Studies of polar mesosphere summer echoes over EISCAT using calibrated signal strengths and statistical parameters

Abstract

The EISCAT (European incoherent scatter) monostatic VHF radar operating on 224 MHz has been used to investigate characteristics of polar mesosphere summer echoes (PMSE) at Bragg scales of around 0.6 m. In particular, we look at four main parameters, two of which have not been studied in any real detail for these echoes. These parameters are absolute backscatter strengths, fading times, amplitude distributions, and turbulent energy dissipation rates. The calibrated signal strengths, combined with theoretical modeling, are used to demonstrate that specular reflection from even the sharpest background density gradients which have been measured thus far by rocket probes cannot be the cause of these echoes at VHF frequencies. We also demonstrate that specular reflections from the edges of the large electron density "bite-outs" (which are often observed in association with these echoes) cannot be responsible for the measured backscatter strengths. However, these same gradients can, in association with turbulence, cause measurable backscatter, but only provided that scatter is from within the inertial subrange of turbulent electron density fluctuation spectra. This requires large Schmidt numbers. We also use statistical studies of the amplitude distributions to show that our echoes were due to an ensemble of scatterers in the radar volume, rather than single entities. A new calculation concerning the expected lifetimes of the scattering entities in the presence of turbulence is also demonstrated, and then our measurements are used to show that the scatterers often have lifetimes in excess of those which are predicted for classical neutral turbulence. We use this latter point to supply additional evidence that the Schmidt number substantially exceeds 1.0.