THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
"Always paying attention to details."
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458688.5
THE DAILY ASTRONOMER
Tuesday, July 23, 2019
Love of the Aurora
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.
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.
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.