The AURORAE BOREALIS and AUSTRALIS have been associated with magnetic activity since the eighteenth century. Most of the light in these displays is produced by excitation and ionization of the upper atmosphere at high latitudes by an influx of electrons with energies about l0keV. Disturbances of the magnetic field con¬figuration within the magnetotail energize these electrons; in their propagation towards the Earth they become deflected towards high latitudes. In each hemisphere they arrive in thin sheets mostly within an area called the AURORAL OVAL. The extent and position of this depends upon magnetic activity, but generally it has a width of about 5° and comes within about 12° of the geo¬magnetic poles on the sunward side and about 22° on the dark side The rotation of the Earth underneath the oval, causes aurorae to be viable over a large zone in the course of a single day Aerial and satellite studies have confirmed the shape of the oval inferred from ground-based observations.

The aurorae in the oval visible to the naked eye often have curtain-like structures. The lower border is at a height of about 100km and may extend several thousands of kilometers. Vertical striations appear in rayed arcs and more complicated forms occur in so-called draperies showing folds and curls. Spectral lines are observed, and in particular a green oxygen line at 557.7nm is present. Rocket and balloon measurements indicate X-ray emission produced by bremsstrahlung from the particles impacting on the atmosphere. The auroral zones march equatorward in times of increased solar and magnetic activity and indeed aurorae have been observed from Singapore.

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