Technicolor Auroras? A Reality Check

[bystander]

Technicolor Auroras? A Reality Check
Universe Today | Bob King | 2013 Oct 14

Click to view full size image 1 or image 2

Beautiful red and green aurora the night of Oct. 1-2, 2013. Mouse-over for how
it appeared to the eye. Surprised? I took the photo above and using Photoshop
adjusted color and brightness to match the naked eye view. Notice the green
tinge in the bright arc at bottom. The rays were colorless. Details: 20mm lens,
f/2.8, ISO 1600 and 25 second exposure. Credit: Bob King[hr][/hr][imghover=http://d1jqu7g1y74ds1.cloudfront.net/wp ... 80x386.jpg]http://d1jqu7g1y74ds1.cloudfront.net/wp ... 80x386.jpg[/imghover]A massive wall of bright purple and green rays from July 20, 2012 and the image
adjusted to better represent the visual view. Most auroras are between 60 and
150 miles high, but occasionally reach to 400 miles. Details: 16mm at f/2.8,
ISO 800 and 20 second exposure. Credit: Bob King[hr][/hr][/i]

[i]A striking coronal aurora in Feb. 1999 photographed on film. The red in this aurora was obvious to the naked eye but appeared more like the Photoshopped version at right. Credit: Bob King[/i]

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I shoot a lot of pictures of the northern lights. Just like the next photographer, I thrill to the striking colors that glow from the back of my digital camera. When preparing those images for publication, many of us lighten or brighten the images so the colors and forms stand out better. Nothing wrong with that, except most times the aurora never looked that way to our eyes.

The colors you see in aurora photos ARE real but exaggerated because the pictures are time exposures. Once the camera’s shutter opens, light accumulates on the electronic sensor, making faint and pale subjects bright and vivid. The camera can’t help it, and who would deny a photographer the chance to share the beauty? Most of us understand the magic of time exposures and factor in a mental fudge factor when looking at astronomical photos including those of the aurora.

But photos can be misleading, especially so for beginners, who might anticipate “the second coming” when they step out to watch the northern lights only to feel disappointment at the real thing. Which is too bad, because the real aurora can make your jaw drop.

A typical aurora begins life as a pale white band low in the northern sky. If we’re lucky, the band intensifies, crosses the color threshold and glows pale green. Deeper and brighter greens are also common in active and bright auroras, but red is elusive because are eyes are far less sensitive to it than green. Often a curtain of green rays will be topped off by red, blue or purple emission recorded with sumptuous fidelity in the camera. What does the eye see? Smoky, colorless haze with hints of pink. Maybe.

Again, this doesn’t mean we only see green and white. I’ve watched brilliant (pale) green rays stretch from horizon to zenith with their bottoms bathed in rosy-purple, a most wonderful sight. Another factor to keep in mind is dark adaption – the longer you’ve been out under a dark sky, the more sensitive your eyes will be to whatever color might be present. At night however we’re mostly color blind, relying on our low-light-sensitive rod cells to get around. Cone cells, fine-tuned for color vision, are activated only when light intensity reaches certain thresholds. That happens often when it comes to auroral green but less so with other colors to which are cells are less responsive.

Auroral colors originate when electrons from the sun spiral down Earth’s magnetic field lines like firemen on a firepole and slam into oxygen and nitrogen atoms in Earth’s upper atmosphere between 60 and 150 miles (96-240 km) high. Here’s a breakdown of color, atom and altitude:

• Green – oxygen atoms 60-93 miles up (100-150 km)
• Red – oxygen atoms from 93-155 miles (150-250 km)
• Purple – molecular nitrogen up to 60 miles (100 km)
• Blue/purple – molecular nitrogen ions above 100 miles (160 km)

When an electron strikes an oxygen atom for instance, it bumps one of the oxygen’s electrons to a higher energy level. When that electron drops back down to its previous rest or ground state, it emits a photon of green light. Billions of atoms and molecules, each cranking out tiny flashes of light, make an aurora. It takes about 3/4 second for that electron to drop and the atom to release a photon before it’s given another kick from a solar electron. Most auroras are rich with oxygen emission.

Higher up, where the air’s so thin it’s identical to a hard vacuum, collisions between atoms happen only about every 7 seconds. With lots of time on their hands, oxygen electrons can transition down to their lowest energy level inside the atom, releasing a photon of red light instead of green. That’s why tall rays often show red tops especially in time exposure photos.

Only during very active geomagnetic storms, when electrons penetrate to low levels in the atmosphere, are they able to excite molecules of nitrogen, giving rise to the familiar purple fringes at the bottoms of bright rays. Bombarded molecular nitrogen ions at high altitude release a deep blue-purple light. Rarely visible to the eye, I did record it one night in the camera.

While videos hint at how wildly dynamic auroras can be, they’re no substitute for seeing one yourself. That’s why I never seem to get to bed when that first tempting glow appears over the northern horizon. Colorful or colorless, you’ll be astonished at how the aurora constantly re-invents itself in a multitude of forms from arcs to rays to flaming patches and writhing curlicues. Don’t miss the chance to see one. If there’s one thing that looks absolutely unearthly on this green Earth, it’s the aurora borealis. Click HERE for a guide on when and where to watch for them.

[saturno2]

This article is very interesting
The first image that I saw in APOD was " Auroras over Alaska"

[rstevenson]

I've seen the Northern Lights several times in my life. Most times I saw only a vivid but pale green. But on one memorable occasion, when I was visiting Dawson City, the sky was covered with whipping lines and fields of yellow, red, green and some faint purple. The colours were both vivid and bright. Residents came out and oohed and awwed along with the tourists, many exclaiming they'd never seen anything like it. I've often wondered if some satellites got fried that night.

Rob

[owlice]

because are eyes

ACK!!!!!

I've seen the Northern Lights several times in my life.

~~~ sigh ~~~

[Beyond]

because are eyes

ACK!!!!!

Now what kind of owl speak is that ((I also noticed the "are", but just let it go. I couldn't think of-->ACK!!!!!))

[rstevenson]

I've seen the Northern Lights several times in my life.

~~~ sigh ~~~

Don't be too envious. Seeing them that often required being "up north" somewhere, and lacking the means to be there at my leisure, that meant working at jobs like driving 85-ton trucks in open pit mines in -30° temperatures. But the aurorae were nice, as was the sight of the sun setting in the north-west, the dusk going around the northern horizon, and then the sun rising in the north-east -- all in the space of a few hours (in late June, at least.) The part I liked least was the cold: first snow of the season the year I was there fell on Sept. 3rd, and the last snowfall that year was on May 27th. So I think I paid in full for those aurorae sightings.

Rob

[Beyond]

Rob, i think you are owed a few more auroras

[geckzilla]

Oh, an 85 ton truck. That's the kind of machine with wheels I come about eye-level with at the axle. A fearsome beast.

[Beyond]

The bigger beast was the -35 tempreture That's easily worth two-->