Two techniques for determining F-region ion velocities at meso-scales: Differences and impacts on Joule heating

We have investigated the difference between two standard techniques for deriving the ionospheric ion velocity using data taken with the EISCAT incoherent scatter radar between 1987 and 2007. For large-scale convection flows, there is little difference between the tristatic and monostatic techniques, though the biggest relative difference occurs during periods when the interplanetary magnetic field (IMF) is strongly northward. At small scales the difference between the two techniques is correlated with a measure of the variability of the tristatic measurement. This suggests that small-scale flow bursts, such as those associated with enhanced auroral arcs, could explain the local time variation in the velocity difference distributions. The difference in velocities obtained from the monostatic and tristatic techniques can make a significant difference in the estimate of the magnitude of Joule heating in the thermosphere. Considering only the electric field dominated component of Joule heating, Q, the difference in the two techniques can be as much as 52% of the tristatic measurement (Qm = 0.48Qt) in the morning sector (0 – 6 MLT), during a moderate to large geomagnetic storm. This reduces to a difference of 36% at non-storm times in the same MLT period. Careful averaging of the velocity field with the future EISCAT_3D radar system will allow us to establish the impact of both spatial and temporal scales on the magnitude of the observations.