Backscatter from decameter-wavelength field-aligned F region irregularities, as measured by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland HF coherent radar, is compared with common volume plasma parameters and the electric field deduced by the European Incoherent Scatter (EISCAT) UHF incoherent radar system, for a 12 hour period from June 18 to June 19, 1996. During this interval we find an excellent agreement between irregularity Doppler velocity and bulk ion drift resolved along the CUTLASS beam. Backscatter is found to exist only in regions of nonzero electric field, as the E¿B instability growth rate is dependent on E. Following a substorm expansion phase onset, backscatter largely disappears for a period of several hours, thought to be a consequence of nondeviative absorption of the HF radio wave in the D region or a quenching of the F region instability mechanism by enhanced E region Pedersen conductivity. Finally, the presence of auroral arcs within the scatter volume increases the intensity of backscatter returns and introduces a subsidiary peak, displaced from the preexisting peak, in the backscatter spectra; this subsidiary peak results in an increase in the apparent spectral width of the backscatter. We show how this allows the location of precipitation features within the field of view to be determined. ¿ 1999 American Geophysical Union