Geometric considerations of polar mesospheric summer echoes in tilted beams using coherent radar imaging

S Sommer,R Latteck,J L Chau,G Stober

doi:10.5194/ars-12-197-2014

Abstract

Abstract. We present observations of polar mesospheric summer echoes (PMSE) using the Middle Atmosphere Alomar Radar System in Northern Norway (69.30° N, 16.04° E). The radar is able to resolve PMSE at high spatial and temporal resolution and to perform pulse-to-pulse beam steering. In this experiment, 81 oblique beam directions were used with off-zenith angles up to 25°. For each beam pointing direction and range gate, coherent radar imaging was applied to determine the mean backscatter location. The location of the mean scatterer in the beam volume was calculated by the deviation from the nominal off-zenith angle of the brightest pixel. It shows that in tilted beams with an off-zenith angle greater than 5°, structures appear at the altitudinal edges of the PMSE layer. Our results indicate that the mean influence of the location of the maximum depends on the tilt of the beam and on the observed area of the PMSE layer. At the upper/lower edge of the PMSE layer, the mean backscatter has a greater/smaller off-zenith angle than the nominal off-zenith angle. This effect intensifies with greater off-zenith beam pointing direction, so the beam filling factor plays an important role in the observation of PMSE layers for oblique beams.

Highlights

Strong VHF backscatter at mesospheric heights in polar regions were described by Ecklund and Balsley (1981) for the first time
Our results indicate that the mean influence of the location of the maximum depends on the tilt of the beam and on the observed area of the polar mesospheric summer echoes (PMSE) layer
At the upper/lower edge of the PMSE layer, the mean backscatter has a greater/smaller off-zenith angle than the nominal offzenith angle. This effect intensifies with greater off-zenith beam pointing direction, so the beam filling factor plays an important role in the observation of PMSE layers for oblique beams

Summary

Introduction

Strong VHF backscatter at mesospheric heights in polar regions were described by Ecklund and Balsley (1981) for the first time As these echoes occur only during the summer months, they were named polar mesospheric summer echoes (PMSE). A review of the understanding of PMSE was done by Rapp and Lübken (2004), where the formation was explained by the presence of heavy charged ice particles. These ice particles lead to a significantly reduced electron diffusivity and explain the existence of PMSE in the presence of neutral air turbulence.

Experimental configurati3on

Conclusions