What is an Aurora?

An Aurora also called polar light or northern light is a natural light display in the sky. They are usually seen in the high latitudes (Arctic and Antarctic) regions. Auroras are produced when the Earth's magnetosphere is disturbed by the solar wind.

An aurora around the North Pole is called the Aurora borealis or 'northern lights'. Around the South Pole, it is the Aurora australis or 'dawn of the south'. It can be seen from long distances, stretching in the sky many hundreds of miles. Auroras can only be seen at night because their light is not as strong as the light of day and faint stars can even be seen through the aurora. However, they can also happen during the day.

An aurora occurs when the Sun sends off particles into space. These particles are mainly electrons, with charge and energy, which means they contribute to electricity. Earth has a protective shield of energy around it. This is called the "magnetic field" and forms an elongated sphere around the Earth called the "magnetosphere". The Earth’s magnetic field keeps off most of the solar wind.

At high-latitude areas (polar areas), the magnetic field is vertical. It does not keep off particles of the solar wind which can come from the magnetosphere and hit the particles of the air (Earth's atmosphere). When they hit, the atmosphere is heated and excited and the excess energy gets away, a phenomenon which we see as moving lights in the sky above 100 km altitude typically. An aurora can be especially bright following a solar event called a coronal mass ejection (CME) when the charged particles rip through the electromagnetic field because of their power.

Auroral phenomena have been observed on other planets that have a magnetic field, such as Jupiter, Saturn and more recently Mars. It is believed to be a widespread phenomenon in the Solar System and beyond. Many legends are associated with the aurora in all countries where this phenomenon regularly occurs.

What causes Aurora’s?

A full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves the interaction of the solar wind with the Earth's magnetosphere. The varying intensity of the solar wind produces effects of different magnitudes but includes one or more of the following physical scenarios.

1. A quiescent solar wind flowing past the Earth's magnetosphere steadily interacts with it and can both inject solar wind particles directly onto the geomagnetic field lines that are 'open', as opposed to being 'closed' in the opposite hemisphere, and provide diffusion through the bow shock. It can also cause particles already trapped in the radiation belts to precipitate into the atmosphere.

2. Geomagnetic disturbance from an enhanced solar wind causes distortions of the magnetotail ("magnetic substorms"). These 'substorms' tend to occur after prolonged spells (hours) during which the interplanetary magnetic field has had an appreciable southward component. This leads to a higher rate of interconnection between its field lines and those of Earth.

3. Acceleration of auroral charged particles invariably accompanies a magnetospheric disturbance that causes an aurora. This mechanism, which is believed to predominantly arise from strong electric fields along the magnetic field or wave-particle interactions, raises the velocity of a particle in the direction of the guiding magnetic field. 

The details of these phenomena are not fully understood. However, it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiation zones and temporarily magnetically-trapped particles confined by the geomagnetic field, coupled with particle acceleration processes. 

Other auroral radiation

In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR), discovered in 1972. Ionospheric absorption makes AKR only observable from space. X-ray emissions, originating from the particles associated with auroras, have also been detected.

Aurora noise

Aurora noise, similar to a hissing, or crackling noise, begins about 70 m (230 ft) above the Earth's surface and is caused by charged particles in an inversion layer of the atmosphere formed during a cold night. The charged particles discharge when particles from the Sun hit the inversion layer, creating the noise.