Northern lights are one of my favorite subjects to photograph. What I will discuss here is based on my own experience and research. Some of it is opinion and you should take it for what it's worth. There are many photographers out there that do aurora photography differently. This is what works for me.

The Aurora are caused by particles in space, coming from the sun, that zip into our atmosphere so fast they cause the molecules in our atmosphere to glow when they crash into them. This activity takes place in the Ionosphere, about 100-300 kilometers above the Earth. The main occurrences on the sun that produce these particles are Coronal Mass Ejections (CME's), solar flares and Coronal holes. Coronal Holes are magnetic disruptions on the surface of the Sun that tend to spew slow, steady streams of particles or Solar Wind. The storms produced from Coronal Holes can last days. If a flare or CME happens to follow a day or so behind the emissions of a Coronal Hole, the faster moving particles can piggy-back the solar wind from the Coronal Hole and produce an especially spectacular storm. CME's are eruptions on the surface of the Sun that send billions of tons of Plasma into space. If these eruptions are Earth-directed, they can produce some magnificent storms. Solar flares are produced when built-up magnetic energy in the Sun's atmosphere is released. These flares can last a few seconds or as long as an hour. Flares produce shorter, intense storms that usually last 12-24 hours.

The different colors of the Aurora are dependent on which elements are being crashed into. The rare red Northern Lights are caused by solar particles crashing with Oxygen molecules very high up in the Ionosphere. The common green Auroras are caused by collisions with Oxygen molecules lower down (about 100-300km) and the red edges you see with green Auroras are caused by collisions with Nitrogen molecules lower in the Ionosphere (around 100km). Collisions also occur with Hydrogen and Helium which produce blues and purples. Our eyes are not as sensitive to these colors so we don't see them. It is sometimes possible to pick these colors up on film.

Well, actually, they're out all the time. Usually they are at such low intensity, that we can't see them. When they reach a certain intensity they become visible to us on the ground. The intensity that is required for us to see the aurora depends on our latitude or, more exactly, our magnetic latitude. Because of the way Earth's magnetic field directs the path of the incoming particles, the incoming particles are directed towards the (magnetic) North & South Poles. This is why people in the far North (like Alaska and Finland) and South seem to be seeing Auroras all the time. While here in the Upper Peninsula of Michigan, we see them less frequently but still more often than most places. If you are interested, you can calculate your magnetic latitude and find out the intensity required for your location at my Aurora Geek Page. Now that we have particles headed toward Earth from the sun, there are a couple more factors to worry about. The first is local weather. The great thing about the Internet is that I can sit here on a totally overcast night and see what I'm missing. I probably miss 90% of all auroras because we have so many cloudy nights.

Yet another factor is the Interplanetary Magnetic Field or IMF. This is basically the magnetic field of the Sun as it is carried along on the Solar Wind. If this field is pointing South, it produces more favorable conditions than when it points North. Dark is another requirement for photographing the Aurora. People who live in the city are going to have a difficult time seeing anything because of all the light pollution. The moon can also interfere with our ability to photograph the lights. This is especially the case in lower intensity storms.

I have 2 cameras that I use for Aurora Photography. The first is a Canon EOS 1N with a 28mm f1.8 lens. The second is a Pentax 6x7 with either a 105mm f2.4 or a 55mm f3.5 lens. I like the canon because it has built-in shutter speeds of 15,20 and 30 seconds. I like the Pentax for it's big negatives. Really, any camera that will do time exposures will do. Most SLR's have a "B" or "bulb" setting which is really what is needed. A sturdy tripod is a must. I have taken pictures on cheap tripods in the past and I was not happy with the results. I now have a beast of a tripod that weighs 10 pounds but it is very sturdy.

Your lenses should be fast. My Canon lens is an f1.8. I would really prefer something faster but it would be way too expensive for me right now so I make do with f1.8 which seems to work just fine. Medium format is a bit more difficult as the lenses are usually not as fast. My Pentax 55mm lens is an f3.5 which forces me to go to ASA 400 film. You might wonder why this matters. After all, we could just use longer exposures with the slower lenses. While this would get you some decent exposures, it leads to another problem... star trails. Now, don't get me wrong, I like photographs of star trails. But I don't like to see them in my aurora photos. To me, the short trails make the stars appear out of focus. Someday, I would like to experiment with trying to get longer star trails in my aurora photos. I know it can be done. I've seen the pictures.