G. C. Hussey

Simultaneous 50-MHz coherent backscatter and digital ionosonde observations in the midlatitude E region

SESCAT, a coherent backscatter radar system located in Crete, Greece, was operated for one summer together with a (CADI) digital ionosonde observing nearly the same scattering volume. The purpose of the experiment was to further investigate the origin of midlatitude E region VHF echoes which occur almost exclusively during summer nighttime. It was found that 50-MHz midlatitude backscatter always occurs in association with sporadic E layers. A statistical analysis indicated significant correlations between SESCAT total echo power and E s characteristics such as the layers top frequency f t E s (a measure of maximum E s electron density) and the apparent E s trace spread which results from range spreading due to oblique reflections from a nonuniform and horizontally inhomogeneous layer. Similar correlations were obtained for SESCAT spectral width and the same sporadic E characteristics. The experiment confirmed that the presence of an E s layer in the scattering volume, which could provide destabilizing electron density gradients perpendicular to the magnetic field, is necessary but not sufficient for the occurrence of 50-MHz backscatter. We suggest that in addition there is need for an enhanced electric field to be present inside the layer as well, a notion that is in line with the observed correlation of backscatter with a dense but strongly inhomogeneous E s layer and a recently proposed mechanism for strong polarization fields at midlatitudes.

Azimuth‐time‐intensity striations of quasiperiodic radar echoes from the midlatitude E region ionosphere

The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86°, from 26° E to 58° W, with ∼2° angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth-time-intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range-time-intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post-midnight hours. The zonal rates, dx/dt, computed from the striation slopes take values between ∼30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.

Wavelength dependence of Doppler spectrum broadening in midlatitude E region coherent backscatter

We use observations from a high-frequency Doppler radar experiment at midlatitudes to study the wavelength dependence of spectrum broadening of backscatter from field-aligned E region irregularities. The radar made multifrequency measurements of high-resolution Doppler spectra simultaneously at four frequencies, that is, 9.23, 11.03, 12.71, and 16.09 MHz, which correspond to backscatter from irregularities with wavelengths of 16.3, 13.6, 11.8, and 9.3 m, respectively. In the analysis we deal with secondary irregularities (type 2 echoes) characterized by small mean velocities that show no dependence on radar frequency and have mean spectrum width to velocity ratios well above unity. The spectrum width is found to increase monotonically with wavenumber k in the range from 0.38 to 0.67 m−1 covered in the experiment. By postulating that the width is determined mainly by the nonlinear growth rate of the secondary short-scale plasma turbulence, we compare the results to the Sudan [1983] theory. Although there is some general agreement, on the average, the measured mean spectrum width follows approximately a k4/3 power law dependence which is considerably stronger than the theoretical k2/3 dependence. This discrepancy may be reduced somewhat but cannot be fully accounted for when additional spectrum broadening effects caused by the irregularity velocity distribution are also included.

Spatial occurrence of decameter midlatitude E region backscatter

This paper provides a statistical analysis of the spatial occurrence of midlatitude E region decameter backscatter. Measurements were made using the Valensole HF (high frequency) radar located in southern France during the summer of 1995 when it operated simultaneously at four frequencies. On the basis of the premise that E region scattering is fully magnetic aspect sensitive, the spatial occurrence statistics show that the aspect sensitive region moves toward the radar (southward) with respect to line of sight propagation calculations, with the lower-frequency echoes being closer toward the radar than the higher frequency ones are, in agreement with refraction theory predictions. Ray tracing inside nighttime midlatitude electron density profiles augmented with dense sporadic E s layers was used to calculate the expected echoing region, and good agreement with the observed region was found. Another finding is the angular distribution of backscatter inside the wide azimuthal sector covered by the radar scan. The spatial distribution of echo occurrence has its maximum at small azimuths at and about the geomagnetic north, suggesting that statistically, the meridional direction is strongly preferred for backscatter. Under the postulation that these are secondary decameter waves, we concluded that the observed angular anisotropy in spatial occurrence is at odds with the concept of strong isotropic plasma turbulence [Sudan, 1983] but in general agreement with the two-step gradient drift instability theory of secondary-wave generation proposed by Sudan et al. [1973].

Spectral width of type 2 coherent echoes at large magnetic aspect angles

The spectral characteristics of smoothly varying type 2 signals were studied using two 50-MHz CW radio links. The echoes observed came from two closely located scattering volumes where the geometrical aspect angles of observations were about 10° for both links but the azimuthal angles differed by 88°. The events reported here were selected when the spectra on both links were wide, with a comparable spectral width, but the mean Doppler shift was almost zero on one link and very high, sometimes in excess of the ion-acoustic velocity, on the other link. Highly shifted Doppler spectra were observed to be more skewed than the spectra with low shifts. The skewness was negative for negative Doppler shifts and positive for positive Doppler shifts in accordance with previous radar observations at large magnetic aspect angles. The variation of spectral width with the radial velocity or with the magnetic disturbance level exhibited two regions of linear dependence, with a larger slope at small values and a much smaller slope at higher ones. These different slopes likely reflect the change in the rate of the irregularity amplitude increase with the electric field.

Polarization of auroral backscatter at 50 MHz

Auroral backscatter from the lower E region was studied using two 50 MHz bistatic, continuous wave radar links which shared a common polarimetric receiver. The polarization parameters were defined in terms of the polarization ellipse, which is described by the ellipticity angle χ orientation angle ψ, and polarization ratio m. Spectral analysis was applied to the intensity measurements and its corresponding polarization parameters. Observations of typical auroral spectral types 1, 2, and 3 indicated that the scattering of a linearly polarized incident wave produced an essentially linear and highly polarized scattered wave. These results imply a small scattering volume and/or a small number of discrete scatterers located close to one another, “scatterer” referring to a volume where radar waves are scattered according to weak coherent scattering theory, and also reaffirms that the scattering process is a weak coherent one. These results are typical of most observations, but not all; an otherwise typical intensity spectrum may also exhibit variable polarization parameters with an appreciable reduction in the polarization ratio and/or signals of significant ellipticity. These anomalous properties can be explained as scatter coming from a number of individual scattering volumes within the scattering region (i.e., the effective radar viewing region), which are each influenced differently by Faraday rotation to and from the scattering region.

On the statistics of SuperDARN autocorrelation function estimates: SUPERDARN ACF STATISTICS

Time domain signal processing techniques are employed by the Super Dual Auroral Radar Network (SuperDARN) to obtain bulk measurements of the velocity and spectral width of F region ionospheric plasma irregularities. The measurements are obtained by fitting estimates of the mean autocorrelation function (ACF) of the radar target. To accurately and consistently extract target parameters from the mean unnormalized ACF, it is necessary to utilize error-weighted fitting algorithms with a weight given by the variance of the ACF. Currently implemented weights are ad hoc, and a detailed description of the statistical characterization of SuperDARN ACFs is needed. Following the discussions in Farley (1969) and Woodman and Hagfors (1969), which describe the variance for the mean normalized ACF used with incoherent scatter radars, we present analytic expressions for obtaining the variance of the real and imaginary components of the mean unnormalized SuperDARN ACF. These expressions are based on models by Andre et al. (1999) and Moorcroft (2004) of the voltage signal received by SuperDARN radars but may be used for other soft target radar systems. An algorithm for obtaining the variance of both the magnitude and phase of the mean ACF is also presented. The results of this study may be directly integrated into existing SuperDARN data analysis software and other pulse-Doppler radar systems that utilize estimates of the mean unnormalized ACF.

Radar observation of kinetic effects at meter scales for Farley‐Buneman plasma waves

[1] Coherent backscatter Doppler measurements, made simultaneously at 144 MHz and 50 MHz from a common volume in the midlatitude E region ionosphere, were analyzed in order to study the phase velocity ratio of type 1 plasma irregularities at 1 m and 3 m wavelengths. In the analysis, high-resolution Doppler spectrograms were used first to identify the type 1 events and then to estimate the mean and spectral peak velocities from averaged power Doppler spectra. The simultaneous spectrogram signatures of type 1 echoes suggested a somewhat higher threshold for instability excitation at 144 MHz than at 50 MHz. Statistically, the measured 144 MHz to 50 MHz velocity ratios attain values above unity, mostly in the range from 1.05 to 1.14 with an overall average of 1.10. This 10% difference in the type 1 velocities at 144 MHz and 50 MHz was attributed to kinetic effects at short plasma wavelengths. For comparison, a linear kinetic model of the Farley-Buneman instability, which includes also a destabilizing plasma density gradient, was used to provide numerical estimates of type 1 phase velocities. It was found that the theoretical predictions for gradient-free Farley-Buneman waves agreed well with the observations, under the suppositions that the strongest type 1 echoes come from E region altitudes where conditions for instability are optimal and that type 1 waves have their phase velocities limited at threshold values equal to the plasma ion acoustic speed. The present study has confirmed the accuracy of the kinetic theory of the Farley-Buneman instability, which strengthens its validity and suitability for meter-scale E region irregularity studies.

Modelling of relative mode power received on RRI experiment on ePOP satellite mission

The Cascade Demonstrator Small-Sat and Ionospheric Polar Explorer (CASSIOPE) satellite is scheduled to be launched in 2011. The satellite will carry a suite of eight scientific instruments comprising the enhanced Polar Outflow Probe (ePOP). One instrument is the Radio Receiver Instrument (RRI) which will be used to receive HF transmissions from ground transmitters such as the Super Dual Auroral Radar Network (SuperDARN) array. Magnetoionic polarization and propagation theory has been used to model the relative power that SuperDARN delivers to the Ordinary (O) and Extraordinary (X) modes of propagation. The geometry of the radars and magnetic field results in the X-mode dominating the transmitted signal when the modelled wave propagates northward and is nearly perpendicular to the magnetic field lines. Other propagation directions (i.e., above or southwards of the radar) results in propagation which is anti-parallel to the magnetic field lines and an equal splitting of transmitted power between the O- and X-modes occurs. For either high transmitting frequencies or low ionospheric electron densities, the range of latitudes that signal will be received at the satellite is quite large (up to ^90° of latitude). Conversely, for lower transmitting frequencies or higher ionospheric electron densities, the latitudinal range that signal will be received over is smaller. These relative mode power calculations will be used to characterize the average electron density content in the ionosphere or to provide a measure of relative absorption in the D- and E-regions when the satellite passes through the field-of-view of a SuperDARN radar.