Abstract
Abstract. Observations of HF radar backscatter from the auroral electrojet E region indicate the presence of five major spectral populations, as opposed to the two predominant spectral populations, types I and II, observed in the VHF regime. The Doppler shift, spectral width, backscatter power, and flow angle dependencies of these five populations are investigated and described. Two of these populations are identified with type I and type II spectral classes, and hence, are thought to be generated by the two-stream and gradient drift instabilities, respectively. The remaining three populations occur over a range of velocities which can greatly exceed the ion acoustic speed, the usual limiting velocity in VHF radar observations of the E region. The generation of these spectral populations is discussed in terms of electron density gradients in the electrojet region and recent non-linear theories of E region irregularity generation.Key words. Ionosphere (ionospheric irregularities)
Highlights
At F region altitudes, both ions and electrons are said to be magnetised; their gyrofrequencies exceed their collision frequencies, νe < e and νi < i, and so both move with a velocity E × B/B2, where B is the Earth’s magnetic field and E is the convection electric field imposed by the magnetosphere
Lester: Spectral populations in HF radar backscatter from the E region auroral electrojets allow E region instability mechanisms to be studied in a new wavelength regime where, it will be demonstrated, a greater variety of spectral types are found and Doppler shifts near 1000 m s−1 are regularly observed
The present paper aims to provide an overall description of the spectral characteristics of backscatter originating in the auroral electrojet regions, predominantly from E region altitudes, as observed by the CUTLASS Iceland coherent HF radar
Summary
At F region altitudes, both ions and electrons are said to be magnetised; their gyrofrequencies exceed their collision frequencies, νe < e and νi < i, and so both move with a velocity E × B/B2, where B is the Earth’s magnetic field and E is the convection electric field imposed by the magnetosphere. Plasma instability mechanisms operating in the F region, primarily the gradient drift instability (Maeda et al, 1963; Knox, 1964), generate field-aligned ionospheric plasma density perturbations or irregularities with a variety of scale-lengths from submetre to kilometres As these irregularities are thought to drift with the bulk plasma motion, the Doppler shift imposed on coherent backscatter (radar aurora) from the F region gives an estimate of the line-of-sight component of the convection speed. Lester: Spectral populations in HF radar backscatter from the E region auroral electrojets allow E region instability mechanisms to be studied in a new wavelength regime where, it will be demonstrated, a greater variety of spectral types are found and Doppler shifts near 1000 m s−1 are regularly observed. This paper presents a context within which future studies can discuss the instability mechanisms and propagation geometry responsible for E region backscatter generation
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