UHF Coherent Backscatter Observations of Mid-Latitude Electric Field Structures and Variability

Mid-latitude ionospheric regions located near the plasmasphere boundary layer (PBL) respond with large spatial and temporal variability to geomagnetic and solar disturbances. In particular, those areas on field lines connected to asymmetric ring currents in the magnetosphere equatorial plane experience large poleward and eastward electric fields during disturbance periods. These fields interact with conductivity gradients in the dusk and evening periods to create large ExB sub-auroral polarization stream (SAPS) flows and steep storm-enhanced density (SED) structures in a complex magnetosphere/ionosphere feedback system. Farley-Buneman two stream irregularities frequently accompany these dynamic conditions, creating coherent wave structures which perturb existing operational systems such as GPS but which can be also exploited for radar remote observations. The Millstone Hill UHF 440 MHz radar, when operated in a high-resolution coherent backscatter mode, is a sensitive diagnostic of these intense electric fields and dramatic variability. We present observations and interpretation of mid-latitude UHF coherent backscatter events at 34 cm wavelength in the L=3.5 to 4 region (55 to 60 degrees invariant latitude) showing a wide range of spatial and temporal variability. Very large electric field gradients of up to 4 mV/m per kilometer and total electric field increases of more than 40-50 mV/m can occur within the 3 to 5 degree wide SAPS region, moving at cross-L-shell velocities of 250-400 m/s. These very narrow sub-auroral ion drift (SAID) configuration structures are superimposed on the main background SAPS velocity channel and have lifetimes as short as 1.5 minutes. We will also illustrate the range of spatial scales seen and discuss their implications for generation of amplitude and phase scintillation disturbances in transiting radio signals.