auˇroˇra n., pl. auˇroˇrae. 1. Aurora borealis. 2. Aurora australis. 3. The dawn.
(American Heritage Dictionary)
These dictionary words hardly depict the awe and mystery felt for the northern (borealis) or southern (australis) lightsdiffuse, luminous bands or streamers of colorful lights sometimes observed in night skies of polar regions. Upon seeing them for the first time, these beautiful shimmering displays, often in varying hues of whitish green to vivid red, can instill fright, disbelief and excitement.
Location of Visibility
On rare occasions these displays can be observed from subtropical latitudes as Florida. Still, most often they are best seen in Earth's polar and subpolar regions. This includes the northern regions of Norway. Therefore, we have selected a Norwegian cruise and dates to maximize the chances of seeing these elusive, mysterious and dramatic light shows, which often peak in spring and fall.
Aurorae occur at altitudes of about 60 to 70 miles forming around two irregular, changing ovals of auroral activity centered on Earth's magnetic poles. Some occur higher, from 125 to 250 or 350 miles. The auroral ovals enlarge and aurorae increase at times of high solar activity. High speed particles (mostly electrons and protons) discharged from the Sun (the solar wind) become trapped in the Earth's magnetic fields. These charged particles spiral in toward Earth to interact with atoms and molecules in the atmosphere of our planet causing these colorful emissionsan "atmospheric light show!"
Strangely, auroral activity seems to increase near the times of the equinoxes (March and September) although solar activity is independent of Earth's seasons. For example, the frequency may double near equinox dates compared with the solstices (June and December).
Explaining this annual variation is not well understood. However, the cause may relate to the tilt of the Earth which allows the variable solar wind to travel more easily between the Earth's and Sun's magnetic fields.
A region near Earth where charged particles are controlled by our planet's magnetic field is known as the magnetosphere. The outer boundary of the magnetosphere is called the magnetopause. Here Earth's magnetic field runs into the Sun's magnetic field and resists the solar wind. Earth's magnetic field points north at the magnetopause. But, if the Sun's magnetic field ends up tilting south near the magnetopause, it can partially cancel Earth's magnetic field at the contact point. This may happen as the Earth's tilted axis seem to wobble slowly back and forth.
Some experts, nevertheless, believe this effect is too small to explain fully increased auroral activity near equinox dates. The jury is still out. Regardless, aurorae never fail to dazzle all who look upon them.
Colors in Aurora
Red and especially green colors often dominate resulting from glowing atomic oxygen. Molecules of nitrogen and its ions produce some low level red (pinkish) and some very high level blue-violet colors. Nitrogen ions can also produce light blue and green, while helium can yield purple. Finally, neon is responsible for rare orangy, rippled edge flares. Varying amounts of solar activity condition color and intensity.
Aurora may display a great variety of shapes including the common homogeneous arc and may loop one or more times at its end. Rayed arcs of several types are also common and may emanate from many points along auroral arcs. Individual rays may occur as pulsating arc or spots. Other types include flamed aurora and glowing spots or patches.
Aurorae are elusive, transitory and in motion. Generally a good camera, with a fast lens on a stable support and patience is needed. A versatile camera with manual settings will increase chances of getting good photographs of the aurora borealis.
Judging exposure times is less problematic today with digital cameras since results can be immediately seen. However, significant exposure bracketing is helpful.
Some General Tips:
- Modern digital SLR much preferred
(Compact or point and shoot cameras can be challengingsee below*)
- Potential for manual exposures (incl. shutter speed, aperture and ISO) and manual focus
- Fast lens, f/4 or faster preferred
- Stable tripod (make sure wind will not blow it over)
- Cable release or electronic remote preferred, or time delay function (but can lose picture waiting for delay)
- Fresh batteries; more than one set (keep in pocket to keep warm)
- Large memory carddon't waste time deleting pictures
(if possible, memory cars designed for cold as San Disk Extreme)
- Flashlight (preferably a headlamp) with red bulb or filter (don't blind others)
- Warm layered clothing (no cotton against skin)
- Hand warmers
Note: Links to products are given as examples. No intent is given for product endorsement.
Brief Photo Tips
*Point and Shoot Cameras: Challenging to Use
- Use wide angle lenses if possible, 24 mm or shorter (aurora can get pretty big)
although longer lenses can isolate part of the aurora
- Use a tripod
- Use manual focus; focus at infinity (tape focus so won't move)
- Set ISO to about 400-800
- Typical exposure times at ISO 400: at f/2.8, 25-40 sec
- Bracket widely. Shorter exposures may be OK. Longer exposures if aurorae faint or using slow lens
- Dynamic nature of aurorae may blur detail for long exposures
(and produce long star trails, which may or may not be objectionable)
- Stopping down lens one stop may produce overall sharper images at expense of longer exposures
- Shoot in manual mode
- Otherwise, shoot in aperture mode (set at smallest f/stop & over expose 1/2 to 1/3 stop)
- Remove (UV or haze) filter
(since narrow auroral spectral lines can produce concentric interference circles)
- Shoot RAW not just jpg (shoot both if possible)
- Be careful of underexposed images (LCD finder images may make image look bright at night)
- Use histogram to judge exposure
- Include foreground objects for interest (some moonlight helpslights up landscape)
- Replace batteries and cards only if auroral activity seems reduced
- Do not breath on lens or LCD finder (they will fog or ice up)
- Beware of condensation when returning to warm places (put camera in bag/pocket, etc)
- Take lots of pictures
- Limited timed exposure time (15 sec may be too short)
- Bulb mode, if available, difficult to use
- Widest lens range too narrow
- ISO quality often too noisy
- Auto focus inoperative at night (need manual focus)