THE DAILY ASTRONOMER
Wednesday, June 15, 2016
Love of the Aurora
We can admit that the scientific explanation is often a few shades shy
of the fantastic. Here, we refer to the plethora of myths and legends
once regarded as having been responsible for natural phenomena. A
coterie of vengeful gods is a more compelling cause of lightning than
the electrical discharge claptrap that atmospheric physics would have us
believe. (Granted, they'd have us believe it because the model has
withstood every test and been confirmed by every experiment, so it has
that going for it.)
Alas, the lamentable price we pay for science is that we must behold an
unpeopled sky and ascribe all phenomena as being the contrivance of
physical laws. To some, replacing the capricious and ill mannered
deities prone to rage fits with electrons devoid of emotions is just
spiffing: we don't have to worry about being felled by a furious bolt
fired in misdirected anger. During violent electrical storms, we just
have to be sure we don't offer the shortest route to the ground. Life
is a bit easier when we can rely on consistent natural laws without the
added complication of bad moods.
A brilliant aurora image. Admire this image while listening to
As for the aurora, well, we did lose something special when we puzzled
out the science. When we realized that celestial fires alighting
across the northern snowscapes were the interplay of charged solar
particles with upper atmospheric electrons, the dragons, fairies and
goddesses passed into nothingness. You see, the
higher latitude communities crafted exquisite mythologies associated
with the Aurora Borealis: they were the nocturnal spirits gathering in
covens amongst the stars; broad chested Nordic goddesses conveying the
shades of slain warriors to the mead-soaked revelries at Valhalla; the
red-emerald encrusted dragons raging out of frozen lairs to terrorize
the protein sources running about the tundra.
Galileo Galilei, not a Nordic fellow, started the investigation into the
aurora. He actually coined that term: "Aurora Borealis," the
"Northern Dawn." Galileo drew from mythology, as Aurora was the
goddess of dawn. The shimmering lights that often illuminated the
northern climes perplexed Galileo greatly, and he described it as a
being similar to the diffuse light preceding the Sun's rise, except it
emerged from the north, not east.
Other investigations followed Galileo's initial inquiry, but it wasn't
until the Eighteenth century that systematic observations were compiled
by astronomers such as Edmond Halley (of comet fame) and French
scientist JJ Dortou de Mairan. It was Mairan (1733) who first equated
the aurora with the Sun, for he noted that few aurora were observed
during the Maunder Minimum (1645-1715), a prolonged period when the
Sun's visible "surface" showed precious few sunspots. Mairan
described the aurora as resulting from the introduction of solar
"fluid," into Earth's atmosphere. He didn't describe this fluid's
characteristics, as, to his mind, he had no means of understanding its
composition. Yet, Mairan provided the first solar-based model
designed to explain the Aurora.
In 1741, another scientist named Hiorter determined that the aurora had a
magnetic component, for he observed a slight compass needle
displacement during an aurora event. (To put this experiment into
historical context, Anders Celsius, himself, provided Hoirter with this
compass needle assembly.) Meanwhile, Mairan suggested that the aurora
event wasn't just confined to the northern hemisphere and should be
observable at an extreme southern latitude. ("Extreme," in this case,
means far from the Equator.) Don Ulloa, a Spanish naval officer,
recorded an observation of the southern aurora in 1745 and in 1770
Captain Cook observed this light, dubbed the "Aurora Australis." These
observations established the aurora's bi-polar nature: that, like a
magnet, it was directed toward opposite poles. Around the time of
Cook's expedition, W. Wagentin concluded that the aurora exhibited a
belt or loop shape: extending longitudinally around an oval.
Confirmation of this hypothesis lent more evidence to support the
aurora's magnetic components; for they seemed to assume a shape
determined by magnetic field lines.
More detailed explanations were offered in the late nineteeth century,
after the electron was discovered and James Clerk Maxwell's presented
the fundamental equations relating electricity and magnetism. In
1896, Birkeland proposed that solar electrons created the aurora after
Earth's magnetic field drew them toward the planet's magnetic poles.
Subsequent electric experiments demonstrated that electrons alone
couldn't produce the aurora due to electrostatic repulsion. A more
complex outflow of solar "plasma" was then described: a constant "wind"
of charged particles suffused through the solar system. Some of it
struck Earth: our planet's complicated magnetic field, generated by the
electron flow within Earth's solid-molten interior, directs this
charged particles toward the magnetic poles: the particles then impart
energy into the gaseous atoms in Earth's upper layers.
For example, when particles strike nitrogen atoms between 20 - 60 miles
above Earth's surface, they "excite" the electrons. Exciting an
electron is as simple as pushing it into a higher energy level. (We can
think of these as orbits if we want to make Quantum theorists cry.)
When the electron settles back into its original lower energy orbit
-where it wants to be- it emits a photon. This photon's energy is
equal to the energy difference between the upper and lower level. A
photon is a bundle of radiant energy whose frequency depends on its
energy. So, the energy of the photons emitted by the once excited
nitrogen atoms have a frequency in the red end and purple (indigo part)
of the visible light spectrum: it produces red-purplelight. Higher
level excited oxygen will emit blue, green and pure red. Aurora
can occur between 20 - more than 160 miles above Earth's surface.
When we admire an aurora, we're seeing the kind of pure color that only
excited atoms can generate: atoms responding to the assaulting outflow
of our furiously energetic Sun. Not exactly a dragon, but powerful
physics nevertheless.
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FROM THE CATACOMBS OF INFINITE KNOWLEDGE
Jogging a mile and walking a mile both burn about 100 calories.
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