Yoshikazu Muraoka

The quantitative relationship between VLF phase deviations and 1–8 Å solar X-ray fluxes during solar flares

Abstract An attempt is made to investigate the quantitative relationship between VLF phase deviations in SPA (sudden phase anomalies) events and associated solar X-ray fluxes in the 1–8 A band during solar flares. The phase deviations (Δφ) of the 18.6 kHz VLF wave transmitted from NLK, USA are used in this analysis which were recorded at Nishinomiya, Japan during the period June 1974 to May 1975. The solar X-ray fluxes (F0) in the 1–8 A band are estimated from fmin variations using the empirical expression given by Sato (1975), because no observed data were available on the 1–8 A X-ray fluxes during the period of the VLF observation. The result shows that the normalized phase variation, Δφ cos min , where min represents the minimum solar zenith angle on the VLF propagation path, increases with increasing log F0. A theoretical explanation for this is presented assuming that enhanced ionizations produced in the lower ionosphere by a monochromatic solar X-ray emission are responsible for the VLF phase deviations. Also it is found that a threshold X-ray flux to produce a detectable SPA effect is approximately 1.5 × 10−3 cm−2 sec−1 in the 1–8 A band.

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.

Lower ionospheric disturbances observed in long-distance VLF transmission at middle latitude

Abstract An attempt is made to investigate disturbances in the relative phase of long-distance, cesium-controlled, VLF radio wave propagation from NLK (Jim Creek, U.S.A.) to HCM (Nishinomiya, Japan). The propagation path covers a longitudinal range of about 100° and ranges from L ~ 1.2 to L ~ 3.2 in invariant latitude. VLF phase disturbances following major geomagnetic storms indicate that the altitudinal and latitudinal extents of ionization enhancement in the D -region depend on the strength of the storm. The after-effect observed in the night-time VLF phase becomes pronounced especially in winter condition of the D -region. There is another type of VLF phase disturbance in the night-time which is not related to storm but associated with the winter anomaly observed in the daytime at middle latitudes (40°–45°N) but below L = 2. This result suggests that the enhanced ionization occurs in the D -region above about 80 km not only in the daytime but also in the night-time for a group of winter anomalous days. It is likely that the main cause of the enhanced ionization is related to the vertical propagation of planetary waves into the mesosphere, resulting in a sudden warming.

A study of velocity fluctuation spectra in the troposphere and lower stratosphere using MU radar

Abstract We present an analysis of the vertical wave number and frequency spectra of atmospheric motions in the height ranges between 5 and 25 km observed using the Shigaraki, Japan, MU radar during a 4-day period in January 1988. The vertical wave number spectrum of the horizontal velocity fluctuation is found to saturate at large wave numbers satisfying power law ~ N 2 2K 3 z while departing from this −3 power law at small wave numbers. Frequency spectra of the oblique radial velocity fluctuations can be fitted by a Garrett-Munk gravity wave model spectrum. However, the vertical velocity fluctuation cannot be fitted simultaneously. The observed spectra are too steep and their energy levels are too low compared with the results from model prediction. Also, the vertical profiles of the energy densities of the horizontal velocity fluctuations are found to be positively correlated to the background wind velocity profile. These characteristics of the observed spectra are satisfactorily explained by dynamic instability and wave-wave interactions in the regions below the critical layer through nonlinear numerical simulations. The correlation between the background wind and the horizontal velocity fluctuations is shown to result from wave-shear interaction.

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.

A new approach to mode conversion effects observed in a mid-latitude VLF transmission

Abstract The seasonal variation of the mode conversion effect has been investigated as observed for more than three years on the VLF signal from NLK 18.6 kHz in Nishinomiya, Japan. A new analytical method is proposed for the further understanding mode conversion effects on long-distance VLF transmission. Using this method for our measurements during the sunrise transition in winter, the modal interference spacing distance D21N = 2.30 Mm has been derived uniquely consistent with VLF waveguide mode theory and suggests that the height of the earth-ionosphere waveguide is somewhat higher than 90 km at night during winter. Moreover, the quantity arg ( S 21 DN S 11 DN ), the argument of the ratio between complex two major mode conversion coefficients for day-night propagation, was found to be dependent on the length of the transition region in an essentially linear manner. This linear relationship indicates that a reference value of solar zenith angle corresponding to the effective sunrise in the ionosphere can be derived uniquely from the requirement of the VLF mode theory. It is noted that the nighttime height of the waveguide is not always constant owing to the geographical configuration of the propagation path. This leads to an effective extension of the transition region during summer.

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.