Kyoto Meteor Radar Research Articles

Characteristics of Mean Winds and Semidiurnal Tides in the Lower Thermosphere Observed with the Kyoto Meteor Radar and the Irkutsk Low Frequency Spaced-Receiver Facility.

Observations of wind fields in the upper mesosphere and lower thermosphere (85-95 km) at Irkutsk, East Siberia (52°N, 104°E), and Shigaraki, near Kyoto, Japan (35°N, 136°E), in 1983-1985 were used to study the behavior of mean winds and solar semidiurnal tides. The meridional mean winds were mostly southward at both sites, but the wind amplitudes in Japan were weaker, except in winter 1983/1984, and autumn 1984 and 1985. The zonal mean winds were generally eastward at both sites, but there were distinct differences in annual and interannual variations. The variations of the amplitudes of semidiurnal tides agreed rather well between the two sites, especially for the zonal component. For the meridional component, the amplitudes of semidiurnal tides were systematically greater in East Siberia. The phases of the zonal component agreed fairly well in summer between the two sites, while they were significantly different in winter months. The results of the comparison could be explained by the latitudinal distributions of thermospheric winds, and partly by possible longitudinal variations in the excitation, propagation and dissipation of atmospheric waves due to climatic differences.

Characteristics of semidiurnal tides observed by the Kyoto meteor radar and Saskatoon medium‐frequency radar

We have compared seasonal variations of semidiurnal tides in the upper middle atmosphere observed by the Kyoto meteor radar and the Saskatoon medium‐frequency (partial reflection) radar in 1983–1985. Amplitudes of semidiurnal tides are generally larger at Saskatoon than at Kyoto. Seasonal variations of phases at 90 km agree fairly well for both zonal and meridional components between Kyoto and Saskatoon. The seasonal variation is mainly composed of two fundamental states appearing in summer‐centered (April/May–September/October) and winter‐centered months (November–February/March), which are characterized by long and relatively short vertical wavelengths, respectively. Clear transitions between the two states occur in equinoxes (February–April and October–November). Using determinations at Kyoto and Saskatoon with 4‐ to 5‐day observation intervals, we have studied the behavior of semidiurnal tides during the transitions in October–November in 1983–1985 and February–April in 1985. Precise comparisons have shown that the transitions are slightly delayed at Saskatoon in both vernal and autumnal equinoxes. Abrupt changes in phases of semidiurnal tides coincide with reduction and pulsating fluctuations of their amplitudes, suggesting the existence of interference between fundamental and higher modes which are dominant in summerlike and winterlike months, respectively.

Long period wind oscillations observed by the Kyoto meteor radar and comparison of the quasi-2-day wave with Adelaide HF radar observations

We have detected wind oscillations with periods ranging from 1.4 to 20 days at 80–110 km altitude using Kyoto meteor radar observations made in 1983–1985. Among these oscillations, the quasi-2-day wave is repeatedly enhanced in summer and autumn. We found that the period of the quasi-2-day wave ranges from 52 to 55 h in summer, and becomes as short as 46 to 48 h in autumn in 1983 and 1984. The change in the wave period seems to coincide with a decrease in the amplitude of the zonal mean wind. A quasi-2-day wave event was simultaneously observed in January 1984 at Kyoto (35° N, 136°E) and Adelaide (35° S, 138° E), which are located at conjugate points relative to the geographic equator. Amplitudes of the meridional component at Adelaide are approximately four times larger than those observed at Kyoto. Comparison observations clearly show that the meridional component is in phase and the zonal component is out of phase, respectively, implying antisymmetry of the quasi-2-day wave between the northern and southern hemispheres. Relative phase progressions with height are similar between the Kyoto and Adelaide results for both meridional and zonal components, and indicate the presence of an upward energy propagating wave with a vertical wavelength of about 100 km.

Mean winds observed by the Kyoto meteor radar in 1983–1985

Mean winds at 82–106 km altitude have been almost continuously monitored by the Kyoto meteor radar over the period from May 1983 to December 1985. The mean zonal wind becomes eastward with amplitudes as large as 30 m s −1 in the summer months (May–August), maximizing early in July at 95 km altitude, while it is less than 10 m s −1 at all the observed altitudes during the equinoxes. It is normally eastward in winter at low altitudes, although it sometimes becomes westward during sudden stratospheric warmings. The mean meridional wind is usually equatorward and is weaker than the zonal component. A southward wind exceeding 10 m s −1 is detected in July and August. The observed mean winds are compared with the CIRA 1972 model and coincidences with sudden warmings of changes in zonal wind direction are pointed out.

Long period wind oscillations in the 80–110 km altitudes observed by the Kyoto meteor radar in 1983–1985

During the MAP period, the Kyoto meteor radar has almost continuously monitored the wind fields in the 80–110 km altitudes. Wind oscillations with various periods ranging from 2 to 20 days are detected. The period of the quasi 2-day wave in 1983 was 2.2 days in summer months but became as short as about 2.0 days in autumn. Antisymmetry in phase profiles is detected by comparing the behavior of the quasi 2-day wave simultaneously observed at Adelaide and Kyoto.

Gravity waves observed by the Kyoto meteor radar in 1983–1985

We have studied characteristics of gravity waves with periods longer than 2 h observed by the Kyoto meteor radar (35°N, 136°E) in the period from May 1983 to April 1985. The annual mean power spectral densities of the eastward wind velocity in 1983 and 1984 agree well with each other. Linearly fitted curves of the spectral density can be described as 1.87 ƒ −1.29 (m 2 s −2 Hz −1) in 1983 and 2.11 ƒ −1.27 in 1984, respectively, where ƒ is frequency. By dividing the observation range of the Kyoto meteor radar into sub-areas according to the horizontal distance from the radar, the apparent eastward phase velocity of a monochromatic gravity wave has been determined. Typical values of apparent zonal phase velocity and wave period are 25 m s −1 and 4 h, respectively. Gravity waves with westward phase velocities have been more frequently detected than those with eastward ones in the whole observation period; they especially dominate in November and December in both 1983 and 1984.

Comparison observations between the MU radar and the Kyoto meteor radar

We have performed a comparison between the MU radar and the Kyoto meteor radar on August 14–15, 1984. The MU radar provided vertical, zonal and meridional wind components in the 65‐ to 85‐km region during daylight hours when echoes from the turbulent scattering were intense. Meteor echoes were also received by the MU radar, and utilized to measure winds at meteor heights. Comparisons among these MU radar measurements and Kyoto meteor radar observations have shown fairly consistent wind profiles at 65‐ to 105‐km altitudes. To interpret differences in the wind fields observed in 2 successive days, it was necessary to consider the behavior of quasi 2‐day oscillation deduced from long‐duration Kyoto meteor radar observations.

Long period oscillations in the meteor winds observed over Kyoto during 1978-1983.

We have performed a harmonic analysis on the Kyoto meteor radar (35°N, 136°E) data taken in 1978-1983, and examined long period oscillations at heights of 80-110km. In summer, an intense oscillation with almost constant phase with height has been detected at a period of around two days, although its amplitude varied in time comparable to a few periods of the wave. Other components with periods of several days have also been detected. We have found that the phases of these long period waves progress upward and their vertical wavelengths are fairly long. Their amplitude maxima took place in between midsummer and autumn, and the longer the period is, the later the maximum occurs.

Seasonal variation of solar atmospheric tides at meteor heights.

Systematic observation of the Kyoto meteor radar (35°N, 136°E) has been continuing since the end of 1977. A seasonal variation of diurnal and semidiurnal solar atmospheric tides at meteor heights has been studied by using meteor radar data. Monthly average behaviours of these tidal components have been summarized for the period from March 1979 to November 1980. Vertical profiles and harmonic dials of these components have been investigated in comparison with other meteor radar data. Complicated behaviour of diurnal tide can be explained by an interference between propagating and evanescent modes. The evanescent mode becomes dominant in summer, while the propagating mode appears in winter. As for semidiurnal tide, the S2, 2 mode is fundamental in summer, and the S2, 4 mode becomes apparent in winter.

Lunar tides at meteor heights

We determined lunar semidiurnal (M2) and diurnal luni‐solar (O1) tides of the meridional winds observed for two years between 1979 and 1980 by the Kyoto meteor radar. At 95 km altitude, the M2 and O1 tidal wind of 2.2 m/sec and 1.2 m/sec were observed. From the downward phase progression with altitude, the vertical wavelength was estimated as 60 and 25 km for the M2 and O1 component, respectively. Seasonal variation of the M2 tidal wind was detected. Lunar variation has also been detected in the mean height of meteor echoes. The amplitude for the M2 and O1 component was 0.22 and 0.18 km, with the maximum height occurring at 9.4 and 14.1 hrs lunar time, respectively.

Observations of lower ionospheric wind by the Kyoto Meteor Radar

This paper describes the results of meteor radar observations which have been carried out at Kyoto University since December 1977. The radar, a coherent pulse Doppler radar with transmitting frequency of 31.57 MHz and nominal peak power of 10 kW, measures wind fields at meteor heights of 80–110 km. The solar semidiurnal tide generally dominates the wind oscillations at these heights. The less regular diurnal tide has comparable magnitude. The variability of the tidal structure suggests the possibility of interaction between respective waves or winds, particularly the influence of disturbed conditions in the lower atmosphere. Other components of winds as mean flow or a quasi 2‐day planetary wave have also been observed. The combination of meteor wind radar and other observational techniques will further the understanding of the global dynamics of atmospheric waves.