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Radar auroral backscatter observed by the Scandinavian Twin Auroral Radar 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 height-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 electron density at the height of the diffuse auroral backscatter. The dependence of the volume cross section for auroral backscatter on the ionospheric electric field and electron density is analyzed, and the following points are shown. (1) For a STARE radar directed approximately perpendicular to the Hall current in the backscatter region and ionospheric electric fields between 15 and 35 mV/m, the turbulence level of 1-m plasma waves associated with diffuse radar echoes has a linear dependence on the field. For larger values of the electric field, the turbulence level seems to saturate to a constant value. (2) A minimum height-integrated Hall current density of 82 A/km was required to observe diffuse backscatter by either of the STARE radars. The threshold Hall current density is constant to within 20% for all electric field strenghs. For threshold backscatter the height-integrated conductivity is inversely proportional to the ionospheric electric field. (3) The threshold current values are determined by the sensitivity of the radar used. The squared threshold Hall current is proportional to the minimum detectable value of the backscatter cross section. The radar sensitivity concept is verified quantitatively for the 144-MHz STARE radar in Finland and for the 90-MHz radar of the Polar Geophysical Institute.