Turbulence scattering layers in the middle‐mesosphere observed by the EISCAT 224‐MHz radar

Abstract

Spectral measurements of signals scattered from altitudes well below 70 km have for the first time been obtained with the EISCAT 224‐MHz radar. Observations with the antenna pointing away from the vertical at times show spectral power distributions departing markedly from those expected from collision‐dominated incoherent scatter theory. Separated peaks in the spectra occur in regions of abrupt shear in the velocity of the neutral wind within the scattering volume. Anomalously large backscattered power levels are sometimes observed in the vicinity of such shears at altitudes near 68 km. It is unlikely that these echoes are due to incoherent (Thomson) scatter. Instead, it is proposed that the abrupt velocity shears develop neutral turbulence in narrow layers and that these layers cause the enhanced backscatter. Calculations of the expected backscatter power levels at 224 MHz for incoherent scatter and for turbulence scatter show that the turbulence component can be the dominant one at heights below about 68 km if the potential refractive index gradient and the turbulence energy dissipation rates are large enough.