Star Points for December 2009 by Curtis Roelle Those Awesome Auroras The Scandinavian peoples inhabit the countries of Sweden, Denmark, Norway, and Iceland. In Scandinavian, or Norse, mythology, Valhalla was the residence of Odin, the all-wise lord of the sky and father of the gods. According to the 19th Century mythology authority Thomas Bullfinch, Valhalla was the most beautiful of the silver and golden palaces in Asgard, the abode of the gods, located on the other side of the Bifrost Bridge – the rainbow. The name Valhalla, incidentally, was included in the lyrics of the British rock band Led Zeppelin's "Immigrant Song," with the cry, "Valhalla, I am coming!" In Valhalla, Odin feasts with chosen heroes who have fallen bravely in battle. Excluded is anyone whose death was peaceful. Odin's agents are the Valkyries, whose name means "choosers of the slain." The Valkyries, commemorated in Richard Wagner's opera Die Walküre (The Valkyrie), were warlike, horse-mounted virgins wearing helmets and armed with spears. Their primary function was to visit every battlefield to select which slain warriors were carried to Valhalla to be honored at the nightly feast. According to Norse mythology, when the Valkyries ride, their armor shines with a shimmering light, flickering and flashing in the northern sky. We know this glow as the Aurora Borealis, or the "Northern Lights," and we know its cause to be solar radiation interacting with Earth's magnetic field. The outermost layer of the sun's atmosphere is a tenuous plasma of ionized particles expanding outward to form the "solar wind." When particles swept along in the wind encounter Earth's magnetosphere -- a region of space surrounding Earth in which its magnetic field dominates – they are forced to detour around the planet. Some particles get trapped within Earth's magnetic field and spiral along the field lines toward the planet's poles, where they interact with the ionosphere, causing a glow typically at an altitude of 60-80 miles above the surface of Earth. Auroras can appear in many shapes. There are curving "arc" auroras, folded "curtain" auroras resembling drapery, irregular "band" or cloudy "patch" auroras, glowing uniform "veils," or linear shaft-like "ray" structures. One particular type of aurora resembles a picket fence in the sky. The colors of an aurora may also vary depending on the type of ion that is being excited. Oxygen can glow either green, yellow, or a brownish-red. The red auroras occur high in the atmosphere, up to 200 miles above Earth, and so are seen above the greenish or yellow auroras. At lower altitudes, ionized nitrogen glows reddish-purple. In my experience green is the most commonly seen color. At any given time an aurora is occurring somewhere on Earth. They are most commonly seen at high northern latitudes, such as in Alaska and Canada. More rarely, when the solar wind is stronger because of activity on the sun, they can become visible farther south. Very intense auroral storms have even been observed from the tropics. Auroras occurring in the southern hemisphere are referred to as the Aurora Australis (the Southern Lights). Auroras are hard to predict, and the secret to spotting one is to search the northern sky on a clear night for anything unusual. Faint auroras can appear whitish and be confused with clouds. A notable difference is that stars shine undiminished right through an aurora, whereas stars are dimmed by clouds. I've observed auroras from various locations in Carroll County, including Bear Branch Nature Center (BBNC), where the Westminster Astronomical Society is hosting a public star party on the night of Friday, December 18. Of course, there is no guarantee that an aurora will be visible on any given night. A planetarium program will be presented at 7:30 p.m. Reservations are recommended for the planetarium program (call 410-386-2103), and there is a nominal fee. However, there is no charge for the free outdoor star party, with observing through Society members' telescopes, weather permitting. Please remember to dress warmly.