A height‐ and time‐dependent model of polar mesosphere summer echoes

Abstract

The generation of VHF radar echoes in the polar summer mesosphere is studied with a height‐ and time‐dependent model including three mechanisms: electron scavenging by microscopic ice particles created by heterogeneous nucleation on meteoric dust, formation of steep gradients in the ice particle and electron concentrations by the Kelvin effect, and reduction of the electron diffusivity by the presence of negatively charged particles. Then turbulent mixing produces a viscous convective subrange in the variance spectrum of the electron concentration, which in turn, can give rise to strong radar echoes. Based on a gravity‐wave‐perturbed model and many sensitivity tests, several conclusions are reached. Taking into account the high variability of relevant parameters, the model can predict the wide range of reflectivities measured on the northern hemisphere and indicates that the virtual absence of detectable radar echoes on the southern hemisphere is probably due to the annual variation of the meteoric mass influx and relatively high mesopause temperatures. The frequently occurring double‐layer structure of the reflectivity is characteristic of almost unperturbed rnesospheric conditions. In the presence of weak harmonic gravity waves, the inherently strong nonlinearity of the model yields steepened and breaking reflectivity structures. In agreement with observational results, the model further predicts a positive (negative) correlation between the vertical velocity variance and the reflectivity of spectrally broad (narrow) echoes. It is suggested that the frequently measured aspect sensitivity of spectrally narrow echoes results from anisotropic turbulence in the electron gas occurring if the inertial subrange is small or absent and the viscous‐convective subrange virtually adjoins the buoyancy subrange. Even under extremely favorable conditions, the calculated reflectivities at radar frequencies near 1 GHz are much weaker than measured ones, confirming an earlier result that in the UHF range, polar mesosphwre summer echoes originate from modified Thomson scattering.