Aurora (astronomy)

In astronomy, an aurora is an optical phenomenon characterised by colourful displays of light in the night sky, caused by the interaction of charged particles from the solar wind with the upper atmosphere of a planet. The most powerful aurorae tend to occur after coronal mass ejections. In Latin, aurora means 'dawn'.

On Earth, Jupiter, Saturn, Uranus and Neptune, aurorae are caused by the interaction of solar wind particles with the planet's magnetic field, and are therefore most prominent in higher latitudes near the magnetic poles. For this reason, the aurora occurring in Earth's Northern Hemisphere is called the aurora borealis, or northern lights; and in the Southern Hemisphere the aurora australis. However, aurorae also occur on Venus and Mars, which lack planetary magnetic fields. On Venus, atmospheric molecules are energised directly by the solar wind; on Mars, aurorae occur near localised magnetic anomalies in the planetary crust which are remnants of a presumed former planetary magnetic field which is now long extinct.

On Earth, aurorae occur when the Van Allen radiation belts become 'overloaded' with energetic particles, which then cascade down magnetic field lines and collide with Earth's upper atmosphere.

Origin and appearance

The origin of the aurorae is 149 million km from Earth at the Sun. Energetic particles from the Sun are carried out into space along with the ever-present, hot solar wind. This wind sweeps supersonically toward Earth through interplanetary space at speeds ranging from 300 to over 1000 km per second, carrying with it the solar magnetic field. The solar wind distorts Earth's magnetic field to create the comet-shaped, plasma-filled magnetosphere. The terrestrial magnetic shield acts as a barrier, protecting Earth from energetic particles and radiation in the hot solar wind. Particle energy and momentum are transferred from the solar wind to the magnetosphere through a process known as 'magnetic reconnection'.

In this process, interplanetary magnetic field lines (originating from the Sun) are coupled to Earth's magnetic field. Particles in the solar wind can enter this newly created magnetic field line. Auroral physicists call this an 'open magnetic field line' (the field line is open into the solar wind). Due to the dynamic pressure of the solar wind, this newly opened magnetic field line will be convected over the polar cap, and into the tail of Earth's magnetosphere. Here, a new magnetic reconnection can occur, creating a new, closed magnetic field line. The convecting field line will contain solar wind particles. Some of these particles will be able to reach the ionosphere before the field line has reached the magnetospheric tail. These particles will create dayside aurorae. Nightside aurorae are created from particles accelerated from the magnetospheric tail towards Earth. These particles will be trapped on the closed field line.

Electrons trapped in Earth's magnetic field (the magnetic mirror effect) are accelerated along the magnetic field toward the polar regions and then strike the atmosphere to form the aurorae. Aurorae are most intense at times of intense magnetic storms caused by sunspot activity. The distribution of auroral intensity with altitude shows a pronounced maximum near 100 kilometres above Earth.

The particles, which stream down the magnetic field of Earth, reach the neutral atmosphere in a rough circle called the auroral oval. This circle, or annulus, is centred over the magnetic pole and is around 3000 km in diameter during quiet times. The annulus grows larger when the magnetosphere is disturbed. The location of the auroral oval is generally found between 60 and 70 degrees north and south latitude. During intense solar activity, the auroral oval expands, and aurorae have been seen from latitudes as low as 25-30 degrees north and south on extreme occasions. For example, on 7 November 2004, following a Coronal Mass Ejection, they were seen as far south as Arizona. At 45 degrees, aurorae are visible approximately five times per year, while above 55 they are visible almost nightly.

Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above Earth's surface and extend upward along its magnetic field for hundreds of kilometres. These arcs or curtains can be as thin as 100 metres while extending from horizon to horizon. Auroral arcs can nearly stand still and then, as though a hand has been run along a tall curtain, begin to dance and turn. After magnetic midnight, aurorae can take on a patchy appearance and the patches often blink on and off once every 10 seconds or so until dawn. Most of the auroral features are greenish-yellow but sometimes the tall rays will turn red at their tops and along their lower edge. On rare occasions, sunlight will hit the top part of the auroral rays creating a faint blue colour. On very rare occasions (once every 10 years or so) aurorae can be a deep blood red colour from top to bottom. In addition to producing light, the energetic auroral particles deposit heat. The heat is dissipated by infrared radiation or transported away by strong winds in the upper atmosphere.

In recent years, the popularity of 'Aurora Travel' has brought growing numbers of tourists to many traditionally inhospitable destinations during their polar winters. Thanks to the warming influence of the gulf stream, and their relative accessibility, Iceland and Northern Scandinavia are popular. In addition to auroral activity, optimal viewing requires cloud free skies and minimal unnatural light; it is therefore largely a matter of luck. Photography of the aurora requires cameras equipped to hold the shutter open in excess of 5 seconds. Digital camera batteries will likely expire very quickly in the cold environment, making spare batteries a good suggestion.

The physics of the aurora

Aurorae are caused by the interaction of high energy particles (usually electrons) with neutral atoms in Earth's upper atmosphere. These high energy particles can excite (by collisions) valence electrons that are bound to the neutral atom. The excited electrons can then return to their initial, lower energy state, and in the process release photons (light particles). This process is similar to the plasma discharge in a neon lamp.

Any particular colour of the aurora depends on a specific atmospheric gas and its electrical state, and on the energy of the particle that hits the atmospheric gas. Atomic oxygen is responsible for the two main colours of green (wavelength of 557.7 nm) and red (630.0 nm) from high altitudes. Nitrogen causes the colour blue to appear, e.g. at 427.8 nm (molecular ions) as well as the rapidly varying red from the lower borders of active auroral arcs.

One of the first scientists to model aurorae was Norwegian Kristian Birkeland. His magnetised terrella (simulating Earth), shows that energetic electrons directed toward the terrella are guided toward the magnetic poles and produce rings of light around the poles. He further suggested 'currents there are imagined as having come into existence mainly as a secondary effect of the electric corpuscles from the sun drawn in out of space' (1908). Such currents were later supported in a paper by Hannes Alfvén, and in 1969 Milo Schield, Alex Dessler and John Freeman, used the name 'Birkeland currents' for the first time, whose existence was finally confirmed in 1973 by the navy satellite Triad.

Variations on the Sun

The Sun is a star with some features that are highly variable on time scales of hours to hundreds of years. The interplanetary magnetic field direction and solar wind speed and density are driven by the activity on the Sun. They can change drastically and influence the geomagnetic activity. As geomagnetic activity increases, the lower edge of the auroral ovals usually move to lower latitudes. Similarly, solar mass ejections coincide with larger auroral ovals. If the interplanetary magnetic field is in the opposite direction of the Earth's magnetic field, there can be increased energy flow into the magnetosphere and thus, increased energy flow into the polar regions of the Earth. This will result in an intensification of the auroral displays.

Disturbances in the Earth's magnetosphere are called geomagnetic storms. These, in turn, can produce sudden changes in the brightness and motion of the aurorae called auroral substorms. The magnetic fluctuations of these storms and substorms may cause surges in electric power lines and occasional equipment failures in the power grid, resulting in widespread power outages. They can also impact the performance of satellite-to-ground radio communications and navigation systems. Magnetospheric storms can last several hours or even days, and auroral substorms can occur several times a day. Each substorm can deliver several hundred terajoules of energy, as much as the electrical energy consumed in the entire United States of America over 10 hours.

Measuring the geomagnetic field

The geomagnetic field can be measured with instruments called magnetometers. Data from many magnetometers allow observers to track the current state of the geomagnetic conditions. The magnetometer data are often given in the form of 3-hourly indices that give a quantitative measure of the level of geomagnetic activity. One such index is called the K-index. The K-index value ranges from 0 to 9 and is directly related to the amount of fluctuation (relative to a quiet day) in the geomagnetic field over a 3-hour interval. The higher the K-index value, the more likely it is that an aurora will occur. The K-index is also, necessarily, tied to a specific observatory location. For locations where there are no observatories, one can only estimate what the local K-index would be, by looking at data from the nearest observatory. A global average of auroral activity is converted to the Kp index.

Auroral sounds

It is frequently claimed that sightings of aurorae are accompanied by humming and/or crackling sounds. The propagation of these sounds through the air (like a speaker vibrating the air molecules) is unlikely. Aurorae occur around 100 km above the earth in extremely rarefied conditions which certainly could not transmit audible sounds well enough for them to reach ground level. One possibility is that electromagnetic waves are transduced into sound waves by objects in the vicinity of the observer, or directly influence the auditory senses of the observer.

Aurora in folklore

In Bullfinch's Mythology from 1855 by Thomas Bulfinch there is the claim that in Norse mythology:

‘

The Valkyrior are warlike virgins, mounted upon horses and armed with helmets and spears. /.../ When they ride forth on their errand, their armour sheds a strange flickering light, which flashes up over the northern skies, making what men call the 'Aurora Borealis,' or 'Northern Lights.'

’

While a striking notion, there is nothing in the Old Norse literature supporting this assertion. Although auroral activity is common over Scandinavia and Iceland today, it is possible that the Magnetic North Pole was considerably further away from this region during the centuries before the documentation of Norse mythology, thus explaining the absent references.

The first Old Norse account of norðrljós is instead found in the Norwegian chronicle Konungs Skuggsjá from 1250 AD. The chronicler has heard about this phenomenon from compatriots returning from Greenland, and he gives three possible explanations: that the Ocean was surrounded by vast fires, that the sun flares could reach around the world to its night side, or that glaciers could store energy so that they eventually became fluorescent.

An old Scandinavian name for northern lights translates as herring flash. It was believed that northern lights were the reflections cast by large swarms of herring onto the sky.