HF radar observations of high-aspect angle backscatter from the E-region

Abstract

Abstract. We present evidence for the observation of highaspect angle HF radar backscatter from the auroral electrojets, and describe the spectral characteristics of these echoes. Such backscatter is observed at very near ranges where ionospheric refraction is not sufficient to bring the sounding radio waves to orthogonality with the magnetic field; the frequency dependence of this propagation effect is investigated with the Stereo upgrade of the CUTLASS Iceland radar. We term the occurrence of such echoes the "high-aspect angle irregularity region" or HAIR. It is suggested that backscatter is observed at aspect angles as high as 30 • , with an aspect sensitivity as low as 1 dB deg −1 . These echoes are distinguished from normal electrojet backscatter by having low Doppler shifts with an azimuthal dependence that appears more consistent with the direction of the convection electric field E than with the expected electron E×B drift direction. This is discussed in terms of the linear theory dispersion relation for electrojet waves.

A first comparison of irregularity and ion drift velocity measurements in the E-region

“…8 and 9), an important prediction in the context of the present study. Milan et al (2004) argued that if the aspect angles are very large (>3 • ) the range widens even more, reaching small flow angles within the instability cone. Again, the prediction was that the l-o-s velocity sense can be opposite to that of the electron drift, which was supported by the observations with the CUTLASS Iceland radar at very short ranges (<400 km).…”

“…Of special interest for this study is the recent work by Drexler and St.-Maurice (2005) who proposed that what appears as large aspect angle echoes in the HF radar data can in fact be successfully interpreted using the nonlocal formulation of Drexler et al (2002). Furthermore, Drexler and St.-Maurice (2005) have derived a formula for the phase velocity (rather than simply adopting the one from the linear theory) that only contains the ion drift term, thus providing a theoretical basis for the interpretation used in Milan et al (2004). Even though the nonlocal theory of Drexler et al (2002) and Drexler and St.-Maurice (2005) strictly speaking applies only to small flow angles, it provides additional insights as to why irregularities at large flow and aspect angles are observed at all as well as why their Doppler velocities appear to be close to that of the ion motion in the E-region.…”

“…More observations casting doubt on the validity of the cosine law have been presented in a series of recent studies by and Uspensky et al (2003) at VHF and by Makarevitch et al (2002Makarevitch et al ( , 2004a and Milan et al (2004) at HF. It has been suggested that the observations could be explained if the ion motion contribution to the phase velocity of E-region irregularities is taken into account as prescribed by generalized formulas of the linear theory in the frame of reference of the neutrals.…”

“…Milan et al, 2004;Makarevitch et al, 2004a). One could argue that, depending on conditions (in particular the solar cycle period, season, and time of observations), the collision frequency can vary considerably and so can the ion drift velocity.…”

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

“…Other modes that have been run include a fixed Surv Geophys (2007) 28:33-109 91 frequency on one mode while sweeping the frequency from one scan to the next on the other channel (e.g. Milan et al 2004). In addition, it is possible to run two modes with different range resolutions, for example 15 and 45 km.…”