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IMAGE: Aurora
This photo of an Aurora was taken by NASA ISS Science Officer Don Pettit.
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Don Pettit Space Chronicles

Expedition Six
Space Chronicles #12

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By: ISS Science Officer Don Pettit


If Iris, the goddess of the rainbow, had a sister she would be the goddess of Aurora. Aurora is nothing short of occipital ecstasy. Glowing green ripples form concaved arcs that constantly transform their shape into new glowing diaphanous forms. There is nothing static about aurora. It is always moving, always changing, and like snowflakes, each display is different from the last. Sometimes, there is a faint touch of red layered above the green. There arebright spots within the arcs that come and go at a whim. These bright spots will transform into upward directed rays topped by feathery red structures. Sometimes there will be six or more rays, sometimes none at all. Red is not always seen but when it is, it usually lies above the green.

Most of the auroral light is emitted by oxygen atoms excited from bombardment by charged solar particles. Charged particles consisting of atomic fragments released by the sun and streaming through space intersect Earth's magnetic field. When a charged particle moves through a magnetic field, a force perpendicular to the motion is created and that force diverts the particle into a spiral path until it collides with atoms in the upper atmosphere. These collisions excite the atoms into emitting light, much like electrons pumped inside of a glass tube filled with neon create a light that says "NO VACANCY." The green is centered around the 558 nanometer line of oxygen while the rarer red is emitted around several lines in the 630 nanometer region.

Aurora follows Earth's magnetic field, thus it is seen more frequently on the Canadian side of the hemisphere than the Siberian side due to the north magnetic pole lying in the proximity of Hudson Bay. It seems to be at its peak 180 degrees from the sun. Thus when your orbit coincides with local midnight at high latitudes, you will be rewarded by turning down the lights and looking out a north-facing window.

The edge-on view in the upper atmosphere allows height scales to be estimated. Using the atmosphere as a ruler where its edge is taken to be about 50 kilometers in altitude, the green emissions extend from the ever present thin-shell of airglow at 2 atmospheric thicknesses to perhaps 6 atmospheric thicknesses. That would place them in the 100 to 300 km range. The red emissions are at higher altitudes. They lie on top of the green and extend beyond that layer by about 4 atmospheric thicknesses, thus placing them in the 300 to 500 km range.

Aurora forms large concaved arcs 30 to 70 degrees along the visible horizon with well-defined edges. From this large scale arc smaller curtain-like structures extend in southerly directions. One time the space station flew through one of these curtains while over northern Canada near local midnight. Glowing green lines, some curvy like a doodle on a scrap of paper and some spotted like a connect-the-dot drawing were seen while looking through a nadir-viewing window. We were most definitely above the aurora looking down onto the structure.

A glance through the north-facing window was a sight to behold. It was as if we were in a dimly glowing fog of red. It was like you had been shrunk down to some miniature dimension and inserted into the tube of a neon sign. And it was just on the other side of the windowpane. You wanted to reach out and touch, but of course you could not. Afterwards, I had to clean a nose print off of the window.

Our orbital altitude was 388 km. These observations of emission altitudes are consistent with the simple atmospheric ruler method for determining their height. For a few days, viewing geometry was such that we could see both aurora and the setting sun terminator at the same time. This occipital treat gave both the sunlit horizon with its iridescent layers of orange and blue and the glowing greens from the auroral arc. It was as if Iris and her sister of the night were having a brief conversation.

Green aurora was visible in the blackness above the sunlit atmosphere. Above the terminator, the fuzzy line that demarks day from night, at about the same altitude as our orbit, was a glowing cloud of red aurora. No green emissions were visible near by, and the red emissions seemed to follow the path of the terminator as it moved westward until it was no longer in sight. I stared as if star-struck. Aurora, out of all other natural phenomena, is the most deserving of goddess stature and makes the sheer beauty of Venus pale by comparison.

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