Proceedings with Extended Abstracts (single PDF file) - Radio ...

HF DIGISONDE AND MF RADAR MEASUREMENTS OF E-REGIONBRAGG SCATTER DOPPLER SPECTRAL BANDS UNDER THESOUTHERN POLAR CUSPR. J. Morris 1 , D. P. Monselesan 1,2 , D. A. Holdsworth 3 , P. L. Dyson 4 , M. R. Hyde 1,2 , andD. J. Murphy 11 Australian Antarctic Division, Kingston 7050, Tasmania, Australia2 IPS Radio and Space Services, Haymarket, NSW 1240, Australia3 Atmospheric Radar Systems, Thebarton 5031, South Australia, Australia4 La Trobe University, Bundoora 3083, Victoria, AustraliaIntroductionThis paper presents new observations of Bragg scatter events in the high-latitude E-regionionosphere using HF digisonde and MF radar. The digisonde transmitted in a swept-frequencymode from 1.2 to 2.7 MHz while the MF radar transmitted at a single frequency at 1.94 MHz.The two independent instruments were programmed to record E-region backscatter. Resultsfrom spectral signal analyses show evidence of Doppler spectral bands in the respective HFdigisonde and MF radar data. The backscattered signals observed from these different radartechniques at two Antarctic stations appear to originate from the same E-region heights.Moreover there is a remarkable tendency for such spectral bands to occur during intervals ofionosphere slant Es condition (SEC) with lacuna. Independent MF radar observations fromDavis (78.0°E, 68.6°S geographic, 74.6°S magnetic) during 2001 and 2003, and HF digisondeobservations from Casey (66.3°E, 110.5°S geographic, 80.4°S magnetic) during 1996, arepresented. The instrumentation used at the Australian Antarctic stations and in this study isbriefly discussed in Morris et al. (1995). The plausibility of these spectral band events beingrelated to E-region ionosphere plasma instability processes will be discussed.ObservationsA comprehensive account of digisonde observations of E-region Bragg scatter spectralsignatures observed at Casey during 1996 is in preparation by Monselesan et al. (2003).Observations show that during summer months at solar cycle minimum, F-region lacuna andslant-Es conditions (SEC) are a common feature of daytime ionograms recorded around localmagnetic noon at Casey. Digital ionosonde measurements of drift velocity height profilesshow that the occurrence of lacuna prevents the determination of F-region drift velocities, andalso affects E-region drift velocity measurements. Unique E-region spectral features revealedas intervals of Bragg scatter superimposed on typical background E-region reflection wereobserved in ionosonde Doppler spectral measurements during intense lacuna conditions.Daytime E-region Doppler spectra recorded at carrier frequencies from 1.6 to 2.7 MHz, belowthe E-region critical frequency f o E, have two side-peaks corresponding to Bragg scatter atapproximately ±1-2 Hz symmetrically located on each side of a central-peak corresponding tonear-zenith total reflections. Both E- and F-region DPS ionospheric drift velocities are usuallycomputed for a refractive index of n=1. Monselesan et al. (2003) show that for mean spectralsignatures of ±1.0 Hz at 2-MHz and echoes coming from 30° zenith angles ( n=0.5 at 110-kmgroup range), that V i ≅ 425 m/s which corresponds to the side-peaks in the Doppler spectra (asshown in the Figure 1) or possibly equivalent to the irregularity phase velocity, which is nearthe ion-acoustic speed Cs expected in the polar E-region ionosphere.122