AURORA

Auroras, also known as northern and southern polar lights, are natural light displays in the sky, and usually observed at night. They typically occur in the ionosphere. They are also referred to as polar auroras. This is a misnomer however, because they are commonly visible between 65 to 72 degrees north and south latitudes, which place them a ring just within the Arctic and Antarctic circles. Aurorae do occur deeper inside the polar regions, but these are infrequent and often invisible to the naked eye. An aurora is a natural display of glowing light in the night sky, mainly in zones around the magnetic north and south poles of the Earth and some other planets. Auroras can be spotted throughout the world and on other planets. They are most visible closer to the poles due to the longer periods of darkness and the magnetic field.

The aurora is formed when charged particles (electrons and protons) are guided by the Earth's magnetic field into the atmosphere near the poles. When these particles collide with atoms and molecules of the upper atmosphere, primarily oxygen and nitrogen, some of the energy in these collisions is transformed into the visible light that characterizes the aurora.

Causes of Aurora

The energy source for the aurora is 149 million kilometers (km) (93 million miles) from Earth at the sun. The sun continuously emits charged particles (mostly protons and electrons), which are the by products of thermonuclear reactions occurring inside the sun. These charged particles make up the solar wind, which travels away from the sun through space at speeds ranging from 300 to 1,000 km/sec.—about a million miles per hour. Traveling at this high speed, the solar particles can reach the Earth in two to three days.

Height of Aurora

Aurora Polaris, or 'polar lights', occurs most often above 60° north or south of the equator. Generally, if an auroral band has an easily discernible lower border, this will be at around 60 to 70 miles in altitude. Auroral rays may extend above the lower border for hundreds of miles. If the lower border has a pinkish edge to it (resulting from an emission of molecular Nitrogen), the altitude may be around 50 to 60 miles. A diffuse red aurora occurs above 150 miles.

Most aurorae occur between 90 and 130 km above sea level, but some, particularly the ray-like forms, extend to several hundred kilometers up. In comparison, the usual altitude for a jet aircraft is around 10km and the ozone layer lies between 20 and 30km so we have to be almost up at the heights of satellites’ orbits to be at the same height as the aurora. A consequence of its great height is that the aurora is visible at horizontal distances of several hundred kilometers. Thus an aurora over Bear Island will be visible from both Spitsbergen and Tromsø, and one over Tromsø can be seen in the northern sky from central Norway.

The above image is taken by NASA. Aurora Australis as taken by the space shuttle Discovery from low Earth orbit. The variation of the colour of the glow of the aurora with height may be clearly seen with the red aurora occurring at greater heights than the green aurora.

Types of Aurora

The aurora can only be seen against a dark sky, and the chances of observing an Aurora are obviously much better in winter when nights are also dark at auroral latitudes. There are two basic types of Aurora, Diffuse and Discrete aurora.

1- DIFFUSE AURORA

Unlike the discrete aurora, which is due to the parallel acceleration of electrons down along the earth's magnetic field lines into the atmosphere, diffuse aurora "dribbles" in from the plasma sheet. This dribbling comes from wave-particle interactions that scatter electrons and give them velocities that are more parallel to the magnetic field line.

2- DISCRETE AURORA:

Discrete auroras are the most intense auroral types, where field-aligned acceleration plays an important role, forming the so-called inverted-V precipitation.

Discrete auroral types include