Aurora

An aurora is a broadened source of light of different forms and colors observable at high altitudes and sometimes at mid-latitudes. The effect is named Aurora Borealis in the Northern Hemisphere and Aurora Australis in the South. Their brightness may reach that of the full moonlight. The auroral oval (one in the Northern and one in the Southern Hemisphere) is the location where most of the auroras are observable. It acts as belt around each of Earth’s magnetic poles with a maximum latitudinal extent at magnetic midnight.

The polar cap auroras (inside the oval) and daytime auroras are usually weak and diffuse while in the oval, they are more intense and variable in time. In both cases, auroras are generated by particles of solar origin interacting with the atoms and molecules of the Earth’s upper atmosphere through direct collisions and chemical processes. However, in the case of the auroral oval, the particles are accelerated by some complex magnetospheric processes leading to phenomena of brighter intensity, of various forms, both characters being highly time dependent.

The auroras are usually observed between 80 and 300 km high depending on the characteristics of the precipitating particles (electron and proton energies) and atmospheric composition, leading to an auroral spectrum made of many lines from atomic and molecular neutral or ionized species. The occurrence and intensity of auroras follow the solar activity; for example the typical 11-year solar cycle is found in the observations. The aurora’s characteristics represent the last stage in a series of processes starting in the solar atmosphere and taking into account the Earth’s magnetosphere and atmospheric physics.

The modern means of auroral observations are spectrometers which analyze their spectrum, photometers which study the behavior of one or several emissions lines, cameras which record the aurora forms as a function of time, and radar. These instruments can be used on the ground or from orbiting platforms where they provide a global survey of auroras.

Auroral intensity is a function of time. The change may be either slow, as for a pulsating surface (0.1 Hz), or reach 10 Hz for certain homogeneous forms. The line of the greatest intensity is due to the emission of atomic oxygen at 557.7 nm frequently referred to in the scientific literature as the ‘green line’. Auroral colors are usually green, red or blue depending on the atmospheric species involved in the excitation processes. Auroras of intensity IBCI or II appear colorless. Yellow auroras are due to the blending of red and green auroras.

Scientific investigations of the aurora began in earnest in the 18th century, with the work of Edmond Halley, Jean-Jacques de Mairan (1678–1771) and others. The first modern account of the aurora Australis was made by Captain Cook in 1773. The long-suspected connection between solar activity and the aurora was confirmed when a major solar ?are was observed in 1859, followed a day or so later by a huge auroral storm. Theoretical work and experiments by Kristian Birkeland (1867–1917) and Carl Störmer (1874–1957) in the early 20th century began to clarify some of the mechanisms by which the aurora occurs. Störmer undertook an intensive programme of parallactic auroral photography from Norway beginning in 1911. Great progress in understanding the global distribution of auroral activity was made during the International Geophysical year of 1957–58. Current models owe much to the work of the Japanese researcher Syun-Ichi Akasofu.

 Questions to Ponder

• Why are Auroras observed only near the Poles?
• What makes the color of the aurora?
• Does the aurora have any effect on the environment?
• Can you predict when and where there will be aurora?
• Is there any difference between Aurora Borealis & Aurora Australis?