IN-SITU MEASUREMENT OF THE SCHMIDT NUMBER WITHIN A PMSE LAYER

During the SCALE campaign in July and August 1993, rocket borne in sit u measurements of neutral and electron small scale density fluctuation s were performed in the mesosphere over Andoya (69-degrees-N 16-degree s-E) at the same time as the EISCAT 224 MHz radar in Tromso observed s trong polar mesosphere summer echoes (PMSE). Strong neutral air turbul ence was observed in the height of the PMSE with a turbulent energy di ssipation rate of epsilon = 630 mW/kg. In the same altitude range, ele ctron density fluctuations were observed extending to scales smaller t han found in the neutrals. From a comparison of the neutral and electr on density fluctuation spectrum we obtained a Schmidt number Sc of 6.8 (Sc = nu/D ; nu = kinematic viscosity ; D = molecular diffusion coeff icient for the electrons). From the above numbers we deduced l0D&K = 5 .2m (l0D&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 (lambda/2 = 0.67m) is therefore well located in the viscous-diffusive subrange of the turbulence spec trum, despite the fact that Sc > 1, which shift l0D&K towards smaller scales. Our results indicate that the theoretical model of Driscoll an d Kennedy [1985], frequently used in this context, is not appropriate under PMSE conditions at scales significantly smaller than l0D&K.