Measurement of Ocean Surface Currents by the CRL HF Ocean Surface Radar of FMCW Type. Part 2. Current Vector

The Communications Research Laboratory (CRL) has been developing high-frequency ocean surface radars (HFOSRs). The CRL dual-site HFOSR system can clarify the distribution of surface currents with a nominal range of 50 km. This paper presents a theoretical and experimental analysis of the measurement error of the current vector obtained by the CRL HFOSR system, using a comparison of instantaneous current vectors acquired by the HFOSR system and current meters moored at a depth of 2 m, taking account of the vertical current shear. The theoretical analysis shows that the probability distribution of the measurement error of the current vector forms concentric ellipses at a spatial scale that depends on the RMS measurement error of radial current velocity and with an aspect ratio that depends only on the azimuthal difference of the radar beams. When the azimuthal difference is a right angle, the measurement error of the current vector is at a minimum. A comparison between instantaneous current vectors measured by the CRL HFOSR system and moored current meters shows that the distribution of the difference vector between the radar current and the meter current agrees well with the theoretical measurement error of the current vector and that the RMS of difference vector length is about 10 cm s^–1 while the azimuthal difference between two radar beams is between 45 and 135 degrees. The accuracy of current measurement by the dual-site HFOSR system is therefore considered to be less than 10 cm s^–1 in this range of azimuthal difference. The theoretical analysis will be applicable for a wider range of the azimuthal difference of the radar beams.     

A model of ‘disparitions brusques’ (sudden disappearance of eruptive prominences) as an instability driven by mhd waves

A model of ‘disparitions brusques’ (sudden disappearence of eruptive prominences) is discussed based on the Kippenhahn ans Schlüter configuration. It is shown that Kippenhahn and Schlüter's current sheet is very weakly unstable against magnetic reconnecting modes during the lifetime of quiescent prominences. Disturbances in the form of fast magnetosonic waves originating from nearby active regions or the changes of whole magnetic configuration due to newly emerged magnetic flux may trigger a rapid growing instability associated with magnetic field reconnection. This instability gives rise to disruptions of quiescent prominences and also generates high energy particles.     

Classification of magnetic reconnection during two current-loop coalescence

We present a classification of magnetic reconnection during two current loop coalescence, which may be quite important for the physical process of both solar flares and coronal loop heating in the solar active region. It is suggested that different kinds of the current loop coalescence processes could be identified from the soft X-ray telescope(SXT) of the Yohkoh satellite and the magnetic field data in the active region.     

Excitation of whistler waves driven by an electron temperature anisotropy

A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T > T ) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T /T . The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration.     

A new approach for estimating seismic damage of buried water supply pipelines

Conventional damage prediction methods for lifeline structures are primarily based on peak ground motion measurements. However, line structures such as lifelines suffer damage that is mainly induced by the strain of the ground and therefore are likely to be vulnerable to sharp spatial changes in the ground motion. In this study, we propose a measure for evaluating the damage incurred by underground water supply pipelines based on the spatial gradient of the peak ground velocity (PGV), in an attempt to quantify the effects of the geospatial variabilities in the ground motion on pipeline damage. We investigated the spatial distribution of the damage caused to water pipelines during the Niigata‐ken Chuetsu earthquake on October 10, 2004 (Japan Meteorological Agency magnitude (M_JMA) of 6.8) and the Kobe earthquake on January 17, 1995 (M_JMA7.3) and compared the surveyed damage with the PGV distribution as well as with the gradients of the PGV calculated around the damage areas. For the Kobe earthquake, we used the PGV distribution obtained by the strong‐motion simulation performed by Matsushima and Kawase 1 . In case of the Chuetsu earthquake, we estimated the ground motion using a broadband‐frequency‐based strong‐ground‐motion simulation method based on a multiasperity source model. In both cases, we calculated the gradients of the PGV along the geographical coordinates, with the amplitude of the PGV gradient vector being employed as the damage estimator. Our results show that the distribution of damage to underground water supply pipelines exhibits a greater correlation with the gradients of the PGV than with the PGV itself. Thus, the gradient of the PGV is a useful index for preparing initial‐screening hazard maps of underground facilities. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of acidified seawater on calcification,photosynthetic efficiencies and the recovery processes from strong light exposure in the coral Stylophora pistillata

The aim of this study was to investigate whether coral photosynthetic efficiencies and recovery processes are affected by CO₂‐driven ocean acidification in symbiont photosynthesis and coral calcification. We investigated the effects of five CO₂ partial pressure (pCO₂) levels in adjusted seawater ranging from 300 μatm (pre‐industrial) to 800 μatm (near‐future) and strong and weak light intensity on maximum photosynthetic efficiency and calcification of a branching coral, Stylophora pistillata, as this species has often been used in rearing experiments to investigate the effects of acidified seawater on calcification and photosynthetic algae of corals. We found that, the photosynthetic efficiencies and recovery patterns under different light conditions did not differ among pCO₂ treatments. Furthermore, calcification of S. pistillata was not affected by acidified seawater under weak or strong light conditions. Our results indicate that the photosynthetic efficiency and calcification of S. pistillata are insensitive to changes in ocean acidity.     

The role of loop footpoints rotation in single loop flares

We consider that single loop flares can be caused by the rotation of loop footpoints. Choosing a typical geometry for this case we find from MHD equations self-consistent expressions and a set equations governing behaviour of all physical quantities. Numerical simulations have revealed that under the determined conditions for the initial azimuthal velocity and current the pinch instability takes place. The most important parameters of the problem are the plasma and the ratio of the initial values of longitudinal and poloidal components of the magnetic field-B ₁. Thus, calculations show that the critical pinch time increases with the increase ofB ₁ and decreases with the increase of plasma . So the most effective flares are probable for the most high loops with strong currents. ForB ₁=10 and =0.01 the critical pinch time is 2.5 s. The critical twist angle for magnetic field depends on the initial one. For low intial twist which corresponds to bigB ₁ the critical one is more less. For exampleB ₁=30 gives 1.8 (when ratio of loop length and radius is 10). Geometrical analysis shows that the present model can explain (for high photospheric rotation) single loop flares taking place on different parts of the loop as on the top of it as closer to one of the footpoints. It depends on the relative rotation momentum of loop footpoints. Subject headings: MHD-Sun:flares.     

Generation of Magnetic Field and Fast Particles During Collision of Electron-Positron Plasma Clouds

We present results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counter-streaming instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and that, in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted to magnetic field energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collision-less change of structure, leading to large scale, long living structures and the production of high-energy particles. These processes may be important for understanding of production of high-energy particles in the region where two pulsar winds collide. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-energy flare explosions driven by 3-dimensional X-type current loop coalescence

We present a model for high-energy solar flare explosions driven by 3-dimensional X-type current loop coalescence. The 3-dimensional X-type current loop coalescence, where two crossed flux-tubes interact at one point, is a fundamentally new process as compared to the 1-D and 2-D cases studied earlier. This process is studied by a first-order approach of the relevant variables near the point of coalescence; it appears to yield reliable information in a sufficiently large area around this point. It is shown that, following a strong plasma collapse due to the pinch effect, a point-like plasma explosion can be driven while fast magnetosonic shock waves can also be excited. We found that the conditions in the area producing the remarkable flare bursts of 21 May, 1984 were indeed such that the many flare spikes could have been due to 3-D explosive X-type current loop coalescence. We also show, by studying the conditions of shock formation in a gamma ray flare, that the time delay of -rays from the impulsive phase could be the time needed for the shock formation in the flaring region.We draw some general conclusions on the question why certain flares do emit -rays in the MeV energy range, and why other, apparently important and energetic flares, do not. We accentuate the fact that a well-developed high-energy flare has three phases of particle acceleration.     

Zircon fission‐track and U–Pb double dating using femtosecond laser ablation–inductively coupled plasma–mass spectrometry: A technical note

We present a new LA–ICP–MS system for zircon fission‐track (FT) and U–Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA–ICP–MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced FT method, and have smaller analytical errors. LA–ICP–MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higher‐precision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U–Pb ages that are in agreement. A chip of the Nancy 91500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix‐matched U‐standard for FT dating. When using the Nancy 91500 zircon as a U‐standard, a zeta calibration value of 42–43 year cm² for LA–ICP–MS FT dating is obtained. While this value is strictly valid only for the particular session, it can serve as a reference for other studies.