Tuesday, May 20, 2014

The Aurora and the Black Aurora

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.

Auroras can display as both diffuse and discrete. In the first case the shape is ill-defined and the aurora is believed to be formed from trapped particles originally in the magnetosphere, which then propagate into the lower ionosphere via wave-particle interactions.  Thus, multiple colored auroras can be explained by emissions from different atoms in the upper atmosphere, mainly in the region of the magnetic poles. This is also why, of course, they are more often seen in the vicinity of the N, S magnetic poles. (Though there have been reports of northern lights being seen as far south as northern Florida, especially during periods of exceptional sunspot activity or large solar flares, massive explosions on the Sun).

A great analogy has been given by Syun Akasofu (formerly of the Geophysical Institute in Fairbanks, AK),  comparing the aurora to images on a TV screen. In this case the (polar) upper atmosphere corresponds to the screen and the aurora to the image that would be projected onto it, say for a TV. The electron beam in the TV (remember we are talking about the old-style cathode ray jobs!) corresponds to the electron beam in the magnetosphere. In the conventional TV motions of the image are generated by the changing impact points of the electron beam onto the screen. Similarly, with the aurora, its motions (i.e. moving sheets or curtains)  are produced by the moving impact points of the magnetospheric electron beams.

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.

Not often seen, and alas one we missed, is a variety called the “black aurora”. One such image taken at Fairbanks, AK in December 13 by Deborah Carter, is shown below:

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!

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