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Aurora Borealis (Northern Lights)

Named in honour of the Greek goddess of the dawn, the transient green and red arcs, rays and nebulous glows that distinguish an auroral display are produced in the Earth’s ionosphere at heights of between 100 and 400 km. Auroral displays occur when high-energy electrons, captured from the Sun’s solar wind, interact with the Earth’s magnetic field and excite through collisions atmospheric atoms. The excited atoms, mainly atomic oxygen and nitrogen, characteristically de-excite by emitting a photon; it is those photons with wavelengths corresponding to visible light that produce the observed auroral display. The green and red auroral colours are the result of the de-excitation of oxygen atoms; the rare blue auroral colour is produced by the de-excitation of molecular nitrogen (N2). Auroral displays are commonly seen from latitudes between 60° and 75° (corresponding to the auroral oval), favouring northern Saskatchewan for the best viewing, but they can on less frequent occasions be seen at much lower latitudes. The times at which aurora displays might be seen at lower latitudes is largely unpredictable, but auroral activity is strongly correlated with the Sun’s 11-year sunspot cycle, with greater numbers of displays being recorded at the time of sunspot maximum.

Martin Beech

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