Temperature fluctuations near the mesopause inferred from meteor observations with the middle and upper atmosphere radar

Abstract

Using meteor echo measurements with the middle and upper atmosphere (MU) radar (35°N, 136°E), operated at 46.5 MHz, we examined time‐height variation of the ambipolar diffusion coefficient D , determined from the decay rate of meteor echoes. The height of a meteor trail was determined with an accuracy of about 1 km, by using an interferometer for determination of the arrival angles of meteor echoes. Profiles of D were obtained about every hour owing to the high rate of usable meteor echoes (3000–5000 per day). The mean profile of D showed an exponential increase with a scale height of 5.7–6.5 km in spring, summer, and autumn, which was generally consistent with the density scale height, while the scale height was as much as 8.1 km in November and December. Using the Boussinesq approximation, we found that the normalized D fluctuations D ′ can be related to those of temperature T ′ as D ′/ D 0 ∼ 3/2 · T ′/ T 0 . The observed D ′/ D 0 involved a wavelike structure with periods shorter than the inertial period, which often showed downward phase propagation. Fluctuations of D ′/ D 0 and the horizontal wind velocity showed phase relations consistent with a linear gravity wave theory. Furthermore, frequency spectra of D ′/ D 0 were generally consistent with those of the horizontal wind velocity for wave components with periods longer than about 8 hours, while for short‐period components the spectral amplitudes of D ′/ D 0 were considerably enhanced.