Observations of unusually broadened HF radar spectra from heater-induced artificial plasma irregularities

H Vickers,T Robinson

doi:10.1029/2010ja015516

Abstract

[1] Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) HF backscatter targets may be artificially produced using the European Incoherent Scatter (EISCAT) HF ionospheric modification facility at Tromso, Norway. Plasma irregularities created in this way are known to be highly field aligned and usually give rise to radar echoes possessing spectral widths that are much lower than those seen in natural radar auroral data. In this study we present CUTLASS Finland and Iceland observations of heater-induced irregularities; these irregularities gave rise to enhanced spectral width in the Iceland radar backscatter signal, while the width observed by the Finland radar, pointing almost orthogonal to the Iceland beams, remained low. The observations coincided with a period of disturbed ionospheric conditions when faster-flowing natural irregularities were present, poleward of the heated volume, as well as a flow gradient across the heated volume itself. A latitudinal variation of the Doppler velocity measurements was exhibited in both the Finland and Iceland radar observations. We suggest that this flow inhomogeneity, taken together with the geometry of the observing radar beams, would explain not only the broad spectrum of flow components across the heated volume detected by the Iceland radar but also the asymmetry between the spectral widths measured at Iceland and Finland.

Highlights

[2] The generation of field‐aligned irregularities (FAI) in the high‐latitude ionosphere by means of artificial ionospheric modification often requires low geomagnetic activity in order to establish a strong interaction between the modifying HF wave and the F region plasma
The Cooperative UK Twin‐ Located Auroral Sounding System (CUTLASS) comprises a pair of bistatic HF radars at Hankasalmi, Finland (62.32°N, 26.61°E), and Pykkvibaer, Iceland (63.77°N, −20.54°E), that together form part of the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemisphere. It has been extensively exploited for the purpose of diagnosing artificial FAI, generated using the European Incoherent Scatter (EISCAT) HF ionospheric modification facility at Tromsø, Norway [Robinson et al, 1997, 1998; Yeoman et al, 1997; Wright et al, 2006]
HF radar spectral widths have, on occasion, been reported as being a good proxy for the open‐closed boundary (OCB) [Moen et al, 2000; Chisham et al, 2005], where reduced spectral width is observed equatorward of the OCB, and enhanced spectral widths are seen in regions close to the boundary because of small‐scale structuring of the VICKERS AND ROBINSON: UNUSUALLY BROADENED HF RADAR SPECTRA

Summary

Introduction

[2] The generation of field‐aligned irregularities (FAI) in the high‐latitude ionosphere by means of artificial ionospheric modification often requires low geomagnetic activity in order to establish a strong interaction between the modifying HF wave and the F region plasma. [4] Bistatic scatter measurements of HF diagnostic waves made during a previously reported experimental investigation at Tromsø revealed variations in the drift velocity of artificial irregularities across the heated volume [Blagoveshchenskaya et al, 2006] These variations were interpreted as being due either to two different modes of propagation generating irregularities that were horizontally separated or to a temperature gradient drift instability leading to grouping of irregularities at different altitudes. Barthes et al [1998] showed that spectra containing ionospheric sources and/or ground scatter can be separated out using the multiple signal classification (MUSIC) statistical numerical technique Further work in this area involving the preprocessing stage of the ACFs has resulted in significant reductions in the spectral width estimates made by the SuperDARN FITACF software and better agreement with values of diffusion coefficients for those conditions [Ponomarenko and Waters, 2006].

Experimental Procedure

Discussion

Summary