Chikao Nagasawa

HF-Doppler observations of acoustic waves excited by the Urakawa-Oki earthquake on 21 March 1982

Ionospheric disturbances caused by the Urakawa-Oki earthquake at 0232 UT on 21 March 1982 have been detected by a network of HF-Doppler sounders in central Japan. The HF-Doppler data, together with the seismic data, have been used to formulate a mechanism whereby ionospheric disturbances are produced by an event of relatively small epicentral distance. Comparison of the dynamic spectra of these data has revealed experimentally that the atmosphere acts as a low-pass filter for the upward-propagating acoustic waves. The cut-off periods of this filter are estimated by applying a digital filter technique to the up-down component of the seismograms and are found to be 10 s from the ground up to 156 km and 20 s from 156–195 km. Considering the transfer function of the atmosphere derived from the theory of Pitteway and Hines, the observed result does not contradict the prediction that the atmospheric filter mechanism is mainly attributable to viscosity.

Lidar observations of a lot of sporadic sodium layers in mid-latitude

We have routinely observed mesospheric Na layers since November 1991 with a Na ground-based lidar above Tokyo Metropolitan University (TMU) at Hachioji, Tokyo (35.6°N, 139.4°E) in mid-latitude. In the past, the sporadic Na layers have been observed commonly at low-and high-latitude lidar sites [Kwon et al., 1988; Batista et al., 1989; von Zahn et al., 1987] but rarely observed at mid-latitude sites [Senft et al., 1989]. Contrary to expectations, we could observe more than 100 events of the sporadic Na layers for two years from November 1991 until October 1993 in this mid-latitude lidar site, and the large sporadic Na layers had been observed especially during spring and summer. Most of these events were accompanied by sporadic E layers and the most enhancement of the sporadic E layers preceded that of the sporadic Na layers by 15 to 30 minutes.

Development of a 1.6 μ m differential absorption lidar with a quasi-phase-matching optical parametric oscillator and photon-counting detector for the vertical CO 2 profile

We have developed a 1.6 microm carbon dioxide (CO(2)) differential absorption lidar utilizing a quasi-phase-matching optical parametric oscillator (OPO) and a photon-counting detector. The operating wavelengths were chosen based on their low interference from water vapor and low temperature sensitivity. The online wavelength was in the (30012<--0001) band of CO(2), which was insensitive to atmospheric temperature. The established OPO laser achieved a 10 mJ, 200 Hz repetition rate at the online and offline wavelengths. Our observations confirmed the statistical error of 2% with 5 h of accumulation for the CO(2) density profile less than 5.2 km. Also, the statistical error of 1% at an altitude of 2 km was demonstrated. The results of the vertical CO(2) concentrations acquired using a 1.6 microm wavelength are presented.

Random modulation cw lidar using new random sequence.

New modulation codes are presented for a random modulation cw lidar. One characteristic of these modulation codes is that for very noisy background conditions, the signal-to-noise ratio is improved by using these new sequences, and is better than for the maximum-length sequence (the M-sequence) which is commonly used as the modulation code. Another characteristic of these modulation codes is that there is no correlation between them. This fact will be useful for the simultaneous multitransmitter of the differential absorption lidar. These two characteristics of the new modulation codes were confirmed experimentally.

Simultaneous observations of mesospheric gravity waves with the MU radar and a sodium lidar

Simultaneous observations of mesospheric gravity waves have been carried out using meteor wind measurements with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (34.9°N, 136.1°E), and density perturbations of the sodium lidar at Hachioji, Tokyo, Japan (35.6°N, 139.4°E). The study utilizes 7 hours of data collected on the night of December 15-16, 1993, during a time period when a fairly monochromatic gravity wave was dominant. Using hodograph analysis, the dominant gravity wave was found to exhibit a vertical wavelength of 16 km, an intrinsic period of 9.1 hours, and a horizontal wavelength of about 1900 km. The horizontal propagation direction of the gravity wave was determined from the phase relations between the horizontal wind components and the temperature perturbations at Shigaraki. The wave propagated southward, being almost orthogonal to the baseline between Shigaraki and Hachioji. Employing the dispersion and polarization relations for linear gravity waves, the wave-induced neutral density perturbations from the MU radar observations were estimated. A comparison with the corresponding density perturbations derived from the sodium density measurements showed good agreement. The amplitudes of the neutral density perturbations observed at both locations, which are separated by 310 km, were similar, with a maximum perturbation of ∼7% and a good correlation of phase. Time variations of the hourly variance of the density perturbations also agreed quite well between the two independent determinations, which again supports the view that the radar and the lidar detected the same gravity wave.

Studies of the MLT region using the MU radar and simultaneous observations with OH spectrometer and other optical instruments

Abstract The MU radar (middle and upper atmosphere radar) has been used since 1984 to study the dynamics of the mesosphere and lower thermosphere (MLT) as well as the dynamics of the lower atmosphere and the ionosphere. Gravity waves in the mesosphere have been studied extensively over various time and vertical scales during the daylight hours using backscatter from turbulent irregularities. Scatter from meteor trails can be used during day or night to study the dynamics of the MLT, and techniques to apply the MU radar for meteor scatter observations have been developed and recently improved. The meteor observation mode of the MU radar can detect about 15,000–20,000 echoes/day and 10,000 echoes/day are from underdense trails with arrival angle determination. This high meteor echo rate enables us to determine the wind velocities and molecular diffusion coefficient with the time/height resolutions of 30 min × 1 km at 80–100 km for 24 hours a day. From the horizontal distribution of the meteor echoes, horizontal gradients of the wind fields can also be detected, as well as vertical shears of horizontal winds. These high resolution meteor echo observations were made simultaneously with OH spectrometer observations. These combined observations yielded information on both the horizontal and vertical structure of the gravity waves. Comparisons were made between the temperature variations determined by OH airglow and determined by the diffusion coefficient of the meteor echoes and excellent agreement was found. Cooperative observations with sodium lidars, FPI, airglow imagers, and an MF radar are also being carried out.

Characteristics of single longitudinal mode oscillation of the 2 μm TM, Ho:YLF microchip laser

The lasing condition and frequency stability of the single longitudinal mode oscillation of a diode laser pumped 2 μm Tm,Ho:YLF microchip laser at room temperature are reported. It is shown that the microchip laser with an output mirror of 99.0% reflectivity had better single longitudinal mode performance than that with an output mirror of 99.5% reflectivity. The frequency tuning rate when varying the crystal temperature was estimated to be 1.9 GHz/°C. Frequency stability of the microchip laser is examined by the self-beating heterodyne detection method for several delay times between 0.48 and 4.8 μs. It is indicated that the spectral fluctuation is in proportion to the delay time and the increasing rate is 2.3 kHz/μs.

Observations of mesospheric sporadic sodium layers with the MU radar and sodium lidars

The dynamical structure of the atmosphere around the sporadic sodium layer at mid-latitude (∼35°N) below 100 km was studied by simultaneous observation with the MU radar at Shigaraki (34.9°N, 136.1°E), and two Na lidars at Shigaraki and in Hachioji (35.6°N, 139.4°E). In the lidar data, fifteen Nas (sporadic sodium layer) events were detected. Wind shear, temperature, and stability indices, at around the time and height of Nas were observed with the MU radar. Strong total wind shear correlated well with Nas, especially when sporadic Es did not accompany. However, no other clear correlations, such as correlations with temperature etc., were found. The result is similar to the report of the lidar observations in Hawaii during the ALOHA-93 campaign (Qian et al., 1998), and suggests a similar generation mechanism between 20°N and 35°N.

All-solid-state high-power conduction-cooled Nd:YLF rod laser

A high-average-power conduction-cooled diode-pumped Nd:YLF rod laser has been developed. A new conduction-cooled side-pumping scheme with a solid prismatic pump-light confinement cavity was employed. A transparent, high-thermal-conductivity MgF>(2) prism was used as a highly efficient pump cavity as well as a low-thermal-resistance heat spreader. The pumping efficiency and thermal resistance of the cavity were 85% and 0.20 degrees C degrees W, respectively. When this scheme was combined with heat pipes for heat removal, a maximum average output power of 72 W was demonstrated, with an optical slope efficiency as high as 49%.

Stimulated Raman Scattering Laser Oscillation around 1.6 µm Carbon Dioxide Absorption Line

A solid-state stimulated Raman scattering (SRS) laser oscillation around the 1.6 µm carbon dioxide absorption lines is demonstrated. The stokes output of the SRS radiation at 1.57 µm is generated from the frequency conversion of the 1.35 µm laser radiation of Nd3+:KGd(WO4)2 (Nd:KGW) in the cavity. The maximum output energy was 13.8 mJ with a repetition rate of 10 Hz, in response to the incident laser pumped from the laser diode to the Nd:KGW. To our knowledge, this result of a 1.57 µm intracavity SRS oscillation at CO2 absorption lines around 1.6 µm is gained for the first time.