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. Plain Language Summary We compare two different methods of using an ionospheric radar to measure the speed of ions moving through the high‐altitude ionosphere. One method measures over a shorter time and smaller space than the other. We find that although both methods show similar large‐scale patterns (100–1,000s of km) in the velocity at different times of day and for different levels of space weather driving, there can be large differences at small scales (10–100s of km). This can affect estimates of the heating of the atmosphere that are important for understanding satellite drag, for example. The difference in heating can be very large at certain times of day (nearly double). Key Points Two techniques using incoherent scatter radars to estimate the ion velocity and hence the ionospheric electric field have been compared Tristatic and monostatic radar methods capture the bulk motion of the ionosphere but small scale differences can reach hundreds of ms‐1 Calculations of Joule heating from the tristatic method are >40% gre