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Stars and solar physics

Stars and solar physics

The green flash

16 Jul 2015
Taken from the July 2015 issue of Physics World

Sunrise and sunset can be spectacular – the result of a beautiful interaction between the light from our nearest star and the Earth’s atmosphere. But for Pete Lawrence, a phenomenon connected with these events holds even more appeal – the elusive “green flash”

The green flash

My interest in the “green flash” began at a young age. I can’t remember precisely when or where I’d read about it, but I do recall being fascinated by the thought of an effect that was supposed to produce an emerald-green flash at around sunset.

My interest was piqued at the age of 10 on a fishing trip off the south coast of England. It was a trip organized by my father and coincided with a sky that was clear and blue. We were at sea as the Sun was setting and I recall my hopes rising, despite knowing that the green flash had been described as rare, and normally best seen over a sea horizon from the tropics. On this occasion, I saw a beautiful sunset, but my hopes of seeing the green flash took a hit as nothing green appeared.

Many years later, having become an established astrophotographer, I found myself again on England’s south coast, this time trying to photograph a really thin crescent Moon in the post-sunset twilight on Selsey beach in West Sussex. I had a digital single-lens-reflex (DSLR) camera attached at the prime focus of my 100 mm refracting telescope – an arrangement where the telescope acts like a super-telephoto lens. As the Sun got lower in the sky, it was visibly contorting due to the thick, unstable atmosphere close to the horizon.

As the bottom edge of the Sun touched the horizon, I swung the telescope into position (see safety note at the end of this article), fine-tuned the focus via the review screen and started photographing. The stuttering animation of the sunset that appeared on my camera’s rear-view screen revealed large ripples running up the Sun’s disc. As they hit the top, some caused the upper edge to briefly magnify and detach. Later, as I viewed the shots on my computer, I was reminded of my childhood obsession – the upper edge of the Sun was significantly green. But what exactly is the green flash?

Emerald emanation

As light passes through the atmosphere, it gets refracted. The thicker the layer it has to pass through, the more refraction occurs, so maximum refraction occurs close to the horizon. (When the Sun appears to be touching the horizon, in reality its disc has already set.) The atmosphere acts like a prism, refracting different wavelengths by varying amounts.

At sunset the Sun’s light smears into a spectrum of colour, most of which overlaps and can’t be seen individually. However, at the vertical extremes, you can sometimes notice a bluer top edge and a redder bottom edge. The effect is small, appearing like colour fringes when seen under magnification. Unless the air is very clean, the atmosphere scatters the blue fringe away leaving a green one. This is known as the “green rim”, with the bottom red counterpart known as the “red rim”. The same effect occurs around the rising Sun and the rising or setting Moon.

For a green rim to become a green flash, the atmosphere through which you’re viewing the Sun needs to have the right temperature structure. The atmosphere’s temperature normally reduces with height, but what is needed for a green flash is an inversion layer – a layer of warmer air sandwiched between cooler air on either side – below the observer’s line of sight. This essentially creates a cylindrical lens, which both magnifies and visually detaches the upper green rim from the Sun’s disc. If the conditions and timings are right, then as the last bit of Sun slips below the horizon, a thin, bright-green blob of detached sunlight can be seen hanging above the horizon – a purist’s green flash.

A more common version occurs when the Sun remains just above the horizon or has partially set. Here, the green rim may simply elongate away from the Sun’s edge, appearing more like a “green appendage”. A less common appearance occurs as the Sun dips behind clouds close to the horizon – here a small inversion layer may be present at the top of the clouds, which can result in a “proper” detached green flash.

Surprise views

After photographing the green rim and green appendages described above, I finally got to see my first proper green flash while visiting the European Southern Observatory’s Very Large Telescope platform in Paranal, Chile, during 2013. I’d taken a small, 66 mm refracting telescope along with me for the trip and decided to see whether I could capture the green flash as the Sun set over the Pacific.

The air was very clear at the platform’s 2.6 km altitude, making estimating a safe time to point the camera at the setting Sun quite tricky. As it slipped ever lower – as in my Selsey beach experience – the Sun underwent a series of impressive contortions indicating that inversion layers were present. Then, right at the final moment of sunset, a layer of green rim detached to give a beautiful green flash. Although I had thought I might get lucky, actually seeing the green flash properly for the first time caught me by surprise and was really rather emotional. (This experience was caught on camera by filmmaker Brady Haran.) Later analysis of my shots revealed that a small amount of blue flash was also present. Under less pristine skies, this rare blue light would have been scattered away. (See top image for a composite image compiled from this green flash observation.)

After seeing my first proper green flash from Paranal in August 2013, I unexpectedly saw my second in November of the same year, and this was a real surprise. I was visiting the Roque de los Muchachos Observatory in La Palma, for filming duties with the BBC’s The Sky at Night television programme. I’d taken time off from filming to watch the sunset, and as we were based around the Isaac Newton Telescope we had been allowed on the roof to take in the view. As the Sun slowly sank towards a bank of cloud, I can remember thinking I really ought to have my camera with me. However, as the cloud’s edge had a decent altitude above the horizon, I wasn’t expecting to see the green flash. Of course, nature has an inbuilt ability to detect such oversights and as the last vestiges of the Sun’s disc slipped below the cloud bank, a dramatic, emerald-green flash appeared.

Safety note

Turning a camera or telescope towards the Sun is not normally a safe thing to do. Under no circumstances ever view the Sun through a telescope or via a camera viewfinder. To do so invites eye damage, even permanent blindness. It’s safe to watch events unfold on the camera’s rear-view screen, but point at the Sun when it’s too high above the horizon and the camera could be damaged.

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