In‐situ measurement of the Schmidt number within a PMSE layer

Abstract

During the SCALE campaign in July and August 1993, rocket borne in situ measurements of neutral and electron small scale density fluctuations were performed in the mesosphere over Andøya (69° N16°E) at the same time as the EISCAT 224 MHz radar in Tromsø observed strong polar mesosphere summer echoes (PMSE). Strong neutral air turbulence was observed in the height of the PMSE with a turbulent energy dissipation rate of ϵ = 630mW/kg. In the same altitude range, electron density fluctuations were observed extending to scales smaller than found in the neutrals. From a comparison of the neutral and electron density fluctuation spectrum we obtained a Schmidt number Sc of 6.8 (Sc = ν/D ; ν = kinematic viscosity ; D = molecular diffusion coefficient for the electrons). From the above numbers we deduced ℓoD&K = 5.2m (ℓoD&K is the ‘break‐off’ scale between the viscous‐convective and the viscous‐diffusive subrange of the turbulence spectrum). The half wavelength of the EISCAT 224 MHz radar (λ/2 = 0.67m) is therefore well located in the viscous‐diffusive subrange of the turbulence spectrum, despite the fact that Sc > 1, which shifts ℓoD&K towards smaller scales. Our results indicate that the theoretical model of Driscoll and Kennedy [1985], frequently used in this context, is not appropriate under PMSE conditions at scales significantly smaller than ℓoD&K.