What are Auroras and What Causes Them?

Aurora over Varbla in Finland — Image Credit: Kristian Pikner

The image above shows a spectacular display of the Aurora Borealis over Finland. Also known as the Northern Lights, auroras typically occur at latitudes of between 10 and 20 degrees from the geomagnetic poles but not typically over the geographic poles themselves. When this phenomenon takes place in the Southern Hemisphere, it is known as the Borealis Australis or Southern Lights.

What causes auroras?

Auroras happen when charged particles that form the solar wind interact with the Earth’s magnetic field. In simple terms, an aurora is the result of charged particles from both the solar wind and the magnetospheric plasma (which consists largely of electrons and protons that surround Earth) being forced into the atmosphere’s thermosphere/exosphere layer by Earth’s magnetic field.

These particles lose their energy when they enter Earth’s upper atmosphere, but 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. This, in turn, produces areas of optical light that can take on the form of sheets, rays, and filaments of varying colors.

How do solar winds affect auroras?

The intensity of an aurora’s optical light depends entirely upon the velocity at which the charged particles from the solar wind enter Earth’s upper atmosphere. While most auroras are visible only at high northern and southern latitudes, during a violent geomagnetic storm the area in which an aurora occurs (known as the auroral oval) can expand and be forced to nearly mid-northern/southern latitudes. One such extreme example is The Great Solar Storm of 1989 in which the aurora usually confined to northern Canada was seen as far south as some Caribbean islands.

What are the different types of 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 reach down almost to the ground. Below are some details that explain why no two auroras are ever the same:

Discrete Auroras

Also known as auroral arcs, these are the most distinctive, and 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

Diffuse auroras are commonly areas of faintly glowing light that often approach the lower limits of human vision. This can make them almost indistinguishable from high-level, moonlit clouds, except that starlight passes directly through a diffuse aurora almost undiminished, whereas a cloud almost totally obscures starlight. Furthermore, diffuse auroras often display fairly regular variations in their brightness, although the period of variation depends on local conditions within the aurora.

Why are auroras 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-

Red light

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.” Carmine, scarlet, and crimson are the most often seen hues of red light in auroras.

Green light

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. This 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 slow variations in the intensity of green auroras.

Blue light

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. 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.”

Ultraviolet light

Some ultraviolet light frequencies fall within the optical spectrum and have been observed to be present in some auroral displays. Not all people have the ability to see ultraviolet light that falls into the optical spectrum, though. Nonetheless, ultraviolet has been observed in auroral displays on Jupiter, Saturn, and Mars.

Infrared light

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.

The Carrington Event of 1895

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 September 1, 1859, Richard Carrington, an English amateur astronomer, observed intense bright white light erupting from sunspots. These turned out to be caused by an enormous and tremendously violent coronal mass ejection event on the Sun. The event is estimated to have had the power of 10 billion atom bombs, and 17.6 hours later Earth witnessed its effect.

The Northern Lights and Southern Lights were witnessed as far as the tropics, instead of near the planet’s poles as usual. The resulting geomagnetic storm, known as the “Carrington Event,” also caused a complete disruption to telegraph systems across North America and Europe, with reports of sparks flying from telegraph machines, and shocking operators across the globe.

This is not just a matter of historical interest, though. 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 today’s age, such an event could result in large-scale disruption to our technological systems, including WiFi and mobile data networks, and a extensive communications breakdown.

In recent times, auroras have caused widespread power outages on Earth, and several satellites in space have also suffered fatal damage as the direct result of violent solar events.

What Are the Northern Lights and Southern Lights?