A statistical study of subauroral E-region coherent backscatter observed near 100 MHz with passive radar

We provide a statistical summary of the Doppler features extracted from 21,000 independent E-region irregularity observations made during 2002 and 2003. These data were acquired using the Manastash Ridge Radar, which detects the scatter of commercial FM broadcasts near 100 MHz and observes at subauroral latitude. In particular, we present histograms of Doppler velocity and spectral width of the observations and discuss trends in these data with respect to local time and the geomagnetic Kp index. We find an asymmetric velocity distribution composed entirely of type 1 Doppler spectra; these waves are associated with the modified two-stream instability. Type 2 echoes are completely absent, suggesting a strong latitude and/or wave number dependence in the conditions for secondary wave instability. We compare our observations with HF and VHF coherent scatter results reported by others. Because the radar is completely passive, it is well suited for extended, unattended observations; in this 2-year period of nearly continuous operation, we find pronounced seasonal variation in our observations. The method of radar operation provides excellent data for statistical summary, as it is completely free of range and Doppler aliasing, while also providing fine time and frequency resolution.