Mapping high‐latitude plasma convection with coherent HF radars

Abstract

In this decade, a new technique for the study of ionospheric electrodynamics has been implemented in an evolving generation of high‐latitude HF radars. Coherent backscatter from electron density irregularities at F region altitudes is utilized to observe convective plasma motion. The electronic beam forming and scanning capabilities of the radars afford an excellent combination of spatial (∼50 km) and temporal (∼1 min) resolution of the large‐scale (∼10 6 km²) convection pattern. In this paper, we outline the methods developed to synthesize the HF radar data into two‐dimensional maps of convection velocity. Although any single radar can directly measure only the line‐of‐sight, or radial, component of the plasma motion, the convection pattern is sometimes so uniform and stable that scanning in azimuth serves to determine the transverse component as well. Under more variable conditions, data from a second radar are necessary to unambiguously resolve velocity vectors. In either case, a limited region of vector solution can be expanded into contiguous areas of single‐radar radial velocity data by noting that the convection must everywhere be divergence‐free, i.e., ▽ · v = 0. It is thus often possible to map velocity vectors without extensive second‐radar coverage. We present several examples of two‐dimensional velocity maps. These show instances of L shell‐aligned flow in the dusk sector, the reversal of convection near magnetic midnight, and counterstreaming in the dayside cleft. We include a study of merged coherent and incoherent radar data that illustrates the applicability of these methods to other ionospheric radar systems.