The Azimuthal Distribution of Ultra-High Frequency Radar Echoes From Aurora

Abstract

The distribution in range and azimuth of auroral echoes obtained with an ultra-high f r e q~~e i~c y radar a t Prince Albert is examined. Such distributions may be controlled (i) by the effect of volume integration in the auroral configurations, (ii) by the geographical distribution of aurora, and (iii) by aspect sensitivity of the auroral scatterers. With the radar parameters chosen for this experiment, (i) can be neglected. Of the remaining two effects, (ii) appears to be Inore important in the period imnlediately before local midnight and (iii) in the period immediately following local midnight. INTRODUCTIOSThe ~necl~anism by which radio waves are scattered in the lower ionosphere during auroral disturbances is stilI very poorly understood. The size and shape of the scattering centers have not been established. Several attempts have been made t o interpret, in terms of the "aspect sensitivity" of the scatterers, the distribution in range and azimuth of echoes observed with a specific radar (the so-called azimuthal distribution). This approach assumes that the preferential occurrence of echoes in one direction (geomagnetic north for stations near the northern auroral zone) is due t o the elongation of the clouds of auroral ionization along the geomagnetic field lines. While aspect sensitivity undoubtedly exists, it was pointed out by Forsyth (1960) that the degree of aspect sensitivit)~ (or elongation of the scatterers) could not be accurately estinlated from existing radar data. Most of these data were obtained using very lligh frequency radars having wide antenna beam-widths and short pulse durations. These two characteristics combine to produce a type of observational selection that seriously inodifies the azi~lluthal echo distribution.Recently, new radars operating in the ultra-high frequency band have come into use. These radars are capable of much better spatial resolution than are the older v.11.f. radars and so may shed new light on the reflection illechailisrns iilvolved in radio-auroral reflections.