Northern lights moving across the sky

Northern lights... what do you know about them? Even if it is totally alien to you, you are excused. Because, this magnificent view of the sky occurs only almost once in 200 years, in the countries close to the equator, like ours! Without wasting more words, let us tell you what this is.

This is a light effect that colours the sky around the South and North Poles. How do you think these colours originate?

The origin of Northern Lights also go to our Sun, just as all life. During large explosions and flares or eruptions, huge quantities of solar particles are thrown out of the Sun and into deep space.

These plasma clouds travel through space with speeds varying from 300 to 1000 kilometres per second. And do you know, even with such speeds, it takes these plasma clouds around two to three days to reach our planet?

When they get closer to the Earth, they are trapped by the Earth's magnetic field which is called the magnetosphere (obviously, how can we allow an alien object to enter our Earth?), and guided towards the Earth's two magnetic poles; the geomagnetic South and North Poles.

On their way down, the solar particles are stopped by the Earth's atmosphere, which acts as an effective shield against these deadly particles.

When the solar particles are stopped by the atmosphere, they crash with the atmospheric gases present, and the collision energy between the solar particle and the gas molecule is emitted as a photon - a light particle. And when you have many such collisions, you get an aurora or a northern light - lights that may seem to move across the sky.

Here is the most amazing fact; in order for an observer to actually see the aurora with the naked eye; about 100 million photons need to gather.

Process of the Sun

As we mentioned earlier, the Sun is the energy source of these Northern lights. The Sun or the burning gas globe has provided light and warmth to the Earth for nearly five billion years.

The energy of the Sun generates in its inside, where the temperature is believed to exceed 15 million degrees Kelvin(K), and the pressure is also 250 billion times greater than on the Earth's surface.

This tremendous heat transforms hydrogen, the lightest element in the known universe, into helium. The Sun's surface has a temperature of approximately 5800 K.

The solar activity varies over periods of around 11 years. When the number of spots peak, we call it as solar maximum; a solar minimum is when the sunspot activity is low.

The more sunspots, the more solar particles are driven out into deep space, and then more auroras occur on Earth. The latest solar maximum was in 2001-2002, the next is expected some- time around 2011-2012.

Sunspots

Sunspots are patches created by strong magnetic fields on the surface of the Sun. These areas are cooler than their surroundings (about 1000 degrees cooler), so, they appear darker. Sunspots are visual indications of the process that send charged particles into space. These particles may be captured by Earth's magnetic field to create the aurora.

Solar wind

A gas of electrons and ions - at supersonic speeds (refer to The World of Science of November 19, 2006) - is continuously emitted from the Sun. This stream of gas is called the solar wind.

The solar wind is filled with breezes and hurricanes, and when a strong eruption from the Sun hits the Earth, the stable situation - which may have existed for several days - is strongly disturbed. The most spectacular result of this interaction is an intense aurora significantly closer to the equator, which occurs once in 200 years, than during normal conditions.

The aurora oval

The aurora zones are the places on Earth where auroras occur most often and with the greatest intensity. It was first found by Swiss physicist Herman Fritz (1829-1902).

He showed that the Northern lights have a maximum zone close to 67 degrees north and called this belt the aurora zone.

During the 1960s, the distribution of auroras were mapped. Then, it was discovered that the auroras display a continuous oval zone around the magnetic pole in both hemispheres.

The aurora oval can be regarded as fixed in space with reference to the Sun. As the Earth revolves underneath, the daily variations in the aurora's position occur. Modern studies have clearly shown that the shapes and locations of the ovals vary greatly with solar activity.

With increasing activity on the Sun, the oval widens and spreads, mainly towards the equator.

Read more about Northern lights in our next Geo facts page.

Janani Amarasekara