Observations of an enhanced convection channel in the cusp ionosphere

Abstract

Transient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high‐latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar‐orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line‐of‐sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.

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

“…The cusp and boundary layer, or more generally the dayside auroral zone, are believed to be distinguishable by high and varied radar backscatter spectral widths . The initial observation in the Southern Hemisphere (Pinnock et al, 1993) was of ā ow channel zonally aligned, with greater longitudinal extent (>900 km, 3 h of MLT) than latitudinal ( 100 km or 1°MLAT) and a poleward turn at the western extremity. Higher temporal and spatial observations of the cusp revealed transients with an average temporal spacing of 7 min and also occurring nearly simultaneously across 2 h of MLT .…”

“…These include pulsed auroral phenomena (Sandholt et al, 1985) and associated transient ows (Lockwood et al, 1989;Pinnock et al, 1993). The association with bursty reconnection has partly been made on the basis of their occurrence under southward IMF (B z < 0) conditions when equatorial magnetopause reconnection is predicted to occur (e.g.…”

“…HF radars have observed high velocity¯ow channels in the ionospheric F-region that have been assumed to be the response to transient reconnection (Pinnock et al 1993Rodger and Pinnock 1997). Their spatial extent,¯ow orientation, MLT occurrence, and dependence on IMF B y have been extensively examined (Provan et al 1998.…”

Signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above Casey, Antarctica

“…Of course, unlike satellite detectors, HF radars do not measure cusp activity in terms of changing particle energies and fluxes. Increased cusp activity has been understood to be a consequence of an increased flux transfer rate driven by patchy reconnection [e.g., Pinnock et al, 1993] and thus is favored under IMF B z southward conditions [Rijnbeek et al, 1984] Menk et al [1992] report the results of a detailed analysis of ground-based Pcl-2 pulsation, ionosonde, and riometer measurements made at Australian Antarctic bases and thereby provide further evidence for the proximity of the cusp to Casey during our austral summer campaigns.…”

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

“…The Halley radar commenced operation in January 1988, and the joint project was named the Polar Anglo-American Conjugate Experiment (PACE), providing simultaneous conjugate observations of ionospheric and magnetospheric phenomena. PACE was particularly successful in studies of the cusp, identifying signatures of the cusp in HF backscatter , illustrating the differences in transitions in the cusp-region convection pattern in the two hemispheres in response to changes in the direction of the y-component of the interplanetary magnetic field (IMF) , and identifying the first spatial signatures of the footprints of flux transfer events (FTE) in the ionospheric cusp (Pinnock et al , 1993. It was also used to study interhemispheric asymmetries in the location of the Harang discontinuity in the nightside ionosphere .…”