Small-scale vortices in the high-latitude F region

The dayside outer magnetosphere is a region of direct interaction with the shocked solar wind in the magnetosheath, resulting in plasma and hydromagne tic 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 dete cted directly by radar scattering from the associated electron density vari ations, In a previous paper by Schiffer et al. [1997] it was reported that double-peaked (D-P) velocity spectra measured by the Super Dual Auroral Rad ar Network (SuperDARN) HF radars are a signature of backscatter from the io nospheric footprint of the outer low-latitude boundary layer (LLBL), In the present statistical study, Saskatoon HF radar data are used to determine t he ionospheric location at which D-P HF spectra originate and to confirm th e 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 similar to 26 km on the scale size and similar to 4 s On the lifetime of the structure s producing the echoes showing D-P spectra. We postulate a number of mechan isms that could cause D-P spectra and conclude that ionospheric vortices ap pear to be the most likely explanation of the radar observations.