The Aurora – Introduction
One beautiful manifestation of the interaction of the solar wind with our atmosphere is coloured light displays observed in the night sky. They are most often seen within a band centred on the north and south magnetic poles and are known as the Aurora Borealis and Aurora Australis, respectively. (Aurora is the Roman goddess of the dawn, Boreas is the Greek name for the north wind, and Australis is the Latin word for the south.) The Aurora Borealis is often called the northern lights as it tends to be seen as a green or reddish glow in the northern sky. It is most commonly seen around the vernal and autumnal equinoxes, though it is not known why this should be so.
When the Sun is observed in the light emitted by excited hydrogen at a time of the solar eclipse, bright columns of gas are often seen stretching up from the chromosphere into the corona. These are called prominences. They are caused by dense ionized gas that is suspended by the Sun’s magnetic field but ‘rains’ back from the corona into the chromosphere.
Solar flares, originating in the Sun’s corona, are violent explosions whose energy is believed to be derived from the ‘breaking’ and then ‘reconnection’ of the Sun’s magnetic field lines and the resulting ‘release’ of magnetic energy. The total energy of between 1022 J and 1025 J, released over periods of minutes to hours, accelerates electrons, protons and heavier ions to relativistic speeds – that is, close to the speed of light. Flares tend to occur above the active regions around sunspots, which is where intense magnetic fields emerge from the Sun’s surface into the corona and thus tend to be more frequent at the time of solar maximum.
Flares are related to what are called coronal mass ejections, which are the ejection of material from the solar corona consisting largely of electrons and protons with small quantities of heavier elements such as helium, oxygen, and iron. The material carries with it elements of the coronal magnetic field. The streams of highly energetic particles have been observed to take as little as 15 min to reach the Earth (so travelling at about one-third the speed of light). They can pose a threat to astronauts and have, in the past, destroyed satellite sub-systems.
Interference with short-wave radio communication can also occur, and the interaction of the entrained magnetic field with electricity power transmission cables can cause problems and the possible shutdown of electricity grids –as happened in the Quebec province of Canada in March 1989.
How Aurora be Created
Auroras are caused by the collision of charged particles with atoms high in the Earth’s upper atmosphere. As the Earth’s field lines open out into space above the north and south magnetic poles, charged particles may more easily reach the upper atmosphere in the regions near the magnetic poles. There they collide with atoms of gases in the atmosphere and lift electrons into higher energy levels. The electrons then cascade down to their ground states so emitting light. Most light appears to be emitted by emissions from atomic oxygen, resulting in a greenish glow at a wavelength of 557.7 nm and a dark-red glow at 630.0 nm. Amongst the many other colours that are sometimes observed, excited atomic nitrogen gives a blue colour whilst molecular nitrogen produces a purple hue. Often the auroral glow is in the form of ‘curtains’ that tend to be aligned in an east-west direction. Sometimes these curtains change slowly but at other times they seem to be in continuous motion. Their shape is determined by the direction of the Earth’s field in the region of the observer and observations have shown that electrons from the solar wind spiral down magnetic field lines towards the Earth. The author can testify to the awesome sight that results when field lines guide electrons down to a bright auroral patch directly above the observer. Due to perspective, the converging auroral rays appear as vertical rays of light reaching upwards to what is called a ‘corona’ overhead.