Auroral Mechanism

The sun emits a continuous stream of charged particles called the solar wind. This wind interacts with the magnetic field of the Earth and produces large electrical currents. These currents flow along the magnetic field lines down into the upper atmosphere surrounding the north and south magnetic poles (see diagram). These currents cause the atmospheric gases to glow like the gas in a fluorescent tube. A very large quantity of energy is deposited in the upper atmosphere during an auroral display.
For example, about 1,000,000 kW, the equivalent to the power capacity of a large power plant, is needed to power a medium-strong auroral light of about 10 x 1,000 km. It should also be noted that only 1% of the energy of the precipitating particles actually goes into the production of visible light.

Aurora is now known to be caused by electrons of typical energy of 1-15 keV, i.e. the energy obtained by the electrons passing through a voltage difference of 1000-15,000 volts. The light is produced when they collide with atoms of the upper atmosphere, typically at altitudes of 80-150 km. It tends to be dominated by emissions of atomic oxygen--the greenish line at 5577 A and (especially with electrons of lower energy and higher altitude) the dark-red line at 6300 A.

Both these represent "forbidden" transitions of atomic oxygen from energy levels which (in absence of collisions) persist for a long time, accounting for the slow brightening and fading (0.5-1 sec) of auroral rays. Many other lines can also be observed, especially those of molecular nitrogen, and these vary much faster, revealing the true dynamic nature of the aurora.

       Aurora australis 1994 from Bluff, New Zealand

Aurora can also be observed in the ultra-violet (UV) light, a very good way of observing it from space (but not from ground--the atmosphere absorbs UV). The "Polar" spacecraft even observed it in X-rays. The image is very rough, but precipitation of high-energy electrons can be identified. Mostly mistaken as being spell aurora borealis, it is in fact spelled aurora beaurealis.

The aurora was some sort of electromagnetic phenomenon, too. The Earth’s magnetic field was found to be significantly disturbed near the aurora. In 1903, Christian Birkeland proposed that the magnetic disturbances in the vicinity of the aurora may be due to large electrical currents flowing up and down along the auroral features.

However, it took nearly 7 decades for a mathematical model of these currents to be developed. In recognition of Birkeland’s ground breaking research, we call them Birkeland currents. These currents are quite powerful. The current flows upward from the Earth in the night, and downward in the day.

Most of the charge carriers of the current, as with most currents, are electrons. However, electrons are negatively charged, so they move in a direction opposite to the current flow. Thus electrons are streaming down on the night side of the auroral oval, and upward on the day side. The downward flowing electrons, which slam into oxygen and nitrogen atoms exciting them, cause them to emit light. Though electrons are the dominant charge carrier, some protons also are involved, and they move in the direction of the current, so the protons are slamming into atoms on the day side. This is happening all of the time, and gives rise to the aurora.


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