What’s that glow?
Over the past several years, Mark Weller and I have collaborated on a project where we’ve been capturing the Milky Way over visually interesting landscapes in northern Wisconsin. For the past four years, this trek has taken us to the Apostle Island National Lakeshore in southwest Lake Superior.
For the astro-photographically challenged among you, taking pictures of the sky is pretty hard work and involves knowing a lot about cameras, astronomy, a little luck, and being in the right place at the right time. Pictures like the Outer Island shot (above) are usually long exposures. We shoot digital so our exposures are generally between two and a-half to four minutes long. Back in the film days, it was not unusual for amateurs like us to require exposures of 20 to 30 minutes, so digital has definitely made things much easier.
Above is the sand island shot we did in 2010. Note the vivid appearance of the milky way, but note also that there is something else: a diffuse pink glow that appears perpendicular to the axis of the milky way. There had been some faint auroral activity earlier this same evening. We had noticed the characteristic green glow far off on the northern horizon. There was no question about its nature – it was clearly a faint display of aurora borealis. The pinkish glow in the above photo was not visible to the naked eye and was in fact not noticed until we had a chance to examine the shots later. The exposure of this shot is 3 minutes so whatever the pinkish glow was, it was very faint. I finished processing the Sand shot and thought little more about the spurious glow until the following year.
July 2011 found us at Raspberry island, home to one of the most beautiful lighthouses in Lake Superior.
We again saw evidence of the spurious glow, this year appearing in some time lapse shots captured by my daughter Andrea.
Though the video is short, the greenish glow rippling wave-like through the several seconds of video reminded me strongly of the strange pinkish glow we had seen the year before at Sand Island.
Upon returning home from Raspberry Island, I began to do some research and found that the glow we had seen both at Sand and Raspberry islands was likely air glow, also called atmospheric air glow and sometimes banded air glow, due to the appearance of the characteristic wave-like behavior. Googling around the internet turned up several other examples of the same effect:
It’s not very well known, and not written about that much, but a determined search does turn up plenty of examples, and more than enough discussion of the science to establish that it’s a fairly well understood phenomenon, at least among atmospheric scientists. There are many more references available on the web, and enough discussion to satisfy most curious amateur scientists. And there the matter would have rested, until the summer of 2012.
This past July found Mark, Ian and myself back in the Apostles, this time at Devils Island, again attempting to get a shot of the milky way over the somewhat utilatarian Devils Island light. What we weren’t expecting was a display of airglow that absolutely left us speechless due to its unbelievable brightness. Indeed, it completely dominates the image:
The sheer brilliance of the airglow in this shot stopped us in our tracks. In our quest to present images of the milky way to a new audience, we now had a problem: how would this image be received? Would people understand what they were looking at? The risk, of course, is that many would assume they’re looking at aurora borealis, which is a very good guess. The green color is very characteristic of aurora displays. But airglow is something entirely different, and very novel. We thought it was important to accurately describe for our audience and educate our public about what they were seeing. Here is the description that I finally settled on after reviewing all the applicable science and trying to boil it down for a general audience:
The spectacular green banding seen in the Devils Island photo at first glance look like aurora borealis (northern lights), but in fact is something different. It’s called banded air glow.
Even absent light from the stars or other celestial objects, the night sky is never completely dark. Earth’s atmosphere itself glows faintly all the time as atoms of oxygen, having been excited during daytime hours by UV radiation from the sun, emit faint light as they drop back to their less energized state.
On the night we shot this photograph, this air glow display was unusually bright. To the unaided eye, it looks like a very thin cloud layer, and usually escapes notice altogether. Only with the exceptionally dark skies of the Lake Superior region and the long exposure capability of the camera did the color emerge. The banded structure is caused by atmospheric gravity waves, a common wave-like movement of air high in the thermosphere (more than 50 miles up).
The Devils shot, and all the “lights of” island series, are available for purchase at the Friends of the Apostle Islands website. All proceeds from the sale of these images go directly to support the Apostle Islands National Lakeshore; Mark and I do not profit at all from these shots.
For more images from the island shoots, check out my pBase gallery page.