There are two types of aurora: diffuse and discrete. Most aurorae occur in a band known as the auroral zone, which is typically 3 to 6 degrees in latitudinal extent and at all longitudes. The auroral zone lies typically 10 to 20 degrees from the magnetic poles. During a geomagnetic storm, the auroral zone will expand to lower latitudes. The diffuse aurora is a featureless glow in the sky which may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora which vary in brightness from just barely visible to the naked eye to bright enough to read a newspaper at night.
Unlike the discrete aurora, which is due to the parallel acceleration of electrons down along the Earth’s magnetic field lines into the atmosphere, diffuse aurora "dribbles" in from the plasma sheet. This dribbling comes from wave-particle interactions that scatter electrons and give them velocities that are more parallel to the magnetic field line. This change of velocity will put the electrons in the loss cone in phase space and they will precipitate into the atmosphere, exciting atoms and molecules to produce the diffuse aurora. Discrete auroras are the most intense auroral types, where field-aligned acceleration plays an important role, forming the so-called inverted-V precipitation.
If you follow the quick tips given below you should be fine when photographing aurorae:
- Use a tripod to keep your camera steady.
- Use a remote control to minimize the camera shake when taking long exposures and try to set the camera to use the mirror lock-up feature if available.
- Cold winter temperatures eat batteries, so always carry spares that are charged.
- If you have a DSLR, or a point and shoot with the option, either use M (Manual) exposure mode, so you have full control of exposure and aperture, or use Av/A (Aperture Priority) mode, so you control how wide the shutter is open, and the camera decides the exposure length.
- Use a fast (an f-stop of 1.4 or greater) and wide angle lens from 18mm to 35mm.
- Open your lens aperture as wide as you can, lower aperture/f-stop number, f/2.8 as opposed to f/8. Plan on exposing anywhere from 4 to 30 seconds depending on the ISO chosen (800 to 3200) and longer if you’re using a tighter aperture like 3.5 to 5.6. If you’re using an f/2.8 aperture lens, then a good starting point would be ISO 800, at 15 seconds. However these settings will inevitably vary according to the intensity of light being emitted by the aurorae.
- If you’re shooting on a digital camera, you should set your camera up to shoot RAW images and activate noise reduction if your camera has this facility.
- Don’t use auto focus. Aurora often do not give off enough light for most camera’s sensors to focus on anyway. Find your infinity focus point ahead of time or focus on distant mountain peaks during the day. You can also focus on the brightest star you see, then fine tune with live view.
- And don’t forget to wear warm clothing, with a windproof shell, but dress in layers to prevent overheating. Disposable hand warmer packets are a huge help to warm cold spots like fingers and toes. Last be aware of your surroundings, listen and watch for wildlife and have an "exit strategy" planned.