A high-resolution study of mesospheric fine structure with the Jicamarca MST radar

Abstract

Abstract. Correlation studies performed on data from recent mesospheric experiments conducted with the 50-MHz Jicamarca radar in May 2003 and July 2004 are reported. The study is based on signals detected from a combination of vertical and off-vertical beams. The nominal height resolution was 150 m and spectral estimates were obtained after ∼1 min integration. Spectral widths and backscattered power generally show positive correlations at upper mesospheric heights in agreement with earlier findings (e.g., Fukao et al., 1980) that upper mesospheric echoes are dominated by isotropic Bragg scatter. In many instances in the upper mesosphere, a weakening of positive correlation away from layer centers (towards top and bottom boundaries) was observed with the aid of improved height resolution. This finding supports the idea that layer edges are dominated by anisotropic turbulence. The data also suggests that negative correlations observed at lower mesospheric heights are caused by scattering from anisotropic structures rather than reflections from sharp vertical gradients in electron density.

Radar cross sections for mesospheric echoes at Jicamarca

“…We show spectral width only for echoes above −10 dB SNR and for widths greater than 1 m/s, since values for very weak echoes are less reliable, and beam broadening effects contribute 0.1 to 0.7 m/s to the spectral width (Guo et al, 2007). Wide spectra with positive power-width correlation are interpreted as caused by turbulent fluctuations (e.g., Hocking, 1985;Sheth et al, 2006) and have been used to derive turbulent energy dissipation rates for Jicamarca (Guo et al, 2007). Median turbulent energy dissipation rates estimated from spectral widths are around 10 mW/kg, but can vary between 1 and 100 mW/kg, with values generally increasing with altitude throughout the mesosphere.…”

“…Spectral fitting allows us to estimate very small signals with spectral widths down to about 1 m/s. (Sheth et al, 2006;Guo et al, 2007). Figure 1 shows a fairly broad spectrum fitted by a generalized Gaussian to the logarithm of the power spectrum.…”

“…For our model, we assume propagation through a stratified ionosphere, which is divided into 100 horizontal slabs from 75 to 870 km. The antenna patterns for each of the four beams contain side lobes, which partially overlap each other (Sheth et al, 2006), and must be carefully modeled. In particular, receiving beams may contain different polarizations stemming from different transmitted beam patterns.…”

“…For the very strong summer echoes, it is the extension and enhancement of the turbulent spectrum for electron fluctuations caused by reduced diffusivity and described by a large Schmidt number (Kelley et al, 1987). For aspect sensitivity, it is the presence of anisotropic scatterers, such as filaments or corrugated sheets of quasi-two-dimensional irregularities, formed in narrow layers by instabilities, that may cause partial or diffuse reflection or other deviations from isotropic scattering (Sheth et al, 2006).…”

“…Coherent and incoherent spectra were added for each minute and 150 m range resolution. The smooth line is a generalized Gaussian fitted to the logarithm of the power spectrum (Sheth et al, 2006).…”