D. R. Moorcroft

HF detection of slow long-lived E region plasma structures

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].

Finding gravity wave source positions using the Super Dual Auroral Radar Network

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.

Resolute Bay VHF radar: A multipurpose tool for studies of tropospheric motions, middle atmosphere dynamics, meteor physics, and ionospheric physics

A VHF radar has been established at a site near Resolute Bay in Nunavut, Canada (75°N, 95°W), which has the capability to make a variety of measurements relating to the atmospheric and ionospheric environment in the polar regions. The site is very close to the north geomagnetic pole, and therefore the radar is well situated to make some unique measurements. The system is a multipurpose instrument with good remote control capabilities. It can be used as a wind profiler radar to study the lower troposphere, as a mesospheric radar to study polar mesosphere summer echoes (PMSE) in summer, as a meteor radar to determine winds in the altitude region of 80–100 km, and as an ionospheric radar to study 3 m scale irregularities in the E and F regions. The radar has some unique design features, partly dictated by the rough terrain in which it is sited. In this paper, the radar system is described, including description of some unusual approaches to deal with special conditions at the site, and then some key early results are presented. Important findings include error determinations for tropospheric wind measurements, detection of PMSE, correlations between PMSE and atmospheric temperatures at 86 km altitude, measurements of mean winds and tidal characteristics over a full year, and detection of various normal modes of oscillation in the 80–100 km region, especially in nonsummer months. Some of these features will be discussed here, but more detailed discussions will be left to related papers in this issue.

Magnetic aspect angle effects in radar aurora at 48.5 MHz, corrected for refraction

It is now widely recognized that at lower VHF frequencies, refraction by electron density structures in the auroral E region can greatly affect the character of radio auroral backscatter, making the quantitative study of magnetic aspect angle effects difficult or impossible. In the present study of data taken with the 48.5-MHz Bistatic Auroral Radar System (BARS) in central Canada, these difficulties have been minimized by using only data obtained from spatially and temporally uniform events. After correcting for effects of refraction, it has been found that true large aspect angle backscatter occurs for magnetic aspect angles exceeding 6°. Doppler velocities from the smaller aspect angle Red Lake radar behaved as if proportional to the line-of-sight component of the convection drift velocity; the resulting flow directions were found to be in good agreement with estimates obtained from magnetometers located in the BARS field of view. The constant of proportionality between the radar velocity and the drift velocity component appears to decrease by a factor of approximately 2 as the aspect angle increases from 3° to 6°. At 3° aspect angle, the backscatter power decreased with increasing aspect angle by about 13 dB/deg (the aspect sensitivity), similar to what has been found in other studies at UHF. In common with those other studies this aspect sensitivity is found to decrease with increasing aspect angle, but more rapidly, falling to less than 4 dB/deg at aspect angles of 5°. In contrast to previous studies at small aspect angle, there is no evidence of a dependence of backscattered power on the flow angle, the angle between the radar line of sight and the direction of E × B drift.

Ion acoustic HF radar echoes at high latitudes and far ranges

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 C s 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 C s , in disagreement with these observations. However, the observations are consistent with a new nonlinear theory of St.-Maurice and Hamza [2001].

Flow angle effects in E region 398‐MHz auroral backscatter at small aspect angle

A study of flow angle effects on small aspect angle backscatter from the auroral E region has been made using data obtained in the 1970s with the 398-MHz radar at Homer, Alaska. Several findings differ from previous results obtained at 140 MHz with the Scandinavian Twin Auroral Radar Experiment and the Sweden and Britain Radar-Auroral Experiment radars: (1) backscatter power is virtually independent of flow angle; (2) there is much less difference between type 1 and type 2 echoes; (3) as found in previous UHF studies, Doppler velocities of type 2 (low Doppler velocity) echoes vary with flow angle more rapidly than predicted by the “cosine law”. These differences are considered in the light of the recent strong turbulence theory developed by Hamza and St-Maurice [1993a, b].

Global and local equatorward expansion of the ion auroral oval before substorm onsets

[1] 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.

A statistical study of UHF auroral backscatter at large magnetic aspect angle: A reanalysis of unpublished results from 1968

In 1968 a joint study of radar-auroral backscatter was carried out by Bell Laboratories, Western Electric, and Lincoln Laboratory using the 448-MHz radar of the Prince Albert Radar Laboratory. They made observations covering a range of magnetic aspect angles from 4.5° to 16°, but the results were not published in the open literature. These data have been reanalyzed and used to obtain results on the aspect sensitivity, flow angle dependence, absolute scattering cross section, Doppler velocity distributions, and the azimuthal variation of Doppler velocity for large aspect angle auroral backscatter. The average aspect sensitivity was found to be independent of aspect angle and to decrease with increasing backscatter power. Estimates of the largest absolute scattering cross sections were much greater than previous estimates at the same aspect angles, but these cross sections appear to be consistent with previous values once the long duration of the experiment is taken into account. Backscatter power was found to be essentially independent of flow angle.

The effect of multiple scattering on the aspect sensitivity and polarization of radio auroral echoes

A Monte Carlo model of radio wave scattering in the auroral electrojet has been developed to investigate multiple scattering of radio auroral echoes. Using this model, predictions of the aspect angle behavior of first-, second-, and third-order scattered power have been made. The results indicate that multiple scattering may be an important effect for VHF radars which observe the auroral E region at large magnetic aspect angles. The model shows that linearly polarized radio waves can become depolarized because of multiple scattering if the radio transmitter is horizontally polarized but not if the radio transmitter is vertically polarized. 52 refs.