Oscillations having periods of 5.5 days are found in polar mesospheric summer echoes (PMSEs) observed during 1980 to 1984 with the MST radar at Poker Flat, Alaska. Simulations of noctilucent cloud (NLC) formation are made in the case that embryos for cloud ice particles are proton‐hydrates H+(H2O)n (PHs). Results show that cloud formation bifurcates from a stably steady structure into a quasi‐steady and a stably oscillatory structure in accordance with an increase of humidity. By capturing electrons, descending crowds of ice particles may cause PMSEs. The cyclic behavior of PMSEs is considered to be an indication of the in situ origin of NLCs.
Spectral analysis was made for mesospheric radial wind velocities estimated from the measurement of Doppler‐shifted echo power by the MU radar at Shigaraki (34.9°N, 136.1°E), Japan. In this paper we give an overview of the data and present the frequency spectra of the radial winds at altitudes of 74 to 77 km. Under quiet wind conditions, the frequency spectra for the vertical winds show an almost flat response in between the inertial and Brunt‐Väisälä (fB) frequencies with the peak at a frequency a little lower than fB. The frequency spectra for the other oblique winds have an approximate f−2 spectral slope with a weak peak at the same frequency as the vertical one. These features are consistent with the results given by a theoretical spectral model of gravity waves and suggest that the observed wind fluctuations are caused by gravity waves.
We present mesospheric wind data from very helpful for studying large-and small-scale scattering echo layer observed simultaneously was closely connected with the vertical propagation of the gravity waves.
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