THE RELATIONSHIP BETWEEN VHF RADAR AURORAL BACKSCATTER AMPLITUDE AND DOPPLER VELOCITY - A STATISTICAL STUDY

A statistical investigation of the relationship between VHF radar auro ral backscatter intensity and Doppler velocity has been undertaken wit h data collected from 8 years operation of the Wick site of the Sweden And Britain Radar-auroral Experiment (SABRE). The results indicate th ree different regimes within the statistical data set; firstly, for Do ppler 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 Doppl er velocity for velocities between 200 m s(-1) and 700 m s(-1). At vel ocities greater than 700 m s(-1) the backscatter intensity saturates a t a maximum value as the Doppler velocity increases. There are three p ossible geophysical mechanisms for the saturation in the backscatter i ntensity at high phase speeds: a saturation in the irregularity turbul ence level, a maximisation of the scattering volume, and a modificatio n of the local ambient electron density. There is also a difference in the dependence of the backscatter intensity on Doppler velocity for t he flow towards and away from the radar. The results for flow towards the radar exhibit a consistent relationship between backscatter intens ity and measured velocities throughout the solar cycle. For flow away from the radar, however, the relationship between backscatter intensit y 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 w ith the westward electrojet. The difference in the backscatter intensi ty Variation as a function of Doppler velocity is attributed to asymme tries between the eastward and westward electrojets and the geophysica l parameters controlling the backscatter amplitude.