particles came from,

b)   what caused them to precipitate into the atmosphere, (examples include solar storms and reconnection of the magnetotail, [refer to section 3])

c)    and the amount of acceleration imparted to the charged particles.

Dramatically different auroral shapes can be seen in a single night.

Auroral shapes fall in six categories-

 

Shape	Explanation
Curtains	Curtains have a hanging, wavy appearance, which occur when the particles disperse heterogeneously.
Arcs and bands	An arc is a simple curve of light with a smooth lower border. Auroral displays often start as an arc. A band is a type of arc with an irregular, wavy lower border. They look serpentine, curving and twisting.
Veils	The veil aurora is a uniform area of diffuse pale light covering a wide expanse of sky. It is a faint white, pale green or even a stronger green.
Coronae	Coronae are multiple rays converging to a zenith above the viewer. They occur during strong overhead activity of solar particles.
Patches	Patches are regions of diffuse, less concentrated glow that look like clouds. It occurs when electrons are scattered into the atmosphere by a plasma wave called a whistler.
Rays	Rays are shafts of luminosity coming down from above. When the aurora is more active these rays can appear to move or pulse in the band, due to intense magnetic activity.

3. How solar particles crash into the Earth’s magnetosphere

Earth’s magnetosphere is the region of space within the influence of Earth’s magnetic field, shielding us from the solar wind.

The sunward edge of the magnetosphere is the magnetopause which is located 700,000 km from the Earth. On the side away from the Sun, the magnetosphere trails away like the tail of a comet, called the magnetotail.

Immediately to the sunward side of the magnetopause is a shock wave (which is called the bow shock) that is caused by the solar wind being deflected by the magnetopause. In the event of a solar storm, or reconnection of the magnetotail, the disturbances in Earth’s magnetosphere become strong enough, and the trajectory of charged particles is changed. They get trapped in a doughnut-shaped radiation zone called the Van Allen Belt. They are then funneled down along the geomagnetic field lines into the Earth’s magnetic dipole.

4. Earth’s magnetosphere: How is it different from the magnetic fields of a bar magnet and a solenoid?

• Earth’s magnetosphere

1. The Earth’s magnetosphere is generated by electric currents produced due to the motion of convection currents of a mixture of molten sulfur and other light elements in the Earth’s outer core, called the dynamo effect, which sustains the Earth’s magnetic field in a positive feedback loop.
2. The Earth’s north pole is actually the magnetic south pole, and vice versa, due to the property that unlike poles of a magnet attract each other. At the surface, the Earth behaves like a giant bar magnet with its dipole tilted at an angle of 11 ° in relation to Earth’s rotational axis.
3.Earth is a temporary magnet. It is sustained by electric currents in the outer core and behaves like an electromagnet. The bottom line is, if the Earth's core freezes over, we would be witness the formation of Mars 2.0 :)
4. The Earth’s magnetic field is shaped into an ellipsoid by the solar wind- this is a trivial point, but important nonetheless, as it distinguishes the Earth's magnetic field from the magnetic fields of a bar magnet and solenoid as we will see.
5. The Earth magnetic dipole randomly flips, but it cannot change the intensity of its magnetic field, without changing the current.
6. The intensity of the Earth’s magnetic field depends on the circulation of molten iron in its outer core, producing current, and its rate of rotation on its axis. 

5. The Earth’s magnetosphere: Why is it weakening?

Over the past 200 years, Earth’s magnetic field has weakened by 9%, which can affect satellites and electronics, but would not be catastrophic for life on Earth. Some of the possible explanations of this unknown phenomenon are—

ESA swarm satellites

1)   Changes in the South Atlantic Anomaly- The South Atlantic Anomaly is a large region stretching from Africa to South America, which is one of the weakest spots of the Earth’s magnetic field. It has been growing weaker, has grown in area, and moved westward at a rate of around 20km per year. This seems to be connected to the total weakening of Earth’s magnetic field. Over the past five years, a second center of minimum intensity has emerged southwest of Africa – indicating that the South Atlantic Anomaly could split up into two separate cells.

2)   According to the data retrieved by the swarm satellites of the European Space Agency, ESA, one theory explaining the current weakening is a sign that Earth’s geomagnetic field is about to reverse – in which the north and south magnetic poles switch places, roughly, every 500, 000 years. The last geomagnetic reversal occurred 780, 000 years ago, suggesting that the magnetic field is weakening to prepare for this geomagnetic reversal sometime soon. However, this is still speculative and not a certain sign.