Ground-To Bistatic Radar Measurements of Auroral Irregularities

The second Radio Aurora Explorer (RAX) satellite completed two dozen experiments with the incoherent scatter radars in Poker Flat, Alaska and Resolute Bay, Canada. Coherent radar echoes occurred during three of the passes. Following inspection of summary range-time-intensity plots, segments of raw radar data containing echoes (collected at 1 MHz sampling rate) were downloaded for high resolution power and Doppler velocity measurements. An auto-correlation analysis mapped the distribution of E region backscatter (which spans the altitude range of 90-130 km) with 3 km resolution in altitude and sub-degree resolution in magnetic aspect angle. To our knowledge, these are the highest resolution UHF radar measurements made in the auroral region. The measured mean Doppler shift of the UHF backscatter corresponds to a saturated speed of meter-scale plasma waves generated by the Farley-Buneman instability. The observed Doppler velocity along the line-of-sight is in excellent agreement with the empirical formula (Cs cosθ) that the irregularities propagate at the ion acoustic speed times the cosine of the flow angle. The measurements here extend the validity of this empirical relationship to the UHF frequencies. Furthermore, the magnetic aspect angle distribution narrows down with increasing altitude and approaches to the half of the radar beam-width above the altitude of 110 km, implying extremely fine magnetic field alignment (after the radar main lobe is de-convolved from the measurements). Finally, despite the strong (over 1500 m/s) F region ion drift velocities, we did not find any evidence of F region sub-meter scale irregularities that are predicted to be generated through the Post-Rosenbluth instability.