No insurmountable difficulties: The mysterious light

If anyone ever asked me what is the thing that you want to do the most before you die, I would say " I want to witness the aurora display!". It has always been my dream to witness the occurrence of the northern and southern lights since I was a child. There is an unexplainable mysterious force that attracts me to this kind of lights and I find them beyond words to describe how beautiful they are. There is no more regret if I could watch these lights live, be it the aurora borealis (northern lights) or the aurora australis (southern lights).

Aurora Borealis

An aurora is a natural light display in the sky (from the Latin word aurora, "sunrise" or the Roman goddess of dawn, predominantly seen in higher latitude ( Artic and Antartica) regions. Auroras are caused by charged particles, mainly electrons and protons, entering the atmosphere from above causing ionisation and excitation of atmospheric constituents and consequent optical emissions. Incident protons can also produce emissions as hydrogen atoms after gaining an electron from the atmosphere.

Visual forms and colours

Auroras take many different visual forms. The most distinctive and brightest are the curtain-like auroral arcs. They eventually fragment or 'break-up' into separate and rapidly changing, often rayed features which may fill the whole sky. These are the 'discrete' auroras which are at times bright enough to read a newspaper by at night.

Red aurora

Occurs at the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this colour visible only under more intense solar activity. The low amount of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains".

Green aurora

At lower altitudes the more frequent collisions suppress this mode and the 557.7 nm emission (green) nominates, fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to high stability of the nitrogen molecule) plays its role here as well as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. ( Red and green can also mix together to produce pink or yellow hues). The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains.

Yellow and pink are a mix of red and green or blue.

Blue aurora

At yet lower altitudes atomic oxygen is, uncommon and ionised molecular nitrogen takes over in producing visible light emission; it radiates at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of the solar activity.