Takuya Sugiyama

Ion‐recombination nucleation and growth of ice particles in noctilucent clouds

Formation of ice particles in the polar summer mesosphere is simulated with the proton hydrate, H+(H2O)n, as the origin. In an ion-recombination nucleation scheme, electrons in the mesosphere play a role in removing heavy proton hydrates from the ionic system to the background neutral system, making an ionic system a steady source of neutral embryonic nuclei for condensation. Referring to the laboratory data of n ≤ 6, a model of reaction coefficients up to the hydration order of n = 1000 is constructed with the help of both the nucleation theory and the unimolecular reaction theory. For a model nucleation layer 600 m in thickness with a minimum temperature of 125 K, produced fluxes of embryonic nuclei are calculated as a strong function of [H2O], 10³ cm−2 s−1 at [H2O] = 2.16 parts per million by volume, for example. In the time dependent simulations, stable limit cycle oscillations of the cloud formation are found with periods of 3 to 4 days depending of the magnitude of diffusion. A cloud becomes bright after an active nucleation with a delay of ∼ 1.3 days which is a sedimentation time of embryonic nuclei. Deposition of water vapors in ice particles switches off the nucleation, and evaporation of particles at the bottom of a cloud recovers the nucleation activity with a lag of diffusion-advection time. Out of phase oscillations are predicted between the cloud brightness and the polar mesospheric summertime echo from the region of heavy proton hydrates, which will be a crucial evidence for the in situ origin of the clouds.

Formation of mesospheric VHF echoing layers due to a gravity wave motion

Abstract We present mesospheric backscattered VHF echo power and wind velocity data indicating the co-existence of a threefold strongly echoing layer and a wave motion, observed on 20 September 1985 with the MU radar at Shigaraki (34.9°N, 136.1°E), Japan. The echoing layers are clearly connected with the vertical and horizontal wind perturbations due to the wave. The analysis of the wind data have shown that the wave motion is due to an internal inertia-gravity wave with the vertical and horizontal wavelengths of 6 and 400 km, respectively, and period of 5.6 h. Evaluating the atmospheric stability in the wave field with the estimated wave parameters, the echoing layers are shown to be consistent with statically stable regions generated by the wave. It is suggested from our results that Fresnel scattering is a dominant echoing mechanism for a VHF radar beam in the mesosphere, as well as in the lower stratosphere.

Oscillations in polar mesospheric summer echoes and bifurcation of noctilucent cloud formation

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.

Seasonal and interannual variability of mesospheric echoes observed with the Middle and Upper Atmosphere Radar during 1986–1995

To investigate long-term variabilities of radar echo intensities from the mesosphere, we have analyzed a 10-year data set obtained with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (34°51′N, 136°6′E) during the period from 1986 to 1995. Two types of echoes have been identified: The echoes are primarily due to isotropic scattering from 70 to 85 km in altitude in summer, whereas they are mainly due to specular scattering from 60 to 70 km in other seasons. Clear annual variabilities have been observed in both occurrence frequency and intensity of the echoes. The upper end of summertime mesospheric echoes seems to be located just below the summer mesopause. An interannual variability of summertime mesopause height has also been suggested.

Synthesis of (La,Sr)MeO3 (Me=Cr, Mn, Fe, Co) solid solutions from aqueous solutions

Abstract Methods for forming films from aqueous solutions at ordinary temperature and pressure are advantageous because no vacuum, no high temperature and no expensive apparatus will be required and substrates even with wide areas and/or complicated shapes are available. (La,Sr)MeO 3 (Me=Cr, Mn, Fe, Co) solid solutions are very important as practical materials for solid oxide fuel cells and high-temperature steam electrolyzers. (La,Sr)MeO 3 solid solutions prepared by usual solid state reaction were dissolved in hydrofluoric acid and the solutions of fluorocomplexes were obtained. Boric acid was added to the solution, the fluoride ions were consumed by the formation of BF 4− , and then the fluorocomplexes were hydrolyzed to (La,Sr)MeO 3 solid solutions in order to increase the amount of fluoride ions. The high crystallinity of the solid solutions were confirmed by sharp and strong X-ray diffraction peaks. A number of synthesized particles of (La,Sr)MeO 3 solid solutions were observed on the substrates in scanning electron microscope images.

Features of a mesospheric inertio-gravity wave observed with the MU radar

Abstract We present an analytical result on an outstanding gravity wave motion observed with the MU radar at Shigaraki (34.9°N, 136.1°E), Japan, on 5 September 1988. In the analysis the three components of wind velocity, estimated from the Doppler shift of the radar echo backscattered in the mesosphere, are used. The velocity data set shows that a long-period wave motion is dominant only in the horizontal component while short-period wave motions are commonly seen in the vertical and horizontal components. The large amplitude ratio between the horizontal and vertical components of the long-period wave motion seems to be due to the propagation property of an inertio-gravity wave. Our data also show that the wave amplitude is saturated at altitudes higher than 75 km while it is kept anomalously large (~50 m s −1 ). The wave parameters estimated from our hodograph analysis lead to the conclusion that the behaviour of the wave is not inconsistent with the linear saturation theory for monochromatic inertio-gravity waves.

MU radar observation of a mesospheric gravity wave breaking caused by convective instability

Abstract In recent mesospheric observations with the MU radar at Shigaraki (34.9°N, 136.1°E), Japan, we obtained a couple of wind velocity data showing that a monochromatic inertia-gravity wave was propagating vertically in the mesosphere. The data shows that the velocity amplitude of the wave did not increase exponentially above a height and further shows that the wave-form had broken down at a level. Evaluating the changes of the atmospheric stability in the wave field from the measured wind data and the estimated wave parameters, we show that the breakdown of the wave can be connected with the occurrence of convective instability in the wave field.

Saturation of frequency spectra for mesospheric wind velocity observed with the middle and upper atmosphere radar

A spectral analysis was made of the radial (line of sight) wind velocity observed in the mesosphere by the middle and upper atmosphere radar at Shigaraki (34.9°N, 136.1°E), Japan, during the daytime of September 5 and October 6, 1988. The radial velocity was measured every 1 min at 300-m range resolution along the three beam directions of 0°, 10°, and 20° from the zenith. In this paper we first give an overview of the analytical results, in particular, on the height dependence of frequency spectra obtained in a wide range of altitudes in the mesosphere. The frequency spectrum of the vertical velocity shows an almost flat response at frequencies less than the Brunt-Vaisala frequency, and the spectral shape hardly changes over the entire height range of the analysis. On the other hand, the frequency spectra of the oblique velocities considerably change their shape and magnitude with height. Although the oblique spectra show a rather flat response at low frequencies when they are taken at the lowest altitudes in this analysis, the overall shape asymptotically approaches a −2 power law with increasing height. It is further shown that the height dependence of the oblique spectra is closely associated with the vertical propagation of an inertio-gravity wave with a period of ∼12 hours. In addition, the overall f−2 dependence observed at around 76 km indicates a saturation of the oblique spectra as the upper limit. Finally, it is suggested that the frequency spectra for the vertical and oblique velocities can be well interpreted in terms of gravity wave theory.

Pre‐sunrise mesospheric echoes and turbulent wind structure observed with the MU radar

Summer mesospheric observations using the MU radar at Shigaraki, Japan show that radar echoes could be detected before sunrise under some circumstances. The pre-sunrise radar echoes observed at heights above 80 km are weak but clearly occur in a layer. Strong radar echoes observed after sunrise at similar heights show an unusual broadening of the spectral width in the echo power. This indicates the presence of strong atmospheric turbulence. In addition, frequency spectra for the vertical wind fluctuations show an enhancement of the spectral power density in a wide range of periods shorter than the local Brunt-Vaisala one. This is consistent with the occurrence of strong turbulence layer in the upper mesosphere though not immediately identified with the turbulence activity. Thus the presence of strong turbulence layer might yield us a possibility of detecting radar echoes before sunrise.