THE USM SOUTHWORTH PLANETARIUM
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"Always paying attention to details."
70 Falmouth Street Portland, Maine 04103
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43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458688.5
THE DAILY ASTRONOMER
Tuesday, July 23, 2019
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 machinations. 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.
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.