Initial backscatter occurrence statistics from the CUTLASS HF radars

S E Milan,T K Yeoman,T B Jones,E C Thomas,M Lester

doi:10.1007/s005850050486

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> A statistical study of the occurrence of ground and ionospheric backscatter within the fields-of-view of the CUTLASS HF radars, at an operating frequency of 10 MHz, during the first 20 months of operation has been undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be highly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. In general, ionospheric backscatter is observed at far ranges during the local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more near-range E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on geomagnetic activity and radar operating frequency are also investigated. The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as well as geophysical processes. A brief assessment of the possible impact of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the `instrumental' aspect of backscatter occurrence, is essential for a full interpretation of HF coherent radar observations.

Highlights

The SuperDARN coherent HF radars (Greenwald et al, 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities as tracers of the bulk plasma motion under the inuence of the convection electric ®eld, and as a diagnostic tool for the investigationCorrespondence to: S
The advantage of HF radars over VHF radars, which were previously employed for convection studies (e.g. Greenwald et al, 1978; Nielsen et al, 1983a), is their ability to achieve the orthogonality condition with the magnetic ®eld in the F region as well as the E region due to the refractive nature of radio wave propagation in the 3 MHz to 30 MHz frequency band
Constant solar illumination during summer months and constant darkness in winter months lead to a large seasonal variation in the propagation environment

Summary

Introduction

The SuperDARN coherent HF radars (Greenwald et al, 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities (radar aurora) as tracers of the bulk plasma motion under the inuence of the convection electric ®eld, and as a diagnostic tool for the investigation. The Doppler velocity gives an estimate of the radar line-of-sight component of the plasma convection velocity (Nielsen and Schlegel, 1983; Villain et al, 1985; Ruohoniemi et al, 1987), and two radars with overlapping ®elds-of-view, such as CUTLASS, can create vector maps of convectionow across large regions of the high-latitude ionosphere (Nielsen et al, 1983b). For this reason SuperDARN radars are generally deployed in pairs, so as to give as wide a vector coverage of the auroral and polar cap region as possible. The Halley Bay radar has been employed to determine the seasonal dependence of the occurrence of polar cap patches (Rodger and Graham, 1996)

The CUTLASS radars and the statistics data set

Ground backscatter

Ionospheric backscatter

Findings

Conclusions