Tomizo Itoh

Amplification of ion-acoustic solitons by an ion beam

Ion-acoustic solutions in a plasma traversed by a weak ion beam are studied experimentally by a double plasma device. Large amplitude compressive pulses are amplified and steepened to become solitons of which the amplitude n/n0 is comparable or less than 40%. The amplitude of the soliton is maximum at a beam velocity of Vb/Cs=1.7 while the linear growth rates is maximum at Vb/Cs=1.3. Theoretical explanations are presented with the use of the fluid model.

Measurements of the stratospheric carbon dioxide concentration over Japan using a Balloon-borne cryogenic sampler

Stratospheric air samples have been collected once per year since 1985 over Japan using a balloon-borne cryogenic sampler and then analyzed for their CO2 concentrations. The measurements showed high values of CO2 in the lowest part of the stratosphere, then a decrease with height to 20.0–25.0 km, and almost constant values above this level. From the comparison with the results of our aircraft measurements in the upper troposphere and lower stratosphere, it is suggested that the vertical profile of stratospheric CO2 over Japan is strongly related to transport processes that are associated with tropospheric air being introduced into the stratosphere in the tropical region and then moving poleward with different speeds at different heights. The average rate of the CO2 increase from 1985 to 1991 was estimated to be 1.4 ppmv/year; this is similar to the rate found for the troposphere over the same period.

Maximization in the damping rate of the ion acoustic wave in a two‐ion plasma

The characteristics of dispersion and Landau damping of ion acoustic waves are experimentally investigated in an argon‐helium plasma with varying ion composition ratio. The dispersion relations change continuously from the argon mode to the helium mode. The damping rate has a maximum when the ion number density ratio (r = nHe/nA + nHe) is about 0.08. This maximum is attributed to the resonance of helium ions with the argon mode ion acoustic wave and is consistent with theoretical predictions.

Long-term monitoring of carbon and oxygen isotope ratios of stratospheric CO2 over Japan

We have been measuring stable isotopic ratios of carbon (delta-C-13) and oxygen (delta-O-18) of carbon dioxide in the stratosphere (18-35 km altitudes) over Japan since 1985, presenting here the first temporal delta-C-13 and delta-O-18 records between 1985 and 1991. The stratospheric delta-C-13 is gradually decreasing due to anthropogenic effects with a rate of approximately -0.03%/y, which is similar to the tropospheric value. The previously found O-18 enrichment is stratospheric CO2 has been maintained almost steadily during the 6 years, with a monotonous delta-O-18 increase with increasing altitude up to 35 km over Japan.

Propagation of Electron Waves in a Now-Maxwellian Plasma

The dispersion relation of electron waves in a non-Maxwellian plasma has been investigated experimentally. It is observed that the dispersion relation of the Landau mode is modified and that a new wave mode with the dispersion relation ω/ k ≃(1/2) V c ( V c : truncated velocity) is observed for frequencies less than the electron Langmuir frequency. Those experimental results are compared with a simple theoretical model.

Reception Characteristics of Monopole Antennas for Electron Plasma Waves

The radiation patterns of a longitudinal electron plasma wave excited in a warm unbounded plasma by a grid and parallel plane grids and the reception patterns for wire antennas and the grid having a grounded plate were measured experimentally. The results agreed well with theoretically predicted radiation patterns based on a fluid model description for electrons. The dependence of the received amplitude of a gridded parallel plate on the D. C. potential of the antenna was studied. The possibility of estimating the wavelength from measured amplitudes of the wire antenna and the gridded parallel plate is also discussed.

Interaction of plasma streams with a neutral gas cloud

Interaction of plasma streams with a neutral gas cloud was studied using gun-produced plasma and laser-produced plasma. The interaction of a quiet plasma stream shows a stable normal shock structure, but that of a plasma stream having turbulent magnetic fields shows a very complicated structure which resembles the ray structure of the type-i comet tail. From time resolved photographs of the interaction of the quiet plasma stream, one can obtain valuable information on the process of the growth of shock waves, and the experimental results are explained in terms of the snowplow model. Similarity relation concerning the type-i comet tail is also discussed.

Rocket‐borne spectrometer to measure the near infrared absorption spectrum of the upper atmosphere

A new type of spectrometer has been developed for performing a rocket measurement of the near infrared (NIR) absorption spectrum of the upper atmosphere. The spectrometer is based on multichannel spectroscopy with the use of a one‐dimensional solid state image sensor. The time of scanning for a spectrum is 10 ms which is much shorter than a usual mechanical scanning. This spectrometer which was borne on the S‐310‐8 rocket measured the NIR absorption spectrum of atmospheric constituents such as O2 and H2O with negligible distortion due to the rocket motion. A PCM (pulse code modulation) transmission is preferable in order to obtain a large amount of data in the spectrum measurement. The PCM data processing system is also described.

Measurement of stratospheric aerosol near Sanriku (39°N, 142°E) in Japan on May 31, 1979

The altitude profile of the scattering extinction of stratospheric aerosols was measured by using the balloon-borne spectrometer with the solar occultation method near Sanriku (39°N, 142°E) in Japan on May 31, 1979. The result confirmed that the altitude profile of the scattering extinction turned out to be close to that of the SAM II measurement near Alaska during July 16–19, 1979.

Propagation of the higher-order Landau modes of electron plasma wave

Propagation of electrostatic electron waves whose frequency is smaller than the electron plasma frequency in a large unmagnetized plasma is investigated both experimentally and theoretically. When a receiver is close to a transmitter, free-streaming electrons are detected owing to their large capacity for excitation. When the distance between the receiver and the transmitter becomes large, the third-order Landau mode is observed due to its smaller damping than that of free-streaming electrons. Finally, a dip in amplitude of the wave, caused by interference by the higher-order Landau modes, is seen. The results are in reasonable agreement with numerical calculation assuming a dipole excitation for the wave.