Soji Ohara

Chaos in Cosmic Ray Air Showers

Unexpected chaotic features are found in time series of arrival time intervals of successive air showers with (E > 3 × 1014 eV). Over 99 % of air shower arrival time intervals obey the Poisson distribution law representing stochastic behaviors, but occasionally there are air showers showing real chaotic behaviors as distinguished from both random and colored noises. With two systems of the Kinki university installations, we found 13 cases showing chaotic time series in 3.36 yr with the system-1 and the 1.37 yr with the system-2. Five out of 10 chaotic air showers of the Kinki installation are detected during the same time zone also by the Osaka City university installation which is at 115 km distance from the Kinki one. In a remarkable example of September 19, 1991, the correlation dimension was observed to have dropped from about 4 to the minimum of 1.3 and recovered smoothly in about 38 h. The chaos structure in this case is detected in nearly the same time zone at the Ohya station of the Institute for Cosmic Ray Research, University of Tokyo, which is separated from the Kinki one by 460 km. Formation of chaos structure due to energetic cosmic ray dust particles is suggested. Progress of cosmic ray physics may be expected with the study of air showers marked with chaos.

Observation of time correlation in cosmic air shower network

In order to measure time correlation of primary cosmic rays, a network observation of extensive air showers started in Japan. The network as Large Area Air Shower (LAAS) group consists of nine air-shower-stations (seven institutes and 26.000 km2) using the Global Positioning System (GPS). We propose an analytical procedure to extract time-limited fractal structure from time series of arrival time intervals of successive air showers and present the results of the analysis by using the network data.

Statistical tests for zenith angle dependences of up-going μ-like neutrino events for multi-GeV region

At the Neutrino 98 conference the Super Kamiokande collaboration presented a reduction of up-going μ-like events compared to down-going μ -like events by a factor of 0.54 −0.05 +0.06 for an exposure of 33 kton-years. Also, the zenith angle distribution observed by MACRO for -1 ≤ cos Θ ≤ −0.1 does not fit well for the no oscillation expectation. On the basis of x 2 tested results (shape, absolute value), however, their claim for neutrino oscillations cannot be considered to be firmly established from the standpoint of statistical evidence.

Numerical Simulation of Artificial Circulation in a Reservoir

貯水池を対象に, 人工循環によって生じる流れと水温変化の数値シミュレーションを実施した.計算量を削減するために貯水池の形状を単純化し, 計算時期も安定度の変化を指標として4月4日から5月15日の41日間とした.貯水池に設置された曝気装置内の流速は, 20秒周期で変動するが, 計算には1周期の平均流速0.67m/secを用いた.Boussinesg方程式はコントロールボリューム法により離散化された.数値解を得るアルゴリズムは, PatankerのSIMPLER法によった.水温分布の計算結果は, 測定値とよい一致が得られた.流れの計算結果から装置近傍と貯水池全体にわたる2つの循環流が形成されることが分かった.また, 全体の循環水量は装置による揚水量の70%程度であった.

Three Dimensional Flow Velocity of Reservoir Calculated with the Data of Temperature Distribution

貯水池内の3次元的流速分布を計測することは, 一般に種々の困難が伴う.比較的機器計測が容易な温度分布のデータをもとにして数値計算で流速分布が推定できれば貯水池の環境管理上の意義は大きい.本研究ではこの手法の効果を理論的に確認するたあ2種類の計算を行った.まず非定常密度流の計算機シミュレーションを行い, 流速水温の時系列データを確保する.次に水温分布のみから定常流近似計算で流速を計算する.その結果を先に求めた非定常計算結果の流速と比較し再現性を調べた.数値モデルは, Navier-Stokes方程式の, Boussinesq近似をPatanker法により離散化したものを用いた.流入開始後8時間程で流速分布はほぼ安定化し, 温度分布より求あられた流速ともよい一致が得られた.