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

Abstract

Abstract. During summer months at solar cycle minimum, F-region lacuna and slant-Es conditions (SEC) are common features of daytime ionograms recorded around local magnetic noon at Casey, Antarctica. Digisonde measurements of drift velocity height profiles show that the occurrence of lacuna prevents the determination of F-region drift velocities and also affects E-region drift velocity measurements. Unique E-region spectral features revealed as intervals of Bragg scatter superimposed on typical background Eregion reflection were observed in Digisonde Doppler spectra during intense lacuna conditions. Daytime E-region Doppler spectra recorded at carrier frequencies from 1.5 to 2.7 MHz, below the E-region critical frequency f o E, have two side-peaks corresponding to Bragg scatter at approximately ±1-2 Hz symmetrically located on each side of a centralpeak corresponding to near-zenith total reflections. Angleof-arrival information and ray-tracing simulations show that echo returns are coming from oblique directions most likely resulting from direct backscatter from just below the total reflection height for each sounding frequency. The Bragg backscatter events are shown to manifest during polar lacuna conditions, and to affect the determination of E-region background drift velocities, and as such must be considered when using standard Doppler-sorted interferometry (DSI) techniques to estimate ionospheric drift velocities. Given the Doppler and spatial separation of the echoes determined from high-resolution Doppler measurements, we are able to estimate the Bragg scatter phase velocity independently from the bulk E-region motion. The phase velocity coincides with the E×B direction derived from in situ fluxgate magnetometer records. When ionospheric refraction is considered, the phase velocity amplitudes deduced from DSI are comparable to the ion-acoustic speed expected in the E-region. We briefly consider the plausibility that these previously unreported polar cap E-region Bragg scatter Doppler spectral signatures, observed at Casey in December 1996 during SEC/lacunaCorrespondence to: R. J. Morris (ray.morris@aad.gov.au) conditions may be linked to ionosphere irregularities. These irregularities may possibly be generated by primary plasma waves triggered by current-driven instabilities, that is to say, a hybrid of the "modified two-stream" and "gradient drift" instability mechanisms.

F‐lacuna at cusp latitude and its associated TEC variation

“…The overall F region electron density decreases markedly with the increasing electric field. Monselesan et al [2004] presented observations of E region Bragg scatter signatures during the SEC/lacuna events in the daytime polar cap/cusp. The Bragg backscatter events manifest during polar lacuna conditions and affect the determination of E region background drift velocities.…”

“…Monselesan et al . [] showed the diurnal and seasonal occurrence distributions for lacuna as scaled from Casey (66.3°S, 110.5°E, 80.8°S corrected geomagnetic latitude) Digisonde ionogram records for 1996. In their statistical study, the lacuna is a mixture of F1, F2, and total lacuna rather than being distinguished into the three types.…”

“…Monselesan et al . [] reported E region Bragg Scatter Doppler spectral signatures observed at Casey during SEC/lacuna conditions. These signatures were considered to be linked to ionospheric irregularities which might be generated by primary plasma waves triggered by modified two‐stream instability.…”

“…Comparison of Temporal Characteristics of F-Lacuna Occurrence Among Casey[Monselesan et al, 2004], Dumont d'Urville[Sylvain et al, 1978], and ZhongshanStationHigh-incidence Time of F1-Lacuna High-incidence Time of F2-Lacuna High-incidence Time of Total Lacuna…”

E‐region bifurcation and magnetic field related Bragg backscattered spectral events observed at high‐latitude by MF radar

“… Monselesan et al [2004] reported the existence of unique E‐region spectral features observed in Digisonde Doppler spectra during intense lacuna conditions at the polar cap/cusp station Casey, Antarctica (66.3°S, 110.5°E geographic; 80.8°S magnetic) during 1996. These authors reported on summer day‐time Doppler spectra, recorded at carrier frequencies ∼1.5–2.7 MHz (i.e.…”