The image above shows a spectacular display of the Aurora Borealis over Finland. Known as the Borealis Australis in the Southern Hemisphere, auroras typically occur at latitudes of between 10 and 20 degrees from the geomagnetic poles, and can be observed at all local times if the sky is clear and sufficiently dark. Note that auroras typically do not appear directly over the geographic poles of Earth.
While it is known that auroras occur when charged particles that form the solar wind interact with Earth’s magnetic field, much of the actual mechanisms that cause the repeated magnetic connections, disconnections, and reconnections between the particles and molecules from the upper atmosphere and Earth’s magnetic field remain either unknown or not well understood.
Nonetheless, in simple terms, an aurora occurs when charged particles from both the solar wind and the magnetospheric plasma (which consists largely of electrons and protons that surrounds Earth) are forced into the atmosphere’s thermosphere/exosphere layer by Earth’s magnetic field. Note that these particles lose their energy when they enter Earth’s upper atmosphere.
The interaction between the injected particles and Earth’s magnetic field excites various atmospheric constituents to the point where ionization of some atmospheric constituents occurs, which in turn, produces areas of optical light that can take on the form of sheets, rays, and filaments of varying colors. Note though that the intensity of the optical light depends entirely upon the velocity at which the charged particles entered Earth’s upper atmosphere, with this velocity sometimes being a function of the energy of the solar event that drives the solar wind to beyond “normal” levels.
While most auroras are visible only at high northern and southern latitudes, in some circumstances such as during a violent geomagnetic storm, the area in which an aurora occurs (known as the auroral oval), can be expanded by the storm, which can force the aurora to nearly mid-northern/southern latitudes. While an aurora may appear to be directly overhead at high latitudes, many observers at lower latitudes have reported seeing faint green and sometimes reddish glows towards the poles, almost as if the Sun were rising from an unusual direction at the wrong time.
What are the Main Types of Visible Auroras?
Some auroras appear as relatively static sheets of light, but many others constantly change their shapes into arcs, filaments, ray-like structures, and even blobs of slowly pulsating light. Moreover, some auroras appear to end abruptly high above the surface, while others seem to fill the entire observable sky, and even to reach down almost to the ground. Below are some details that explain why no two auroras are ever the same:
Aka, discrete auroras, these auroras are the most distinctive, and the brightest of all visible auroras. In fact, some are so bright that it is possible to read a book by their light. This type of aurora typically resembles a curtain, and consists of many parallel rays that each spiral around a part of Earth’s magnetic field as they descend towards the ground. Because Earth’s magnetic field constantly breaks up, and reconnects in complex and largely unpredictable patterns on a local scale, the body of charged particles that spiral around them also break up, and reform in equally complex patterns that make it look as if the aurora is “dancing” around the entire sky.
Diffuse auroras, on the other hand, are most commonly relatively featureless patches of faintly glowing light that frequently approach the lower limits of human vision. In some cases, the only way to distinguish between a diffuse aurora and high-level, moonlit clouds is the fact that starlight passes through a diffuse aurora almost undiminished, whereas a cloud almost totally obscures starlight. Moreover, diffuse auroras often display fairly regular variations in their brightness, although the period of variation depends on local conditions within the aurora.
Why Auroras have Different Colors
The interplay between the various factors that create visible light in a typical auroral display is an exceedingly complex subject, and one that falls outside the scope of this article. However, below are some details on the main drivers of the processes that create optical light in auroras-
At very high altitudes, red light is created by exited atomic oxygen that radiate visible light at 630 nanometres. However, the combination of low atomic oxygen levels at high altitudes, and the fact that the human eye has a low sensitivity to red light, means that the red component of an aurora only becomes visible under conditions of intense solar activity, and then only near the top of the auroral “curtain”. Note that carmine, scarlet, and crimson are the most often seen hues of red light in auroras.
At lower altitudes, the red emission at 630.0 nanometres is largely replaced by the emission of green light at 557.7 nanometres. The reason why green light predominates at lower altitudes is that atomic oxygen is more plentiful here, as well as the fact that human vision is more sensitive to green light than to any other color, which explains why green light is more commonly seen in auroras than other colors.
However, green auroras often seem to end abruptly, which is caused by the fact that atomic oxygen levels decrease very rapidly below about 100 km or so. Nonetheless, green auroras often appear to be pulsating, which is caused by the fact that both the 630.0 nanometre and 577.7 nanometre optical light frequencies fall into the “forbidden transition” zone of atomic oxygen. In simple terms, the forbidden transition zone is one where the transition between energy states of atomic oxygen is very slow, which results in the slow variations in the intensity of green auroras.
Typically, atomic oxygen is rare at altitudes below about 100 km or so, and at these altitudes, blue and (sometimes) violet emissions are produced by both molecular nitrogen and ionized molecular nitrogen, both of which radiate in a large number of frequencies in both the blue and red parts of the visible spectrum. Note that although blue light at 428.0 nanometres is the most common, blue light only becomes visible during conditions of very intense solar activity, and then only at the lower edges of an auroral “curtain”.
Although some ultraviolet light frequencies fall within the optical spectrum, and have been reported by some observers to be present in some auroral displays, not all people have the ability to see ultraviolet light that falls into the optical spectrum. Nonetheless, ultraviolet has been observed in auroral displays on Jupiter, Saturn, and Mars.
As with some ultraviolet light frequencies, some inferred frequencies also fall within the optical spectrum, and many observers have reported seeing hues of red in auroral displays that clearly fall within the (visible) infrared part of the optical spectrum.
Yellow and pink light
Under favorable conditions, red, green, and/or blue light within an auroral display can mix to produce various shades of pink, orange, and yellow-green light. In theory though, almost any color can be produced by mixing the red, green, and blue light in an auroral display, but in practice, the forces that drive the creation of auroral displays limit the observed colors in auroras to the ones listed here.
Auroras in History
There is no doubt that some spectacular auroral displays have occurred throughout Earth’s history, but in recorded history, the displays that occurred on August 28th, and again on September 2nd in 1895 must surely take pride of place as being the most powerful such event ever recorded.
On this occasion, the aurora was caused by an enormous and tremendously violent coronal mass ejection event on the Sun, and provided the first unambiguous evidence that auroral displays and electricity are inextricably linked.
On this occasion, parts of the telegraph network across the USA were of an appropriate length and orientation to conduct the electricity caused by the aurora, and some telegraph operators were able to communicate over long distances across the network solely through the electricity provided by the aurora. Below is a partial transcript of one such conversation-
Boston operator (to Portland operator): “Please cut off your battery [power source] entirely for fifteen minutes”.
Portland operator: “Will do so. It is now disconnected.”
Boston: “Mine is disconnected, and we are working with the auroral current. How do you receive my writing?”
Portland: “Better than with our batteries on. – Current comes and goes gradually.”
Boston: “My current is very strong at times, and we can work better without the batteries, as the aurora seems to neutralize and augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work without batteries while we are affected by this trouble.”
Portland: “Very well. Shall I go ahead with business?”
Boston: “Yes. Go ahead.”
This conversation carried on for more than two hours, and with no battery power on the network, to boot. While this is very interesting, it should serve to remind us that auroral displays are not only pretty and fascinating to look at but are also potentially dangerous and destructive. In fact, they have even caused widespread power outages in recent times on Earth, and several satellites in space have also suffered fatal damage as the direct result of violent solar events.