Evaluation of mesospheric VHF echoes observed with the middle and upper atmosphere radar



[1] Mesospheric radar echoes in midlatitudes, which are generated by refractive index irregularities at half the radar wavelength, are considered to be caused by electron density irregularities induced by neutral turbulence. In this paper we construct a model to evaluate intensities of VHF mesospheric echoes due to electron irregularities with the help of an empirical model profile of mean electron densities and temperatures. We simulate the electron density deviations in a parcel displaced by neutral turbulence, the intensity of which is evaluated from the Doppler spectral width measured with the mesosphere-stratosphere-troposphere radar. Our results show that the estimated reflectivity increases up to the height range of 70–80 km mainly because of the increase of the electron density gradients and decreases above 80 km because of the suppression of turbulence at half the radar wavelength by increasing molecular viscosity. Here 70–80 km is the critical height for the mesospheric echoes where the inner scale of inertial turbulence is equal to half the radar wavelength. Our model reproduces well the characteristics of observed echo intensities. Particularly mesospheric echoes are most frequently found around and just below the critical height, which is much higher than those estimated in the earlier studies.