WHAT IS AN AURORA?

Auroras, sometimes called the northern and southern lights are natural light displays in the sky. In order to see an Aurora, one usually needs a dark sky (no bright moon, no city lights) and relatively clear weather. Auroras usually occur near the magnetic poles of the earth and occur most often during the equinoxes. Note that the magnetic poles of the earth do not coincide with the geographic poles. For example, the Magnetic North Pole is located in the Arctic Islands of Northern Canada.

The name Aurora Borealis is attributed to Pierre Gassendi, a 17th century French philosopher, astronomer and mathematician. He named the natural light display for Aurora, the Roman goddess of dawn and Boreas which was the Greek name for the north wind. The Southern lights were named Aurora Australis because Australis is the latin word for South. As with most unexplained natural phenomena, historically there have been all kinds of legends and quite silly religious and superstitious beliefs associated with the Auroras.

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WHAT CAUSES AN AURORA?

Charged “solar wind” particles (mostly electrons) travel from the Sun towards the earth at high speeds. When these charged particles encounter the earth’s magnetic field, the particles are “funneled” along the magnetic field lines towards the magnetic poles.

As these particles enter the upper layers of the earth’s atmosphere, they collide with the atoms and molecules in that region, mostly Oxygen and Nitrogen. The exact composition and density of the atmosphere where the collisions occur determines the types of light emissions we can see.

When an electron hits an atom in the upper atmosphere, it can either knock out an electron out of the atom (ionization) or it can excite the atom by moving an electron into a higher orbit. When an excited electron returns to a lower orbit (ground state), it emits a photon of a very specific wavelength – in other words, a very specific color. Very specific colors are also emitted when an electron is re-captured by an ion.

The photons that come out of solar wind collisions with the upper atmosphere have therefore the signature colors of nitrogen and oxygen molecules and atoms.

Oxygen atoms, for example, emit photons in two typical colors: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it. The Aurora obviously consists of a mixture of all the colors emitted. To the naked eye it usually gives the impression of a greenish-whitish light (although I have experienced yellow, orange, red, purple and blue with very intense Auroras). Digital sensors (and film) have a different sensitivity to colors, therefore we often see more red and brighter and more saturated greens and purples in photographs versus the unaided eye.

The red Aurora tends to be at higher altitudes than the green Aurora. This is due to a peculiar characteristic of Oxygen atoms: Usually, an excited atom or molecule returns to its ground state and emits a photon very quickly, in a matter of microseconds or less. The Oxygen atom, however, is special.

Oxygen takes about 3/4 of a second to emit a green photon and almost 2 minutes to emit a red photon. If the atom happens to collide with another particle during this time, it often just passes its excitation energy over to the collision partner and thus never radiates the photon. Since collisions are more likely in the denser part of the atmosphere (lower altitudes), less red photons get emitted down below. This is why the red color usually appears only at the very top of an Aurora, where collisions between air molecules and atoms are rare.

Below about 100 Kilometers (roughly 60 miles) in altitude, even the green photons do not have a chance to be emitted; the green emissions get quenched by collisions. All that is left is a blue/red mixture caused by molecular Nitrogen emissions. This is why very intense Auroras tend to have a purple (mixture of blue and red) edge at the bottom.

By the way, Auroras are not unique to our planet and they have been detected and photographed in other planets.

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LOCATION AND TIMING

As mentioned above, the magnetic North Pole is in Northern Canada, and the ideal time to see the Aurora is when the sky is dark during the equinox. Therefore, this year I chose early February in Yellowknife Canada to photograph it. Yellowknife is not only a great location, but it is easy to fly there. There are quite a number of tourists, mostly Japanese, that go there just to see the Aurora, so the town is too touristy and commercial for my taste.

On the other hand, transportation to and from Yelowknife is excellent, and the restaurants are surprisingly good. Luckily, one can get away from most tourists at night, in order to observe and photograph this magnificent spectacle in total peace and quiet.

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EQUIPMENT

The temperature in Yellowknife was expected to average minus 40 degrees (both, Fahrenheit and Celsius. The scales cross at this temperature). For some reason, this year (2010) was extremely unusual, and I landed there to find a “heat wave”: The temperature was a balmy minus 10 degrees Centigrade.

Unfortunately, when the temperature gets this “warm”, there are usually heavy clouds in the area, but again, the weather was very strange and I was lucky to encounter several nights of clear weather.

In terms of clothing, layers are a must, and extremely warm clothing is required, as one is likely to spend several hours out in the cold in one location without moving at all.

In terms of camera equipment, a good solid tripod is mandatory since most exposures are too long to hand-hold the camera. The other mandatory requirement is to have at least one spare battery (I would recommend at least 2) kept in a warm pocket. Lithium Ion batteries have a bad habit of dying instantaneously with no warning in these types of cold temperatures. I missed several once-in-a-lifetime shots because one of my batteries that was almost full died at the worst possible time. Li Ion batteries start to operate normally again once they return to warmer temperatures.

Beyond that, the type of camera or equipment you use is a personal choice.

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TECHNIQUE

My personal preference for these types of images is to shoot with a moderate wide angle. I do not like man-made objects in my images, and I also do not like images of just the sky with no reference point. Therefore, I consistently tried to find a pristine untouched foreground under the sky.

Because of the brightness of the Aurora itself, there is usually more than enough light to capture foreground detail while exposing for the Aurora. Sometimes the color of the foreground looks strange as a result of the deep red or purple that a sensor can capture, but is basically invisible to the naked eye. One can also use a flashlight or strobes to enhance foreground detail during the exposure if so desired.

I found that exposures between 30 seconds and 1 minute at relatively low ISO settings worked best. It is quite difficult to see an image or to focus in darkness outdoors with an SLR, so my choice of camera for this trip was a Leica M9. The combination of rangefinder focusing, portability and excellent wide angle performance seemed to be the right decision. Although I used a Leica Noctilux for some of my shots, I found that the 35 mm APO-Summicron at about F/4 worked best. My typical exposure was 30 seconds at ISO 320 or ISO 640.

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BOTTOM LINE

A good Aurora is one of nature’s most magnificent displays of beauty. It is impossible to capture the noises and the smells from ionization, or the incredibly graceful movements of the Aurora in a still image.

I was lucky enough to experience an unusually intense and extremely rare Aurora on this trip. It looked like a giant multicolored mushroom that occupied almost the entire sky descending on us. It had yellow, orange, blue, green, purple and red formations. It felt like a giant jelly fish or an alien spacecraft had landed. It was so magnificent and awe inspiring, and it kept moving so gracefully that I decided to enjoy the experience to the max as opposed to trying to photograph it.

I would highly recommend to all nature lovers that they try to experience the Aurora first hand. I would also recommend that one spend the time and effort to try and catch a really intense Aurora, as opposed to an average or below average one. Unfortunately, the probability of being in the right place, at the right time, with the right atmospheric conditions is low. It may take patience, perseverance and a number of trips before one is lucky enough to experience an intense Aurora, but the effort is definitely worth it. To experience an intense Aurora is nothing short of phenomenal.

April, 2010
Mark Dubovoy

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About Mark Dubovoy

Dr. Dubovoy is highly regarded as a technical expert in many aspects of printing technology and photography. As such, he is a regular writer of technical articles for The Luminous Landscape, PHOTO Techniques magazine, and a lecturer at various workshops.

His photographs are included in a number of private collections, as well as the permanent collections of the Museum of Contemporary Art in Mexico City, the San Francisco Museum of Modern Art, the Monterey Art Museum, the Berkeley Art Museum and the Museum of Modern Art in Nanao Japan.