Aurora Borealis (January 2010)
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Nature's Neon Lights Auroras are natural light displays in the night sky caused by charged particles which the Sun emits in addition to light and heat. The charged particles, a "solar wind" of protons and electrons, generate high-voltage electricity when they pass Earth's magnetic field. This electricity is concentrated by Earth's magnetic poles and causes gases in the upper atmosphere to glow through a process similar to a neon sign. The light display is concentrated in an auroral oval around Earth's magnetic poles, which are near the geographic poles. Around the North Pole, the auroral phenomenon is called aurora borealis. Around the South Pole, it is known as aurora australis. To see an aurora (Figs. 1–3), proper location, clear weather, and darkness are required. Auroras occur at extremely high altitudes, beginning at approximately 50 miles (Fig. 4). Because auroras occur at such high altitudes, they theoretically can be seen at any latitude if they are bright enough. Auroral intensity is primarily determined by the level of sunspot activity, which was extremely low during early 2010. Therefore, we had to travel to an arctic location in order to have a chance to see the aurora. Alaska's northern interior is one of the best places on Earth to see auroras because it is directly underneath the auroral oval. In January, on average, over half of the nights are clear. The high latitude offers plenty of darkness in the winter, and the remoteness offers freedom from light pollution. Technical Considerations Choosing a proper location and time can improve the odds for seeing and photographing the aurora, but there are no guarantees because one cannot control the weather or the solar wind. One certainty, however, is the cold. Temperatures in arctic Alaska regularly exceed −40°F, a temperature that could render digital cameras inoperable because they are typically rated to work in temperatures warmer than +32°F. Through some Internet research, we discovered that arctic photographers routinely use digital SLRs at temperatures below 0°F. Unlike film cameras, the electronic shutters of modern digital SLRs do not require heavy lubricants which freeze at arctic temperatures. The main problem, however, is the battery. Rechargeable lithium batteries freeze, sometimes quickly, between 0°F and −20°F. The solution we found was to use non-rechargeable lithium batteries in an external battery pack with a voltage regulator borrowed from a previous trip. This setup worked to at least −55°F, the coldest night of our trip. Amazingly, the small lithium coin battery in the time-lapse controller also continued to work under these conditions, long after its LCD display became frozen and inoperable. Listed below are the technical details of our imaging setup (Fig. 5), very optional reading as usual.
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 Fig. 1. Aurora borealis from our lodge in Bettles, Alaska, on 2010 Jan 11 00:47 AKST (09:47 UT), our first night in Bettles. Exposure was 5 s at f/1.4, ISO 1250. The red marker lights were from the airfield's runway. Air temperature was −28°F with winds gusting to 20 mph (wind chill −59°F).  Fig. 2. Aurora borealis from a pond near Bettles, Alaska, on 2010 Jan 13 00:33 AKST (09:33 UT), our last night in Bettles. Exposure was 30 s at f/1.4, ISO 800. The pond's white snow reflects the aurora's greenish light. Reddish colors represent light due to different excitation energies of oxygen atoms in the upper atmosphere. Air temperature was −55°F with calm winds. 
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