THE EFFECT OF RADAR SENSITIVITY ON AURORAL BACKSCATTER THRESHOLD PARAMETERS

Radar auroral backscatter observed by the Scandinavian Twin Auroral Ra dar Experiment (STARE) radars is compared with the current density in an eastward electrojet as seen by the Scandinavian Magnetometer Array (SMA) for about half an hour on March 27, 1977. The value of the heigh t-integrated Hall conductivity, obtained from the ionospheric electric field measured by the STARE radars and from the SMA estimates of the ionospheric current density, is considered as a measure of the mean el ectron density at the height of the diffuse auroral backscatter. The d ependence of the volume cross section for auroral backscatter on the i onospheric electric field and electron density is analyzed, and the fo llowing points are shown. (1) For a STARE radar directed approximately perpendicular to the Hall current in the backscatter region and ionos pheric electric fields between 15 and 35 mV/m, the turbulence level of 1-m plasma waves associated with diffuse radar echoes has a linear de pendence on the field. For larger values of the electric field, the tu rbulence level seems to saturate to a constant value. (2) A minimum he ight-integrated Hall current density of 82 A/km was required to observ e diffuse backscatter by either of the STARE radars. The threshold Hal l current density is constant to within 20% for all electric field str enghs. For threshold backscatter the height-integrated conductivity is inversely proportional to the ionospheric electric field. (3) The thr eshold current values are determined by the sensitivity of the radar u sed. The squared threshold Hall current is proportional to the minimum detectable value of the backscatter cross section. The radar sensitiv ity concept is verified quantitatively for the 144-MHz STARE radar in Finland and for the 90-MHz radar of the Polar Geophysical Institute.