With the potential use of SuperDARN radars in mind and to test the theoretical predictions for dependence of the phase velocity of Farley-Buneman waves on radar frequencies in the HF range, a statistical analysis was made of over 11,000 specifically selected spectra from multi-frequency observations by the SuperDARN ykkvibaer radar in September-October 2000.Good qualitative agreement was found between the observed and predicted frequency dependence for slightly disturbed magnetic conditions.Assuming that increased magnetic activity (higher K p ) manifests itself via enhanced electron temperature and applying the algorithm of the control parameters of Kagan & Kissack [1], it was shown that in agreement with observations, the dependence of the FB waves phase velocity on the irregularity wave number (radar frequency) should decrease with increasing electron temperature (K p ).The results make it clear that specially designed multi-frequency SuperDARN experiments would be a valuable tool in studying the HF Farley-Buneman waves at high latitudes.

: SuperDARN is an array of HF radars, which covers most of the northern and southern high-latitude regions. The primary goal of this array is to study the dynamics of the large-scale convection pattern in order to understand the Solar wind - Magnetosphere - Ionosphere coupling (SW-M-I). Wide area coverage made it possible to detect some of the proxies for the magnetospheric land marks and boundaries on a global scale and shed some light on the on the some of the fundamental problems in the SW-M-I coupling process. One discovery is that SuperDARN radar E region backscatter boundary in the dusk-midnight sector can be used as a proxy for the inner boundary of the ion plasma sheet. This made it possible to study the boundary dynamics on a more global scale. The boundary undergoes seasonal, diurnal, and substorm associated variations. One question in the SW-M-I coupling research is how fast Magnetosphere-Ionosphere system reacts to the changes in the Interplanetary Magnetic Field (IMF). There are two schools of thoughts on changes in ionospheric convection one being instantaneous and the other being delayed response. We present a study of the response of the equatorward boundary of the ion auroral oval on a global scale to the changes in the IMF. We have used the wide area coverage of the SuperDARN radar to investigate the response of the boundary to the changes in the upstream IMF. Estimation of the delay from the changes in the solar wind and IMF from an upstream satellite to the ionosphere is sometimes ambiguous. To avoid this ambiguity we have also used the changes in the central polar cap convection related to the changes in the IMF. This method helps to test the hypothesis of the fast and or slow changes. We will also compare the response of the ion auroral oval and the open/closed field line boundary to better understand the sequence of response from the changes in IMF.

The temporal variation of the equatorward boundary of the proton aurora/high‐energy ion precipitation is a manifestation of diurnal and seasonal (i.e., dipole tilt) effects as well as magnetic activity. In particular, during the substorm growth phase this boundary moves equatorward, an effect due primarily to thinning and earthward motion of the cross‐tail current in the inner magnetosphere as the field evolves toward a more stretched topology. Recent advances in monitoring this boundary using ground‐based instruments have opened up the possibility of following its temporal evolution across several hours in local time. This in turn allows one to explore whether this magnetotail stretching is a global or local phenomenon. We have examined this boundary evolution during the growth phases of 68 substorms over the Canadian sector. We use the equatorward boundary of SuperDARN E region echoes as a proxy for the proton auroral boundary as described by Jayachandran et al. (2002b). We find that in 21 of the 68 substorms the equatorward motion of the auroral boundary is restricted to several hours of local time in the evening sector. In the remaining 47 substorms, the equatorward motion was global so that the boundary retained its shape throughout the growth phase. Our results indicate dramatically different growth phase phenomenology in these two classes of substorms. In one, the growth phase involves stretching in the inner magnetosphere that is most pronounced around the onset meridian. In the other, the stretching extends many hours in local time away from the onset meridian.

Recent observations of auroral backscatter at frequencies from 10 to 933 MHz have found spectral shapes which are intermediate between Lorentzian and Gaussian. To account for these observations a model has been developed consisting of a random distribution of scatterers in the radar field of view. Spectral shape is discussed in terms of the shape of the magnitude of the temporal autocorrelation function (ACF–the Fourier transform of the spectrum), and within the assumptions of the model it is shown that the observed shapes arise from either the average ACF associated with the signal from individual scatterers or from the Doppler spread in the scattering volume. The observed spectral shapes are easily accounted for by a model for the individual scatterer signals consisting of an exponential growth, a quasi‐steady section and an exponential decay.

A study of polar cap/dayside convection response to substorm intervals using Canadian Advanced Digital Ionosondes (CADIs), situated well within the polar cap, and SuperDARN radars during steady and prolonged southward IMF Bz conditions showed three distinct features: (1) gradual prolonged decrease of dayside/polar cap convection speed until the substorm onset; (2) sudden decrease of convection following the onset of the substorm; and (3) increase of convection during the recovery phase of the substorm. We hypothesize that the observed features of the convection are due to the modulation of the Region 1 current system associated with the substorm. The ground magnetic response of magnetometers situated inside the polar cap is at least consistent with our postulate.

When their growth rate becomes too small, the E-region Farley-Buneman and gradient-drift instabilities switch from absolute to convective. The neutral density gradient is what gives the instabilities their convective character. At high latitudes, the orientation of the neutral density gradient is close to the geomagnetic field direction. We show that this causes the wave-vector component along the geomagnetic field to increase with time. This in turn leads to wave stabilization, since the increase goes hand-in-hand with an increase in parallel electric fields that ultimately short-circuits the irregularities. We show that from an equivalent point of view, the increase in the parallel wave vector is accompanied by a large upward group velocity that limits the time during which the perturbations are allowed to grow before escaping the unstable region. The goal of the present work is to develop a systematic formalism to account for the propagation and the growth/decay of high-latitude Farley-Buneman and gradient-drift waves through vertical convective effects. We note that our new formalism shies away from a plane wave decomposition along the magnetic field direction. A study of the solution to the resulting nonlinear aspect angle equation shows that, for a host of initial conditions, jump conditions are often triggered in the parallel wave-vector (defined here as the vertical derivative of the phase). When these jump conditions occur, the waves turn into strongly damped ion-acoustic modes, and their evolution is quickly terminated. We have limited this first study to Farley-Buneman modes and to a flow direction parallel to the electron E × B drift. Our initial findings indicate that, irrespective of whether or not a jump in aspect angle is triggered by initial conditions, the largest amplitude modes are usually near the ion-acoustic speed of the medium (although Doppler shifted by the ion motion), unless the growth rates are small, in which case the waves tend to move at the same drift as the ambient electrons.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)

Radar observations at 440 (Millstone Hill) and 933 MHz (European Incoherent Scatter (EISCAT)) and at both small and large magnetic aspect angle have been used to study the spectral characteristics of backscatter from the auroral E region. The predominantly single‐peaked power spectra were found to have very similar shapes at both frequencies and at both small and large aspect angle. Spectral moments were estimated directly from the autocorrelation functions (ACFs) and were also characterized in terms of a correlation time (related to spectral width) and a decay exponent which quite accurately described the observations. For the most part, spectral shapes were found to be intermediate between Gaussian and Lorentzian, at both large and small magnetic aspect angles. Similar ACF shapes have been observed at much larger wavelengths at HF [Villain et al., 1996], suggesting that this may be a widespread and possibly fundamental characteristic of radar auroral backscatter. In addition to confirming and extending a number of previous observations on UHF spectral characteristics, correlation times were found to decrease sharply near an aspect angle of 3°, having a nearly constant value at all larger aspect angles. This behavior is consistent with a simplified picture of the energy balance for these large aspect angle waves.

An examination of the substorm onset location inferred from Polar UVI and the location of the equatorward boundary of the proton auroral oval inferred from ground based SuperDARN radars is presented. A study of 96 individual substorm events reveals that the substorms can be initiated either near the equatorward boundary of the proton auroral oval or far poleward of the equatorward boundary of the proton auroral oval depending on the preceding interplanetary conditions. When the Interplanetary Magnetic Field (IMF) is predominantly southward prior to the substorm onset, the onset location is near the equatorward boundary of the proton auoral oval; when the IMF is predominantly northward prior to the onset; the onset location is far poleward of the equatorward boundary of the proton auroral oval. The latitudinal separation (ΔΛ) between the onset location and the equatorward boundary of the proton auroral oval shows a linear dependence on the IMF.

We have examined the location of the SuperDARN E region ionospheric backscatter boundary in the dusk‐midnight sector and compared it with the equatorward boundary of the hard ion precipitation (b2i) determined from the DMSP satellite particle spectrograms in the dusk‐midnight sector of the auroral oval. The locations of these two boundaries were found to be nearly coincident. This study evinces the capability of the SuperDARN radars to monitor the equatorward ion precipitation boundary (diffuse auroral boundary) and its dynamics irrespective of magnetic activity.

We compare the locations of the equatorward boundaries of SuperDARN E-region backscatter and Hb emissions, focusing on the dusk-midnight sector of the auroral oval where the proton aurora is statistically located equatorward of the discrete electron aurora. We show that, whenever both boundaries can be simultaneously identified, they are coincident. Our result complements earlier studies, which demonstrated the correspondence between the DMSP b2i boundary and both the equatorward boundary of the proton auroral oval (Donovan et al., 2002), and the equatorward boundary of SuperDARN E-region echoes (Jayachandran et al., 2002). Further, our result shows that, provided there is sufficient precipitating proton energy flux, the SuperDARN radars can be used to monitor the equatorward edge of the proton auroral oval.Key words. Ionosphere (auroral ionosphere; particle precipitation; ionospheric irregularities)

A review of the spectral characteristics of radar aurora with a focus on the behavior over a wide range of frequencies gives a different perspective on several issues concerning auroral backscatter. Some of the conclusions from this study are as follows: (1) It appears most likely (but has certainly not been proven) that auroral backscatter at large geometric magnetic aspect angles has the same general characteristics at all frequencies; VHF observations showing other characteristics are most likely refracted to small aspect angle by sporadic E. (2) The broadest spectra are observed at neither HF or UHF, but at VHF frequencies, but it is unclear whether this is a topic for theoreticians to consider, or if it is an observational effect. (3) The flow angle dependence of sub‐ion‐acoustic UHF echoes and large aspect angle echoes both present theoretical challenges which remain unaddressed. (4) In spite of several attempts, there appears to be no satisfactory theory of either type 3 or type 4 echoes, especially when the frequency dependence of these echoes is considered.

Using data taken over 18 months with the Iceland East (CUT‐LASS/Iceland) Super Dual Auroral Radar Network (SuperDARN) HF radar we have made a statistical study of a class of echoes which occur at ranges typically associated with F region echoes, but which have Doppler speeds near the ion acoustic speed Cs typical of E region echoes [Milan et al., 1997]. Comparison of the seasonal, diurnal, and range distributions of these echoes with the predictions of propagation models show that these are, indeed, E region echoes, differing in morphology from similar echoes at nearer ranges mainly because of the propagation conditions which are required to observe them. For the particular radar geometry of this study, conventional theory predicts that the effects of ionospheric gradients will result in phase velocities (radar Doppler velocities) which differ significantly from Cs. in disagreement with these observations. However, the observations are consistent with a new nonlinear theory of St.‐Maurice and Hamza [2001].

During the equinox and winter seasons, and in the range 300–1000 km the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar often detects extended patches of coherent echoes with remarkably uniform properties and low Doppler speeds, in the range 0 to 200 m/s. Typically, these echoes last for ∼3 hours, and are observed between 1300 and 2300 MLT, at times of moderate to high Kp values. The echo Doppler shift changes systematically with azimuthal angle and a vector reconstruction of the implied drift indicates westward velocities in the range 150 to 250 m/s, well below the threshold speed associated with Farley‐Buneman waves. When ionosonde observations are available, they invariably show the presence of a thick sporadic E layer. This feature, plus the facts that the IMF By is always negative and that the echoes are equatorward of the regions of discrete precipitation (as indicated by comparison with coincident DMSP satellite observations), indicate that the echoes are associated with the diffuse aurora in regions where the electric field is of the order of 10 mV/m or less. We infer from these echo properties that the irregularities are triggered by a primary gradient‐drift mechanism which then cascades to the observed structures through weakly turbulent mode‐coupling processes. Several events were observed during special multifrequency experiments using the Saskatoon SuperDARN radar. It was found that the Doppler speed, power, and spectral width all increase systematically with increasing radar frequency. The findings for Doppler speed and power appear to arise, at least in part, from the increase in height of the radar echoes with increasing frequency. The frequency dependence of spectral width may be related to instability lifetimes; it was found to agree well with the results of numerical simulations [Keskinen et al., 1979].

Through the groundscatter process the Super Dual Auroral Radar (Super‐DARN) has become a powerful tool for studying F region gravity waves. However, the measurement of the gravity wave position is not direct and relies on an assumption relating ground scatter distance to reflection distance. In previous studies it has been assumed that the tilting of the ionospheric reflecting layer was negligible. Hence the gravity wave distance has been calculated as if the reflecting layer was strictly horizontal. Using virtual height data from an ionosonde and ray tracing, we show that this assumption leads to a systematic error of about 16% in the positioning of the ionospheric reflection point, with the error more than 30% on occasion. Using ray tracing, we obtained an improved relation between ionospheric reflection and ground scatter distances. With this improved distance calculation, we have found the direction and velocity for a number of gravity waves. These waves were found to be traveling equatorward, usually, with velocities between 50 and 280 m/s, in agreement with previous gravity wave observations and with the notion of filtering by the thermospheric wind. In some cases the source locations were determined by using gravity wave dispersion. These locations were found to be on the poleward side of the auroral oval during periods of weak, but observable, magnetic disturbance. Our ray‐tracing studies found that the strongest features were due to gravity waves of 3–20 km amplitude.

The EISCAT UHF radar system was used to study the characteristics of E-region coherent backscatter at very large magnetic aspect angles (5–11°). Data taken using 60 μs pulses during elevation scans through horizontally uniform backscatter permitted the use of inversion techniques to determine height profiles of the scattering layer. The layer was always singly peaked, with a mean height of 104 km, and mean thickness (full width at half maximum) of 10 km, both independent of aspect angle. Aspect sensitivities were also estimated, with the Sodankylä-Tromsø link observing 5 dB/degree at aspect angles near 5°, decreasing to 3 dB/degree at 10° aspect angle. Observed coherent phase velocities from all three stations were found to be roughly consistent with LOS measurements of a common E-region phase velocity vector. The E-region phase velocity had the same orientation as the F-region ion drift velocity, but was approximately 50% smaller in magnitude. Spectra were narrow with skewness of about –1 (for negative velocities), increasing slightly with aspect angle.

Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short-range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short-range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.