The newly installed Rankin Inlet HF radar is very similar to other SuperDARN radars but uses a new type of antennae with its back lobe overlooking the auroral zone where ionospheric irregularities occur very frequently. Despite the fact that a special screen has been installed, there is a chance to receive echoes from the back/side lobe, which can affect the observed velocities. In this study, Rankin Inlet HF radar (RKN) velocities are compared with measurements from three independent instruments: the HF radar in Saskatoon, the CADI ionosonde at Resolute Bay, and drift meters on board DMSP satellites passing the RKN field of view. Although data spread and the degree of agreement vary from one comparison to another, the overall conclusion is that even if echoes are received from the back/side lobe, their effect is statistically insignificant. RKN velocities were found to be comparable to those inferred from other instrument outputs; the slope of the best fit line and the correlation coefficient can be as high as 0.7 and 0.8, respectively. The majority of inconsistencies are related to the difference in the spatial and temporal resolutions of the instruments involved in the comparison.

Tomographic estimates of the electron density altitudinal and latitudinal distribution within the Hankasalmi HF radar field of view are used to predict the expected heights of F region coherent echoes by ray tracing and finding ranges of radar wave orthogonality with the Earth magnetic field lines. The predicted ranges of echoes are compared with radar observations concurrent with the tomographic measurements. Only those events are considered for which the electron density distributions were smooth, the band of F region HF echoes existed at ranges 700–1500 km, and there was a reasonable match between the expected and measured slant ranges of echoes. For a data set comprising of 82 events, the typical height of echoes was found to be 275 km.

The enhanced Polar Outflow Probe (ePOP) is scheduled to be launched as part of the Cascade Demonstrator Small-Sat and Ionospheric Polar Explorer (CASSIOPE) satellite in early 2008. A Radio Receiver Instrument (RRI) on ePOP will receive HF transmissions from various ground-based transmitters. In preparation for the ePOP mission, data from a similar transionospheric experiment performed by the International Satellites for Ionospheric Studies (ISIS) II satellite has been studied. Prominent features in the received 9.303-MHz signal were periodic Faraday fading of signal intensity at rates up to 13 Hz and a time of arrival delay between the O- and X-modes of up to 0.8 ms. Both features occurred when the satellite was above or south of the Ottawa transmitter. Ionospheric models for ray tracing were constructed using both International Reference Ionosphere (IRI) profiles and local peak electron density values from ISIS ionograms. Values for fade rate and differential mode delay were computed and compared to the values observed in the ISIS II data. The computed values showed very good agreement to the observed values of both received signal parameters when the topside sounding foF2 values were used to scale IRI profiles, but not when strictly modelled IRI profiles were used. It was determined that the primary modifier of the received signal parameters was the foF2 density and not the shape of the profile. This dependence was due to refraction, at the 9.303-MHz signal frequency, causing the rays to travel larger distances near the peak density where essentially all the mode splitting occurred. This study should assist in interpretation of ePOP RRI data when they are available.

Long‐term data (1996–2001) for a number of Super Dual Auroral Radar Network (SuperDARN) HF radars in both the Northern and Southern Hemispheres are used to study the midnight F region echo occurrence. We confirm the previously reported increase of echo occurrence toward the solar cycle maximum for all radars considered and a clear winter maximum for some of them. The echo occurrence rate experiences clear equinoctial maxima for many radar locations, especially at higher latitudes and in Antarctica. We attribute the solar cycle echo increase in the midnight sector to the more frequent occurrence of enhanced electric fields and strong plasma density gradients. The equinoctial maxima are believed to be controlled entirely by the electric field increase due both to the Russell‐McPherron effect and to differences in conjugate ionospheric conductances controlled by the tilt of the Earth's axis. For the low geographic latitude portion of the Saskatoon radar observations, the echo statistics differ from the other radars; there is a clear summer maximum in echo occurrence and no definite signature of equinoctial maxima. A summer maximum in low‐latitude echo occurrence also is observed by the Hankasalmi radar during the solar cycle minima. The effect is attributed to improved propagation conditions for HF radio waves during summer periods for the latitudes where, for other seasons, there is a deficiency in the electron density.

Abstract. Several factors are known to control the HF echo occurrence rate, including electron density distribution in the ionosphere (affecting the propagation path of the radar wave), D-region radio wave absorption, and ionospheric irregularity intensity. In this study, we consider 4 days of CUTLASS Finland radar observations over an area where the EISCAT incoherent scatter radar has continuously monitored ionospheric parameters. We illustrate that for the event under consideration, the D-region absorption was not the major factor affecting the echo appearance. We show that the electron density distribution and the radar frequency selection were much more significant factors. The electron density magnitude affects the echo occurrence in two different ways. For small F-region densities, a minimum value of 1 × 10 11 m -3 is required to have sufficient radio wave refraction so that the orthogonality (with the magnetic field lines) condition is met. For too large densities, radio wave strong "over-refraction" leads to the ionospheric echo disappearance. We estimate that the over-refraction is important for densities greater than 4 × 10 11 m -3 . We also investigated the backscatter power and the electric field magnitude relationship and found no obvious relationship contrary to the expectation that the gradient-drift plasma instability would lead to stronger irregularity intensity/echo power for larger electric fields. Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

HF measurements in Prince George, British Columbia (Canada), at five radar frequencies between 9.3 and 15.7 MHz are considered to study the Doppler velocity of E region coherent echoes. One event showing a regular variation of velocity with radar frequency, slant range, and azimuth of observations is analyzed in detail. For this event, plasma drifts were in access of 700 m s −1 , but the observed velocities were below 250 m s −1 since measurements were performed at large flow angles ( L shell angles 45°< ϕ< 100°). We show that measured Doppler velocity depends on irregularity scale but only within the Farley‐Buneman (F‐B) instability cone (45°< ϕ< 75°). We demonstrate that maximum velocities measured at the highest radar frequency are ∼1.3 times larger than those at the lowest frequency. We also show that for observations inside the instability cone, the velocity magnitude strongly decreases with aspect angle and the rate of the decrease is scale sensitive. The effect can be described by the fluid theory formula if the nominal electron collision frequencies are replaced by anomalous collision frequencies that are ∼5 times larger. However, for observations outside the F‐B instability cone (75°< ϕ< 100°), the Doppler velocity does not show significant variation with aspect angle. For these directions, velocity change with flow angle was insignificant, very similar at all radar frequencies, and not consistent with the expected “cosine” law. The implications of the measurements on the theory of electrojet instabilities and the processes of coherent echo formation are discussed.

The main purpose of the Super Dual Auroral Radar Network (Super‐DARN) is to use paired radars to deduce the F‐region convection from Doppler measurements of backscatter seen at large ranges, typically beyond ∼900 km. Nearer to each HF radar, the nearest ranges at ∼165–400 km are dominated by meteor trail echoes. Once formed, the motion of these meteor trails is normally controlled by neutral winds in the 80–110 km altitude range. By combining the line‐of‐sight velocities from all 16 receiver beams (∼52° in azimuth) of a given SuperDARN radar, it is possible to determine the full horizontal wind vector field over the meteor trail height range. Elevation angles are also measured using an interferometer mode and as such height information can, in principle, be obtained from the combined range and elevation angle data. A comparison with neutral wind measurements from a colocated (Saskatoon, Canada) MF wind radar indicates good agreement between the two radar systems at heights of ∼95 km. Based on these detailed comparisons, a simple common method for determining two‐dimensional winds for all SuperDARN radars, which have extensive longitudinal coverage, was developed. Comparisons with other systems used for dynamical studies of tides and planetary waves are desirable and prove to be essential to obtain a good SuperDARN neutral wind motion analysis. The MF radars at Saskatoon and Tromsø, Norway, are located near the western and eastern ends of the Northern Hemisphere network of six SuperDARN radars. Comparisons between the two types of radars for two seasonal intervals (September and December) show that the SuperDARN radars provide good longitudinal coverage of tides in support of the more detailed MF radar data. The two systems complement each other effectively.

The statistical study of the azimuthal convection flow in the midnight sector, as measured by the Saskatoon and the Stokkseyri SuperDARN radars, reveals the existence of an enhanced eastward convection stream around the poleward boundary of the auroral oval. The stream occupies two to three degrees and is located at geomagnetic latitudes 72–75°, which is indeed significantly poleward of the position of the center of the auroral oval. Poleward of the eastward convection stream, a westward convection stream is detected by the Saskatoon radar though not evident in the Stokkseyri radar measurements. The existence of the eastward convection stream at the poleward edge of the nightside auroral oval is very consistent with earlier results from the Akebono spacecraft. Such plasma flows are the source of possible plasma instabilities in the ionospheric E and F‐regions.

Ionospheric convection inferred from Super Dual Auroral Radar Network (SuperDARN) HF radar measurements is compared with an equivalent ionospheric convection derived from ground magnetometer data in the dayside winter high‐latitude ionosphere. Although there was general agreement between observed convection patterns produced by radars and magnetometers, there were significant differences in details. The orientation of equivalent convection vectors inferred from magnetic data was often opposite to the convection vectors determined by the SuperDARN radars in the poleward part of the convection vortex structure, though the agreement was reasonable in its equatorward part. The magnitudes of convection vectors determined from radar data and those inferred from magnetometer data were often different. The observed differences are attributed to strong horizontal inhomogeneity in the ionospheric conductivity distribution for winter conditions. It is possible that magnetic disturbances in the dark high‐latitude ionosphere are strongly affected by field‐aligned currents at the terminator that separates regions of the sunlit highly conducting ionosphere and dark poorly conducting ionosphere.

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, 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].

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field‐aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high‐latitude wave is responsible mainly for TCVs, whereas the low‐latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.

The SuperDARN radars are now able to measure the angles of arrival of the backscattered radiation. We describe the analysis procedure and present results for meteor scatter from slant ranges below 500 km and ground scatter from either the E region or the F region at greater slant ranges which can be used to determine peak electron densities and their heights. The angle of arrival measurements can also be used to identify “unwanted” backscatter from the backward lobes of the antenna radiation pattern. Electron densities can also be measured in the back lobes.

The asymmetry (skewness) of broad type II Doppler spectra collected by the pre Saskatchewan Auroral Polarimetric Phased Array Ionospheric Radar Experiment (SAPPHIRE) coherent radar system during westward electrojet conditions was studied. The system included two 50‐MHz CW links which monitored spatially close regions of the ionosphere with about the same aspect angles of ∼10° but with an azimuth difference of the bisectors of about 90°. Periods of scatter were selected when both radar links recorded echoes with a wide, type II spectrum. Typically, the spectra were asymmetrical on both links and had a magnitude of skewness in between 0.3 and 0.5. For radar links with a positive (negative) mean Doppler shift, the skewness was observed to be positive (negative) contrary to previous reported auroral zone experiments (positive skewness means a longer spectral tail toward higher velocities). The spectra were more skewed for observations along the electrojet, for higher echo power and also for narrower spectral width. The characteristic features of the spectral asymmetry are attributed to the plasma turbulent processes at large off‐orthogonal angles.

This paper describes a new, 50 MHz auroral radar system, named SAPPHIRE, which observes at two locations in Canada: SAPPHIRE SOUTH, in the auroral zone and SAPPHIRE NORTH at nominal cusp latitudes in the vicinity of the polar cap – auroral zone boundary. SAPPHIRE was designed to offer superior time and frequency resolution for studies of the geophysical phenomena in the observing regions and to provide a better insight into the plasma processes responsible for the radio scattering. The radars are continuous-wave, dual bistatic systems with two transmitters and two receivers (both at a common receiver location) in each radar system. This configuration provides two-dimensional estimates of the phase velocities in the region of the scatterers. The systems were designed for unattended operation at remote sites and are controlled via modems using conventional telephone service. There are a number of innovative parts to the design of this radar. All frequencies used in the transmitter and receivers are generated using direct digital synthesis which gives independent and accurate control of both frequency and phase. The data collection system uses extensive parallel processing via a custom design of INMOS "transputers" in order to handle the large amount of data received per day. This data collection system only stores data for those times when the observed echoes fulfill some minimum threshold conditions. The most interesting aspects of the electronic design are described and some representative data from SAPPHIRE SOUTH are presented.

At least two new types of spectra (types 3 and 4) from coherent VHF scatter radar spectra of auroral E-region irregularities have been identified whose origins are poorly understood. They are often correlated with optical aurorae. The data obtained by a 50-MHz bistatic CW (continuous wave) radar and an all-sky camera suggest that steep electron density gradients (SEDG) associated with discrete auroral arcs can explain the diversity of the 50-MHz radar Doppler spectra, the narrow nonion-acoustic spectra in particular. Depending on the direction of the electric field perpendicular to the arc, such gradients would either raise or lower the threshold of two-stream instabilities that generate the irregularities. The increase in the electron collision rate with the electron temperature tends to decrease the importance of the gradient term in the dispersion relation. The consequence of electron heating is that the gradients must be steeper to affect the threshold conditions significantly. Similarly, as previous studies demonstrated, the threshold drift velocity at which the spectra tend to saturate can also be raised by an increase in the ambient ion-acoustic speed due to elevated electron temperatures (EET) in regions adjacent to auroral arcs. During strongly turbulent conditions the spectral width is likely to increase and the mean velocities can exceed the nominal ion-acoustic speed in the E-region. Several backscatter events from different times during eastward and westward electrojet are presented to illustrate the diversity of auroral spectra. It is shown that the EET and SEDG theories complement each other and can explain some of the composite spectra.

The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter‐scale E region irregularities. SAPPHIRE is a dual 50‐MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross‐beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial target grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna arrays showed a class of scatter characterized by a narrow power spectrum peaking at the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large‐amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the “short discrete radar auroras” which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the large dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3‐dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations.