Small‐scale vortices in the high‐latitude F region

Abstract

The dayside outer magnetosphere is a region of direct interaction with the shocked solar wind in the magnetosheath, resulting in plasma and hydromagnetic waves and spatial turbulence which map along the Earth's magnetic field lines down to the ionosphere where the waves and/or turbulence can be detected directly by radar scattering from the associated electron density variations. In a previous paper by Schiffler et al. [1997] it was reported that double‐peaked (D‐P) velocity spectra measured by the Super Dual Auroral Radar Network (SuperDARN) HF radars are a signature of backscatter from the ionospheric footprint of the outer low‐latitude boundary layer (LLBL). In the present statistical study, Saskatoon HF radar data are used to determine the ionospheric location at which D‐P HF spectra originate and to confirm the previous result that the majority of the D‐P spectra are associated with the ionospheric cusp/cleft region. Employing high spatial (15 km) and high temporal (3 s) resolution modes, we establish an upper limit of ∼ 26 km on the scale size and ∼4 s on the lifetime of the structures producing the echoes showing D‐P spectra. We postulate a number of mechanisms that could cause D‐P spectra and conclude that ionospheric vortices appear to be the most likely explanation of the radar observations.