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During summer months at solar cycle minimum, F-region lacunaand slant-Es conditions (SEC) are common features of daytime ionogramsrecorded around local magnetic noon at Casey, Antarctica. Digisondemeasurements of drift velocity height profiles show that the occurrence oflacuna prevents the determination of F-region drift velocities and alsoaffects E-region drift velocity measurements. Unique E-region spectralfeatures revealed as intervals of Bragg scatter superimposed on typicalbackground E-region reflection were observed in Digisonde Doppler spectraduring intense lacuna conditions. Daytime E-region Doppler spectra recordedat carrier frequencies from 1.5 to 2.7MHz, below the E-region criticalfrequency foE, have two side-peaks corresponding to Bragg scatter atapproximately ±1-2Hz symmetrically located on each side of a central-peakcorresponding to near-zenith total reflections. Angle-of-arrival informationand ray-tracing simulations show that echo returns are coming from obliquedirections most likely resulting from direct backscatter from just below thetotal reflection height for each sounding frequency. The Bragg backscatterevents are shown to manifest during polar lacuna conditions, and to affectthe determination of E-region background drift velocities, and as such mustbe considered when using standard Doppler-sorted interferometry (DSI)techniques to estimate ionospheric drift velocities. Given the Doppler andspatial separation of the echoes determined from high-resolution Dopplermeasurements, we are able to estimate the Bragg scatter phase velocityindependently from the bulk E-region motion. The phase velocity coincideswith the ExB direction derived fromin situ fluxgate magnetometer records. When ionospheric refraction isconsidered, the phase velocity amplitudes deduced from DSI are comparable tothe ion-acoustic speed expected in the E-region. We briefly consider theplausibility that these previously unreported polar cap E-region Braggscatter Doppler spectral signatures, observed at Casey in December 1996during SEC/lacuna conditions may be linked to ionosphere irregularities.These irregularities may possibly be generated by primary plasma wavestriggered by current-driven instabilities, that is to say, a hybrid of the"modified two-stream" and "gradient drift" instability mechanisms

Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions