Comparison of F-Region Ion Velocities Measured by Swarm Satellites and EISCAT Radars

Ionospheric ion flow velocities measured by the Swarm satellites are compared with the ion velocities estimated from the European Incoherent Scatter (EISCAT) radar measurements in Tromsø and on Svalbard. A comparison is carried out between the cross-track horizontal ion velocity component given by the Swarm Electric Field Instrument and the corresponding component by the EISCAT radars. This paper describes the comparison procedure between the two very different measurement methods and discusses the challenges in the comparison. Several events are found with eastward or westward ion flow channels that exceed 1,000 m/s. The example events shown occur between the Region 1 and 2 current sheets in the afternoon and post-midnight sectors, and one event in the vicinity of the dayside cusp. However, since the flow channels are relatively narrow and short-lived, it is difficult to capture the ion flow channel by ground-based radar measurements. A Linear fit for the selected conjunction events shows that on average, the Swarm ion velocities are larger than EISCAT ion velocities by a factor of $1.34\pm 0.07$. The main reason for the smaller ion velocity estimates by EISCAT compared to Swarm is likely the coarser spatial and temporal resolution of the radar experiment, which prevents measurement of the narrow ionospheric flow channels. Ion composition at Swarm altitudes may also play a minor role by affecting the standard Swarm analysis velocity values.