Aurora Borealis

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Auroras (North/South Polar Lights; or aurorae, sing.: aurora) are natural colored light displays in the sky, usually observed at night, particularly in the polar zone. They typically occur in the ionosphere. Some scientists[who?] call them "polar auroras" (or "aurorae polares"). In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis is also called the northern polar lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. The Cree call this phenomenon the Dance of the Spirits.

Its southern counterpart, the aurora australis/southern polar lights, has similar properties. Australis is the Latin word for "of the South".

Benjamin Franklin first brought attention to the "mystery of the Northern Lights." He theorized the shifting lights to a concentration of electrical charges in the polar regions intensified by the snow and other moisture.

Auroras are produced by the collision of charged particles from Earth's magnetosphere, mostly electrons but also protons and heavier particles, with atoms and molecules of Earth's upper atmosphere (at altitudes above 80 km). The particles have energies of 1 to 100 keV. They originate from the Sun and arrive at the vicinity of Earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with Earth's magnetic field, and solar particles enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards Earth.

The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emission from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae. The light blue colors are produced by ionic nitrogen and the neutral nitrogen gives off the red and purple color with the rippled edges. Different gases interacting with the upper atmosphere will produce different colors, caused by the different compounds of oxygen and nitrogen.

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by Earth's magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.

The curtains often show folds called "striations", which are curtain-like. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.

Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named "auroral electrojets" (see also Birkeland currents).

Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) established that the aurora appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km around Earth's magnetic pole, not its geographic pole. It was hardly ever seen near that pole itself. The instantaneous distribution of auroras ("auroral oval", Yasha/Jakob Feldstein 1963) is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest towards the equator around midnight. The aurora can be seen best at this time.

The Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the million-degree heat of the Sun's outermost layer, the solar corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2-5 nT (nanoteslas; Earth's surface field is typically 30,000-50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger.

The IMF originates on the Sun, related to the field of sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.

Earth's magnetosphere is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around it, at a distance of about 70,000 km (before it reaches that boundary, typically 12,000-15,000 km upstream, a bow shock forms). The width of the magnetospheric obstacle, abreast of Earth, is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances.

When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.

The aurora is a common occurrence in the Poles. It is occasionally seen in temperate latitudes, when a strong magnetic storm temporarily expands the auroral oval. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. However, within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of IMF lines (known as Bz, pronounced "bee-sub-zed" or "bee-sub-zee"), being greater with southward slants.

Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes. It is not well understood why geomagnetic storms are tied to Earth's seasons while polar activity is not. But it is known that during spring and autumn, the interplanetary magnetic field and that of Earth link up. At the magnetopause, Earth's magnetic field points north. When Bz becomes large and negative (i.e., the IMF tilts south), it can partially cancel Earth's magnetic field at the point of contact. South-pointing Bz's open a door through which energy from the solar wind can reach Earth's inner magnetosphere.

The peaking of Bz during this time is a result of geometry. The interplanetary magnetic field (IMF) comes from the Sun and is carried outward the solar wind. Because the Sun rotates the IMF has a spiral shape. Earth's magnetic dipole axis is most closely aligned with the Parker spiral in April and October. As a result, southward (and northward) excursions of Bz are greatest then.

However, Bz is not the only influence on geomagnetic activity. The Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. Because the solar wind blows more rapidly from the Sun's poles than from its equator, the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest - by about 50 km/s, on average - around 5th September and 5th March when Earth lies at its highest heliographic latitude.

Still, neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variation.

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