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 (∼106 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.

Excitation of transient lobe cell convection and auroral arc at the cusp poleward boundary during a transition of the interplanetary magnetic field from south to north

“…5. Ion drift vectors have been superposed using a beam-swinging algorithm given by Villain et al (1987) and Ruohoniemi et al (1989). The spatial plot of line-of-sight velocities are reproduced at higher resolution in Fig.…”

A survey of magnetopause FTEs and associated flow bursts in the polar ionosphere

“…This ability confers a geophysical advantage on the system in allowing it to image large-scale ionospheric ow patterns (e.g. Ruohoniemi et al, 1989). Drawbacks are the multi-hop nature of HF propagation so some regions have ground scatter rather than ionospheric scatter, absorption e ects in the D region ionosphere, and lack of irregularities for the HF signals to scatter from.…”

“…velocities, using a beam swinging algorithm based on Villain et al (1987) and Ruohoniemi et al (1989). The algorithm chooses a number of L-shells which cut through the ®eld-of-view of the radar and assumes the¯ow along these L-shells is constant.…”

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

“…Figure 8a shows Polar UVI auroral intensity data for scans beginning 2253:53 and 2303:05 UT. A number of estimates of the horizontal ow vector, calculated using a beam-swinging algorithm where the¯ow component parallel to the L-shell is assumed constant across the scan (Villain et al, 1987;Ruohoniemi et al, 1989) are overlaid. These velocity vectors are derived from beam-swinging constrained by measurements from both the ³ykkvibñr and Hankasalmi radars (the line-of-sight data from which is shown in Fig.…”