The relationship between VHF radar auroral backscatter amplitude and Doppler velocity: a statistical study

Abstract

Abstract. A statistical investigation of the relationship between VHF radar auroral backscatter intensity and Doppler velocity has been undertaken with data collected from 8 years operation of the Wick site of the Sweden And Britain Radar-auroral Experiment (SABRE). The results indicate three different regimes within the statistical data set; firstly, for Doppler velocities <200 m s–1, the backscatter intensity (measured in decibels) remains relatively constant. Secondly, a linear relationship is observed between the backscatter intensity (in decibels) and Doppler velocity for velocities between 200 m s–1 and 700 m s–1. At velocities greater than 700 m s–1 the backscatter intensity saturates at a maximum value as the Doppler velocity increases. There are three possible geophysical mechanisms for the saturation in the backscatter intensity at high phase speeds: a saturation in the irregularity turbulence level, a maximisation of the scattering volume, and a modification of the local ambient electron density. There is also a difference in the dependence of the backscatter intensity on Doppler velocity for the flow towards and away from the radar. The results for flow towards the radar exhibit a consistent relationship between backscatter intensity and measured velocities throughout the solar cycle. For flow away from the radar, however, the relationship between backscatter intensity and Doppler velocity varies during the solar cycle. The geometry of the SABRE system ensures that flow towards the radar is predominantly associated with the eastward electrojet, and flow away is associated with the westward electrojet. The difference in the backscatter intensity variation as a function of Doppler velocity is attributed to asymmetries between the eastward and westward electrojets and the geophysical parameters controlling the backscatter amplitude.

Simultaneous observations at different altitudes of ionospheric backscatter in the eastward electrojet

“…the backscatter volume size and the backscatter altitude, and the electron density and gradients in the electron density. However, such an increase in backscatter crosssection with increasing plasma drift velocity has previously been noted in VHF radar backscatter observations in the d velocity range 200 to 700 m s À1 (Shand et al, 1996, and references therein).…”

“…It must be noted that there is a portion of the ®eld of view ± beams 6 to 8 and between range gates 25 and 31 ± in which little ground backscatter is observed. This`null' in the ®eld of view, the cause of which is thought to be sea ice with a low backscatter cross-section (Shand et al, 1997), is apparent in both examples of Fig. 2.…”

Substorm associated radar auroral surges: a statistical study and possible generation model

“…A reduction in conductivity is equivalent to a reduction in the ambient electron density and, therefore, the scattering cross section presented to the radar which ultimately results in a reduction in the radar backscatter intensity (e.g. Haldoupis et al, 1990;Shand et al, 1996). Such a reduction is one of the characteristics of a SARAS.…”

“…Figure 2 of Freeman et al (1992) illustrates the diversity of the SARAS signature, which resulted in some major diculties in determining a set of selection criteria from which an algorithm could be coded. Many methods were investigated during the development of the algorithm, one of which led to the investigation of the variation of the backscatter intensity as a function of the Doppler velocity measured by the Wick radar (Shand et al, 1996).…”

“…The authors suggested the observation may be a result of a reduction in the local electron density, which would have the eect of reducing the scattering cross section presented to the radar. Shand et al (1996) investigated the statistical behavior of backscatter intensity as a function of line of sight (l.o.s) velocity for the entire SABRE data set. The intensity was found to decrease for l.o.s velocities above 700 m s A1 for¯ow towards the radar which is predominately associated with ionospheric convection observed within the eastward electrojet.…”