| ISS
Science | 
This
photo of an Aurora was taken by NASA ISS Science Officer
Don Pettit. | | Related
Links | |
| | Science
at NASA | |
|
|
Expedition
Six
Space Chronicles #12
<
Previous | Next >
By:
ISS Science Officer Don Pettit
Auroras
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.
| 
