The aurora is one of the most magnificent sights in the sky and we counted ourselves lucky to see a fairly decent one (captured in the attached photo) in March of 2005, while staying at Chena Hot Springs, Alaska.
To understand the aurora, one can visualize the Earth as a giant spherical magnet, with magnetic field lines extending from the north to south (magnetic) pole. These magnetic field lines have the property that any charged particles (+ protons, - electron or ions) that approach, will spiral along them.
The Earth itself, is "bathed" in the solar wind, a stream of high speed charged particles that flows into space, originating from the Sun's corona. (A hot, gaseous envelope that spews these particles out continuously, moreso when there’s a violent explosion known as a Solar Flare)
Around the Earth the speed of these particles can reach 400- 500 km/second. (Because of its high temperature, over a million degrees, the corona gas is ionized so must consist of charged particles, mainly (+) protons, and (-) electrons).
During high solar activity (e.g. near sunspot maximums) a higher flux of these charged particles inundates the solar wind, and the region around the Earth.
The Earth's magnetic field traps the particles, and the highest density is around the polar regions, which we refer to as the "auroral ovals". In these regions, very large electric currents are set up, as the charged particles start moving in unison about the magnetic field lines. These currents can easily reach a few million amperes.
As this discharge occurs, one or more outer electrons is stripped from the atoms, for example from oxygen in the atmosphere then recombines again to form new (e.g. oxygen) atoms. With this recombination there is emission of light, for a certain part of the visible spectrum. This is basically what gives rise to the "northern lights", or what we call the aurora.
For example, in the case of recombination of oxygen atoms, their emitted light is in the green region of the spectrum. The aurora or northern lights we see display a kind of green curtain-like shimmering. The remarkable red aurora is produced by emission at the 630 nm (nanometer) line of oxygen and at relatively high altitudes (e.g. 200-600 km) compared to green - which tends to form below 100 km and the oxygen line at 557 nm is excited.
A great analogy has been given by Syun Akasofu (formerly of the Geophysical Institute in
In gauging the power and intensity of auroras at different times, it is useful to remember that ultimately the aurora derives its power and potential from the Sun and specifically the charged particles of the solar wind. This is why the most spectacular displays are usually near sunspot maximum. Around those times the currents I noted earlier are “amped” up – no pun intended- to 10^6 A or more. To give an example, during the relatively quiet Sun interval from 2002-08, the residual power for the magnetospheric generator is on the order of maybe a tenth of a megawatt. If we see a new cycle coming on and solar wind activated – we may get that power up to a million megawatts for a few hours.
In contradistinction to the normal aurora, the black aurora emerges with dark structures formed when negatively charged particles are sucked up and out of the atmosphere along magnetic field lines that adjoin the normal bright emission aurora. This negative action effectively completes a circuit between the ionosphere and the magnetic field, creating dark vertical shapes that can stretch tens of thousands of miles and remain visible for several minutes.
One of these years we hope to get back to Alaska again and see the black aurora!