Astronomy

One of the most beautiful and also rare things that you will ever be able to see is the aurora. Anyone who has had the chance to see this wonderful site has been truly amazed and in awe. The Aurora is a bright glow that is observed in the night sky, usually in the polar zone. For this reason some scientists call it a “polar aurora” (or “aurora polaris”). In northern latitudes, it is known as the aurora borealis, which is named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas.

Especially in Europe, it often appears as a reddish glow on the northern horizon, as if the sun were rising from an unusual direction. The aurora borealis is also called the northern lights since it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. Its southern counterpart, aurora australis, has similar properties. Australis is the Latin word for “of the South”.

Auroras are now known to be caused by the collision of charged particles (e.g. electrons), found in the magnetosphere, with atoms in the Earth’s upper atmosphere (at altitudes above 80 km. These charged particles are typically energized to levels between 1 thousand and 15 thousand electronvolts and, as they collide with atoms of gases in the atmosphere, the atoms become energized. Shortly afterwards, the atoms emit their gained energy as light.

Light emitted by the Aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm) and especially at lower energy levels and at higher altitudes the dark-red glow (at 630.0 nm of wavelength). Both of these represent forbidden transitions of electrons of atomic oxygen that, in absence of newer collisions, persist for a long time and account for the slow brightening and fading (0.5-1 s) of auroral rays. Many other colors,  especially those emitted by atomic and molecular nitrogen (blue and purple, respectively) can also be observed.

These, however, vary much faster and reveal the true dynamic nature of auroras. As well as visible light, auroras emit infrared (NIR and IR) and ultraviolet (UV) rays as well as X-rays. While the visible light emissions of auroras can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth’s atmosphere tends to absorb and attenuate these emissions. Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction.

At some times, they form “quiet arcs”; at others (”active aurora”), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by the earth’s magnetic field. Indeed, satellites show auroral electrons to be guided by magnetic field lines, spiraling around them while moving earthwards.

The curtains often show folds called “striations”, which are curtain-like. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a “corona” of diverging rays, an effect of perspective. In 1741, Hiorter and Celsius first noticed other evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead.

This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named “auroral electrojets”

Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) established that the aurora appeared mainly in the “auroral zone”, a ring-shaped region of with a radius of approximately 2500 km around the magnetic pole of the earth, not its geographic one. It was hardly ever seen near that pole itself. The instantaneous distribution of auroras is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest towards the equator around midnight.