Phase coherence of foreshock MHD waves: wavelet analysis

The earth's foreshock is a region where particularly large amplitude MHD waves are commonly observed. They exhibit various waveforms, suggesting that nonlinear interaction between the waves is in progress. In a previous paper (Hada et al., 2003) we have introduced a method to quantitatively evaluate the strength of phase coherence among the waves from a given time series data. Here we further develop our method by applying wavelet filtering technique. From the analysis it was found that, although the turbulence is consisted of waves with a wide range of plasma rest frame frequencies, only those frequencies lower than the ion gyrofrequency are responsible for generating the phase coherence. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phase coherence of MHD waves in the solar wind

Large amplitude MHD waves are commonly found in the solar wind. Nonlinear interactions between the MHD waves are likely to produce finite correlation among the wave phases. For discussions of various transport processes of energetic particles, it is fundamentally important to determine whether the wave phases are randomly distributed (as assumed in quasi-linear theories) or they have a finite coherence. Using a method based on a surrogate data technique and a fractal analysis, we analyzed Geotail magnetic field data (provided by S. Kokubun and T. Nagai through DARTS at the Institute of Space and Astronautical Science) to evaluate the phase coherence among the MHD waves in the earth's foreshock region. The correlation of wave phases does exist, indicating that the nonlinear interactions between the waves is in progress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Natural convection driven by buoyancy and surface tension forces under external magnetic field

Two-dimensional natural convection driven both by buoyancy and surface tension in the horizontal layer of an electrically conducting liquid which is subjected to a horizontal temperature gradient and a vertical magnetic field is studied theoretically. The flow and temperature distribution in the liquid layer is analysed for two cases; (1) the top surface is free and the bottom is a rigid wall, and (2) both surfaces are rigid. The effect of the magnetic field on the flow velocity and the temperature distribution is made clear and the simple expressions are obtained for two extreme cases of the sufficiently large magnetic field and the small magnetic field.     

Two-dimensional convection in liquid layer related to crystal growth techniques in space

In this paper, the Marangoni convection including buoyancy convection in two-dimensional fluid model was investigated experimentally and theoretically. The transparent liquid in the test cell was heated and cooled under various conditions. The fluid flow driven by surface tension and by buoyancy force was visualized and measured. The flow patterns, velocity distributions and temperature field were obtained and they were compared with the results of the numerical solutions. The effect of the configuration and the convection in microgravity on crystal growth was discussed. The temperature field of the crystallizing surface was predicted.     

Cross Field Diffusion of Cosmic Rays in a Two-Dimensional Magnetic Field Turbulence

Cross field diffusion of energetic particles (cosmic rays) in a two-dimensional static magnetic field turbulence is studied performing test particle simulations. Qualitatively different diffusion processes are observed depending on the ratio of Larmor radius (ρ) to the correlation length (λ) of the magnetic field fluctuations. The diffusion is found to be composed of several regimes with distinct statistical properties, which can be characterized using Levy statistics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Two-dimensional Marangoni and buoyancy convection related to crystal growth techniques in space

The natural convection in the horizontal liquid layer driven both by surface tension gradient and buoyancy was investigated experimentally and theoretically.The fluid motion was visualized by mixing fine flakes of aluminum into the liquid. At the same time, the temperature field was visualized by the Mach-Zehnder interferometry and the local temperature gradient distributions were visualized by making use of the refraction of parallel incident light.The numerical analysis using the Galerkin method was also carried out which agreed well with the experimental results.It was found that the velocity and temperature fields of Marangoni convection were varied depending on the aspect ratio of the liquid layer and on the coupled buoyancy convection.