SOME RADIO REFLECTION AND TRANSMISSION PROPERTIES OF THE AURORA

Abstract

For more than a decade, it has been known that the aurora is capable of reflecting or scattering a radio wave back to the earth at frequencies above 30 Me, where direct reflection back from the ionosphere is not normally expected.1 The echoes have unusually rapid amplitude fluctuation characteristics of the order of a few hundred cycles per second at 100 Me. The echoes are well correlated with geomagnetic disturbances, and statistics of auroral echoes agree with statistics of the visible aurora. An individual auroral echo is not necessarily accompanied by visible light, although at least some auroral light is usually observed. Early in these studies, it was found that the earth's magnetic field plays an important role in the reflection process.2»3 Echoes are possible at 30 Me only when the radio ray is directed no more than 20° from the normal to the earth's magnetic field lines ; this angle is even smaller at higher frequencies. For example, echoes are not obtainable from aurorae appearing in the zenith, but are most readily observed about 700 kilometers magnetically north of the station at large zenith angles. This suggests that an auroral region consists of bundles of rod-shaped scatterers, each one aligned parallel to the earth's field lines and scattering most effectively at right angles to its long axis (Fig. 1).