Observing the aurora

What can I expect to see and how do I best see it?

Except in an unusually strong display, you are looking for a faint glow in the northern part of the sky. Typically, this glow will be relatively low in the sky - less than halfway up to overhead - and stretch from the north-northeast to north-northwest. Stronger displays will appear higher in the sky and stretch across more of the sky.

At a given latitude, the ideal location is a rural area without any cities or towns to your immediate north. Most auroral displays are subtle and are easily washed out by city lights or strong moonlight. It is also important to allow your eyes to become "dark-adapted"; 5-10 minutes is a good minimum, but your eyes continue to become more sensitive to light the longer you keep them shielded from bright lights. (That's why even a normal room light hurts your eyes if you have to get up in the middle of the night.) If you can't get to a rural area, try to get to a location as far north as possible within your city where there are few if any lights shining directly on you. Even an upstairs north-facing window can be useful if it gives you a clearer northern horizon and there are no bright lights in your face.

Stronger auroral displays can be seen in or near a moderately large city (or even a truly large city in extreme cases), or when there is considerable interference from a nearly full moon. The stronger displays not only reach higher in the sky, but are brighter. These displays do not require rural skies, although the display will always be better from outside of a city. There are many classic photos of the aurora with skyscrapers and city lights in the field of view. I have observed and photographed several auroral displays from the western edge of the city of Boulder, population 95,000, right at 40 degrees latitude.

The aurora varies on short time scales, so if the aurora is expected on a particular night and you don't see anything, consider trying again 30 or 60 minutes later. The longer you are watching, the more likely you are to see something. Even on a good night, there might only be a few hours of aurora visibility. At middle-latitudes, the aurora is only visible on a small percentage of nights. That's why an alert service can be very useful. Auroral activity is triggered by material blown away from the Sun, so it is possible to get a heads-up for future auroral activity by studying solar activity. The alert services allow you to take advantage of other people's knowledge of the conditions that produce the aurora and give you a much better chance of seeing activity than just randomly being outside at just the right time. Auroral activity is slightly more likely near local midnight, but depending on the circumstances, it can be in progress as night falls, or still be going at dawn. Alerts to potential auroral activity will usually give recommended viewing times, although sometimes it can be hard to pin down the exact time of best activity ahead of time.

Specific features you may see

Note that while the photos to which I refer show the colors perhaps a bit more vividly than they will appear to the eye, in a good display the color will be noticable as you are watching. However, if you are observing from a less than dark site, the colors may be washed out. At higher latitudes, the observed features may differ from the descriptions below, but for middle-latitudes, this should gives a good idea of what to expect.

Quiescent arc

This represents the beginning of an auroral display, and in a weak display may be the only thing you see. The 10th photo from May 4th, 1998 shows a particularly bright arc. If a arc is bright enough for a visual sighting of color, it will appear green or yellow. The arc will usually be quite low in the sky and uniform in appearance. Occasionally, the arc will be folded onto itself at a point, producing a "kink" in the arc, as in the 3rd photo from May 4th, 1998, which also shows a ray.

Rayed arc

As the geomagnetic field becomes more active, vertical rays will appear along the arc, which look like spotlight beams. In a major display, the rays may extend to zenith, but heights of less than 45 degrees are more typical. Most of the photos on the aurora photos and descriptions page show rays, with the May 4th, 1998 display being a particularly good example. Short rays will usually appear green or yellow if any color is visible, but tall rays typically have a reddish hue. In a major display the red color can be quite striking. Movement will often be seen in rays.

I have found that a significant brightening of a quiescent arc often signals the appearance of rays, and the rays can appear quite suddenly and evolve rapidly.

Glow or curtain

Very often, especially during quiet moments of a major display, one will see a relatively uniform glow across the northern sky above an arc. As with rays, the higher the glow extends, the more likely you are to see red color. In addition, in photos you will usually see bright rays superimposed on a background glow. Again, many of the photos show this, but the photos from March 21st, 1990 give a good example of the diffuse glows as well as rays superimposed on glows, as does the photo from May 3rd, 1998.

Pulsations

In rare cases you may also see very rapid flickering in the display, like that decribed in the report for August 19th, 1991. However, I've only seen this kind of display once, so I'm not really sure how often this occurs in middle latitude displays. More frequently you will see subtler movement in the display, such as a slow sideways movement of a ray.


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File last modified: 16 December 2004

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