Aurora Facts Part II.AURORAS ARE SIMILAR to color television images. In the picture tube, a beam of electrons controlled by electric and magnetic fields strikes the screen, making it glow in colors that vary with the screen's phosphor. Auroral color depends on the type of atoms and molecules struck by the energetic particles, particularly electrons, that rain down along earth's magnetic field lines in the discharge process. Each atmospheric gas glows with a specific color, depending on whether it is ionized or neutral, and on the energy of the particle hitting the atoms and air molecules.
THE BRIGHTEST and most common auroral color. a brilliant yellow-green, is produced by oxygen atoms at roughly 60 miles altitude. High-altitude oxygen atoms (about 200 miles) produce rare, all-red auroras. Ionized nitrogen molecules produce blue light; neutral nitrogen molecules create purplish-red lower borders and ripple edges.
AURORAL INTENSITY varies from night to night and during a single night, with best viewing usually from late evening through the early morning hours. Strong auroras can be seen in the continental U.S., particularly in the north during sunspot maximum years. The number of sunspots (a sign of solar activity) varies according to an eleven-year cycle: a few years after a maximum sunspot year (such as 1991), auroras in high-latitude are more numerous. There's also a slight tendency for more auroras in spring and fall.
THE MAGNETOSPHERE protects us from direct effects of the solar wind, but auroras can seriously disrupt radio communications, radio navigation. some defense-related radar systems, and power transmission lines. Current created by changing magnetic fields accompanying aurora causes corrosion in pipes, including the trans-Alaska pipeline.