Eye Troubling Auroras
"I saw a terrific auroral display the other morning," my friend said. "You know, one of those that make you think your eyes aren't working right? Off and on, sort of like strobe lights.
I knew exactly what he meant, even though I hadn't seen that particular northern lights show. He'd seen a good display of pulsating aurora.
Not many casual sky watchers are fortunate enough to observe a pulsating aurora. Even though this auroral form is not really uncommon, there are several good reasons for its status as a rarely seen phenomenon. For one thing, it isn't as eye-catching as discrete aurora, the arcs and bands we think of when someone mentions the northern lights. Observed without the help of instruments, pulsating aurora seldom shows the intense colors or captivating, curtain-like shapes of more familiar auroras. In fact, it is generally less intense in all ways; its light is dimmer and its form is patchier. Usually it comes and goes so quickly that the eye can't register any dancing movement within it.
However, the real reason so few people see pulsating aurora is its timing. As a graduate student working on the auroras once explained to me, "the pulsating aurora starts up exactly five minutes after everyone's given up and gone inside." Pulsating aurora rewards the patient. It often marks the closing phase of an auroral display, appearing some time after the other activity has stopped.
The reward can be memorable. Pulsating aurora makes up for its diffuseness and dimness by its mysterious behavior. It truly is a case of, "now you see it, now you don't" because it makes much of the sky seem to flicker on and off. Scientists speak of its periodic or quasi-periodic variations in brightness, which may be as swift as every tenth of a second to as sluggish as every 20 seconds. Poetically minded observers have likened it to the beating of a gigantic light-filled heart in the heavens; the less poetic have thought in terms of a malfunctioning neon sign.
The less poetic view actually is a little closer to the accepted explanation for the odd on-off effect, but there's no malfunctioning involved. The neon sign analogy describes all kinds of auroras. In the sign, electrical current stimulates the contained gases to emit photons. That means the gases give off light, of a color appropriate to the gas within the sign's glass tubing. Neon itself, for example, glows red when its molecules are hit by the streaming electrons of the current. With aurora, the atmospheric gases glow when they are hit by streams of particles hurtling earthward.
Now according to my usual authority on matters auroral, Neil Davis's The Aurora Watcher's Handbook, some experts think pulsating aurora shows the sudden change in energy intensity of the particles entering the atmosphere. This would be analogous to a sporadic power surge (or short circuit) affecting the flow of electrons---the electric current causing a lit sign to change brightness. Davis adds that some observations suggest that the pulsating aurora comes closer to earth during its "on" phase, and that the forms are thinner---having "limited height extent," as the handbook puts it---compared to other forms of diffuse auroras.
If you'd like to see pulsating aurora, remember that the odds favor observers whose eyes are accustomed to the dark. The likelihood of a pulsating aurora appearing is highest after a strong and active display of the more familiar auroral forms. After the main display has broken up, look for patchy glowing areas remaining in the sky. Fix your eyes on one of those patches. Within a few seconds, it will probably disappear. Keep your gaze fixed on the now-empty bit of sky, and within a few more seconds or even less, the patch of aurora should reappear. Once you've recognized what you're seeing, you'll have little trouble recognizing more of these on-and-off lights in the sky.
