An aurora is a natural light display that occurs in the sky, predominantly seen in high-latitude regions like Alaska, Canada, Iceland, Norway, Sweden, Finland, and Russia. Auroras are produced when charged particles from the sun interact with molecules in Earth’s upper atmosphere. These interactions cause the molecules to give off varying colors of light, creating shimmering curtains, rays, or spirals across the night sky.
Auroras typically appear green or red due to the type of gas molecules involved. However, they can also manifest in other hues like purple, blue, or yellow. On very rare occasions, auroras can even appear black. A black aurora is an unusual phenomenon not fully understood by scientists. In this article, we’ll explore what causes black auroras and why they are so uncommon.
What Causes Auroras?
To understand what creates a black aurora, it helps to first review what causes normal-colored auroras. The Aurora Borealis in the north and Aurora Australis in the south are the result of solar wind. The sun constantly emits a stream of charged particles called the solar wind. When these particles interact with Earth’s magnetosphere, they are funneled down magnetic field lines toward the poles.
As the charged particles approach the upper atmosphere, they collide with oxygen, nitrogen, and other gas molecules. These collisions transfer energy to the molecules, causing them to become “excited.” As the molecules return to their normal state, they release photons of light which are seen as the shimmering glows of an aurora.
Different gases emit different colors of light. The most common are:
|Blue, purple, red
The type of gas, the altitude, and the amount of energy transferred determines the colors and shapes seen in an aurora.
What is a Black Aurora?
A black aurora is essentially the opposite of a typical aurora. Instead of colorful lights, a black aurora appears as dark ribbons flowing across the night sky. These unusual dark patches are caused by the absence of light emission from the gas molecules.
During a black aurora event, the charged particles from the sun transfer very little energy to the atmospheric gases. This fails to raise the molecules into an excited state. With the gases remaining in their natural unexcited state, no light-emitting reactions occur to produce an aurora glow.
Black auroras are exceptionally rare. While thousands of typical auroras occur each year, only a few dozen confirmed cases of black auroras have ever been documented. This makes them a subject of intense scientific interest when they do appear.
The first known sighting of a black aurora was made in 1770 by French astronomer Jean-Jacques d’Ortous de Mairan. He theorized the phenomenon may have been caused by intense magnetic activity blocking light from sunspot auroras.
Over the next two centuries, a small number of additional black aurora events were recorded:
|November 17, 1790
|October 24, 1827
|Charleston, South Carolina
|February 4, 1837
|Fredericton, New Brunswick
|December 1, 1876
But it wasn’t until the 1960s, with more advanced auroral monitoring equipment available, that the phenomenon was directly studied in detail.
Famous Black Aurora of 1967
The most thoroughly-documented black aurora occurred on February 11, 1967 above Alaska. It was observed by a research team at the Poker Flat Research Range near Fairbanks. This extremely rare event coincided with the passing of an unusually strong solar wind storm.
The researchers described the aurora as appearing like “black auroral veils.” They reported an almost complete lack of visible light emissions for nearly an hour despite intense auroral activity occurring overhead. Photographs also clearly showed dark auroral ribbons traversing the sky.
Data from imaging equipment verified that this was not just a simple visual effect. The opacity of the atmosphere during this event measurably decreased, confirming the temporary absence of light.
Scientists are still working to uncover what combination of conditions can create black auroras. Some leading theories include:
– **Strong magnetic fields** – Intense magnetic activity may occasionally suppress the interaction between solar particles and atmospheric gases that normally creates visible light.
– **Dense solar wind** – An extremely dense influx of solar particles bombarding the upper atmosphere could temporarily darken it.
– **Gas composition** – Shifting concentrations of oxygen, nitrogen, and other gases may favor interactions producing little or no light.
– **Exhausted gases** – Previous auroral activity could deplete the immediate availability of gases for more light-emitting reactions.
– **Electromagnetic factors** – Complex electromagnetic effects might inhibit emissions for certain visual wavelengths.
However, none of these potential mechanisms have been conclusively proven yet to account for all black aurora observations. Ongoing auroral research hopes to capture new black aurora events as they occur to better analyze them.
Other Dark Aurora Phenomena
In addition to the extremely rare black auroras, there are also other types of darker auroral phenomena that have been documented:
– **Brown auroras** – These display darker brownish hues rather than vivid colors. They are caused by interactions with oxygen molecules at lower altitudes.
– **Aurora twilight arcs** – These very faint, nearly invisible arcs can be detected in twilight skies during intense geomagnetic activity.
– **Black auroral veils** – Gossamer-like dark wisps of aura that are translucent and difficult to see. They were first imaged in 1956.
– **Willow auroras** – Faint auroral veils that undulate like hanging willow branches. Their movement seems unrelated to wind.
These types of phenomena are slightly more common than a true black aurora, but still rare and not completely explained. They serve to illustrate the complexity and mystery still surrounding auroral science.
Black auroras stand out as one of the most unusual and elusive atmospheric events. Their deeply dark appearance contrasts starkly with the more typical brilliant displays of the Northern and Southern Lights. Scientists are still unlocking the combination of space weather conditions and atmospheric chemistry that can produce these brief periods of darkness amid intense auroral activity. Ongoing study will hopefully shed more light on these aptly named black auroras. For aurora enthusiasts, glimpsing one of these ultra-rare events would be an unforgettable thrill and privilege. Their air of mystery invokes a sense of deeper natural wonder awaiting discovery at the frontiers of aurora science.