Longshore current generation by obliquely incident internal waves

Abstract

It is shown that a significant longshore current of maximum amplitude v = (3 × 10^4/v ) cm sec^?1 (for typical oceanic values) can be forced between the breaker zone and shore by internal gravity waves obliquely incident on a plane beach. In the mean longshore momentum balance Reynolds stress terms appear which can be calculated to O (α), α being a bottom slope parameter, using a WKB approach. With appropriate assumptions being made about the amplitude behaviour of the motion after breaking, the divergence of these stresses does not vanish and forces a current whose magnitude is determined by a balance with viscous stresses derived from vertical eddy motions.     

The generation of phytoplankton patchiness by mesoscale current patterns

 Elken et al. (1994) suggested that phytoplankton patchiness can be generated by mesoscale eddies in light-limited, nutrient-replete environments. This hypothesis is explored using two ecological models of different physical complexity. The model results support the idea that the coupling of mesoscale eddy circulation and phytoplankton growth leads to differential growth rates and thus generates variability in phytoplankton distributions. The specific circulation of a cyclonic eddy isolates a phytoplankton population in its core. Due to the reduced vertical mixing, a higher growth rate is supported in the core, and phytoplankton concentrations increase compared to the surrounding environment. A one-dimensional model is used to explore the hypothesis in general and to perform sensitivity studies. A more realistic simulation uses a coupled three- dimensional model for the western Baltic Sea. Starting from vertically and horizontally homogeneous distributions for nutrients and plankton, the models generate patchiness due to the proposed mechanism. The described mechanism may apply for other mesoscale variable environments during light-limited growth periods as well, e.g., the frontal region of the Southern Ocean. Received: 31 March 2001 / Accepted: 31 August 2001     

Energy characteristics of collisionless plasma disturbance caused by an absorbing sphere

The calculated energy density and energy flux density of charged particles of collisionless plasma, disturbed by an absorbing spherical body, are presented, in addition to the known number density and current density calculations. It has been shown that the external radius of a spherical layer, where particle finite motion is possible, is an important problem parameter. The consequences of the equations for the moments of the electron and ion distribution functions are also discussed.     

Nonlinear disturbance of the dipole field by an axisymmetric plasma distribution

The results of numerical modeling of magnetic field variations with increasing pressure in the inner magnetosphere regions in the axisymmetric case with pressure isotropy are presented. The radial dependence of magnetic field disturbance for the given plasma pressure distribution is determined. The dependence of magnetic field disturbance near the Earth on the total pressure with allowance for the influence of magnetic field of the currents in the plasma on the field value and distribution is obtained. The obtained solutions are applied in an analysis of the large magnetic storm of February, 1986, during which the minimum value of the Dst variation was 307 nT. The plasma pressure values experimentally measured near the equatorial plane were used. It is shown that, with allowance for nonlinear effects, the value of observed Dst variation corresponds to the disturbance caused by the axisymmetric part of a ring current.     

水星内磁层等离子体带及电流体系

水星是太阳系中离主星最近的和最小的类地行星, 它具有独特的空间环境.水手10号(Mariner 10)确认水星拥有全球性内禀偶极磁场和磁层.但是由于早期观测有限, 人们通常将水星磁层简单地视作地球磁层的缩小版来研究.不过, 在信使号(MESSENGER)访问水星之后, 人们认识到水星和地球的空间环境有巨大差异.通过更深入的分析研究, 人们发现在水星内磁层中, 夜侧磁赤道面附近有带状的等离子体分布, 并且该等离子体带可能与多种磁层电流体系(分叉环电流、亚暴电流楔和东向电流等)密切相关.在贝彼哥伦布(BepiColombo)计划揭开水星研究的新篇章之际, 本文回顾近些年来与水星内磁层等离子体带及相关电流体系有关的研究, 并指出相关研究将是未来水星空间环境的前沿热点.

     

Dependence of volumes of magnetic flux tubes on plasma pressure and disturbance in the magnetic field in the axially symmetric case

We have analyzed the applicability of the approximation of the axially symmetric magnetic field created by the dipole field and the currents flowing in the plasma for describing the Dst variation value during magnetic storms and the dependence of the position of the pressure maximum on the volumes of magnetic flux tubes on the plasma pressure. We have determined the dependence of the disturbance in the field on the geocentric distance. We have shown that the experimentally obtained dependence on the position of the pressure maximum on Dst is described in the assumption on the correctness of the adiabatic law on changes in pressure with a change in geocentric distance. We have calculated the values of the magnetic field distortion and the value of the Dst variation for the experimentally determined radial pressure profile for three magnetic storms with Dst ∼ 100 nT. We have shown that, with allowance for nonlinear magnetic field distortions, the axially symmetric part of the ring current makes the main contribution to the value of the Dst variation.     

Simplified seismic analysis of asymmetric building systems

The paper reviews the uncoupled modal response history analysis (UMRHA) and modal pushover analysis (MPA) procedure in the analysis of asymmetric structures. From the pushover curves in ADRS format, showing the relationships of base shear versus roof translation and base torque versus roof rotation, a bifurcating characteristic of the pushover curves of an asymmetric structure is observed. A two‐degree‐of‐freedom (2DOF) modal stick is constructed using lump mass eccentrically placed at the end of beam which is connected with the column by a rotational spring. By converting the equation of motion of a whole structure into 2DOF modal equations, all of the elastic properties in the 2DOF modal sticks can be determined accurately. A mathematical proof is carried out to demonstrate that the 2DOF modal stick is consistent with the single‐degree‐of‐freedom (SDOF) modal stick at elastic state. The bifurcating characteristic of modal pushover curves and the interaction of modal translation and rotation can be considered rationally by this 2DOF modal stick. In order to verify the effectiveness of this proposed 2DOF modal stick, a two‐storey asymmetric building structure was analysed by the UMRHA procedure incorporating this novel 2DOF modal sticks (2DMPA) and conventional SDOF modal sticks (SDMPA), respectively. The analytical results are compared with those obtained by nonlinear response history analysis (RHA). It is illustrated that the accuracy of the rotational response histories obtained by 2DMPA is much better than those obtained by SDMPA. Consequently, the estimations of translational response histories on flexible side (FS) and stiff side (SS) of the building structure are also improved. Copyright © 2006 John Wiley & Sons, Ltd.     

Inelastic response and design criteria of plan-wise asymmetric systems

The study of the torsional response of buildings in the inelastic range of behaviour is of great interest since the ability of structures to resist strong earthquakes mainly relies on their ductility and capacity for energy dissipation. Furthermore, an examination of the performance of structures during past earthquakes demonstrates that plan-asymmetric buildings suffered greater damage due to torsional response. The paper deals with this subject by analysing a model which idealizes a one-storey building with resisting elements oriented along two perpendicular directions. In addition to the parameters of the elastic behaviour, the inelastic system response depends on full yield capacity and plan-wise strength distribution. The influence of the criterion adopted for the design of resisting elements on local ductility demand and damage has been evaluated by parametric analysis. In particular, a comparison has been carried out between systems with equal design levels for all elements and systems with design levels dependent on the element location. For a given elastic behaviour and total capacity, the strength distributions in plan have been defined which minimize ductility demand and structural damage. Finally, based on these findings, responses from models designed according to several seismic codes have been compared.     

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth’s magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30–90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = −21±4, is in accord with the value m = −27±6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f/∂W ≥ 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ≥f/≥W ≥ 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.     

地球等离子体片中磁泡对地面磁场的扰动

观测到的爆发流（Busty Bulk Flows,简称为BBFs）有很多主要特征都可以用磁泡图像描述 . 磁泡模型认为电离层中绝大部分电势降落都发生在磁泡中,据此可以估算它周围的霍尔电 流及其在地面引起的地磁扰动. 将本文结果与SMA（Scandinavian Magnetometer Array）雷 达阵列观测的BBFs的特征进行比较之后,发现二者符合得很好. 这表明BBFs与磁泡在引起地 面磁场扰动上的表征是一样的,暗示BBFs很可能就是磁泡.     

Spatial features of current systems of SFE-flares accompanied by gamma radiation

Based on statistical data and a detailed analysis of geomagnetic response to the hard electromagnetic radiation of the X17 solar flare of September 7, 2005, we considered spatial features of current systems producing the geomagnetic solar flare effect (SFE). During flares accompanied by intensive X-rays and gamma rays, SFEs are shown to be observed globally, including the night hemisphere and high latitudes. Cause-effect relations of phenomena under consideration are discussed.     

Response of equatorial ionosphere to episodes of asymmetric ring current activity

We present the characteristics of the response of equatorial ionospheric zonal electric field and F-region plasma density to the asymmetric ring current intensifications that occurred in succession on 16 December 1991, corresponding to the STEP/EITS-2 campaign period. The study is based on high-time-resolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, Vz measurements at Kodaikanal (10.25°N; 77.5°E; dip 4°), India and quarter-hourly ionosonde data of For-taleza (4°S; 322°E; dip -9°), Brazil. It is shown that short-lived disturbances in F-layer vertical plasma drift, Vz and height (hF/hpF2) indicative of westward and eastward electric fields prevail simultaneously in the dusk (18–21 LT) and predawn (02–05 LT) sectors, respectively, in association with the decay phase of asymmetic ring current events. Electric fields of opposite polarity do also seem to manifest at these local times, particularly in the early-morning sector in conjunction with the intensification of the asymmetric ring current. At a given location, electric field disturbances associated with individual asymmetric ring current events are thus bipolar in nature, with fields of opposite polarity during the growth and decay phases. The nature and polarity structure of the observed electric field disturbances are in agreement with the theoretical/model predictions of prompt penetration of high-latitude electric fields to the equatorial region.On sabbatical leave from Indian Institute of Astrophysics, Bangalore, India     

On the role of collective interactions in asymmetric ring current formation

The contribution of resonant wave-particle interactions to the formation and decay of the magnetospheric ring current is analysed in the framework of a self-consistent set of equations which take into account azimuthal plasmasphere asymmetry. It is shown that the cyclotron interaction of westward drifting energetic protons with Alfven waves in the evening-side plasmaspheric bulge region leads to the formation of a ring current asymmetry located near 18:00 MLT. The time-scale of this asymmetry is determined by the proton drift time through the plasmaspheric bulge and is about 1 - 3 h. A symmetrical ring current decays mainly due to charge exchange processes. The theory is compared with known experimental data on ions and waves in the ring current and on low-latitude magnetic disturbances. New low-latitude magnetometer data on the magnetic storm of 24 - 26 July 1986 are also discussed. The model presented explains the observed localization of an asymmetrical ring current loop in the evening sector and the difference in relaxation time-scales of the asymmetry and the D_st index. It also explains measured wave turbulence levels in the evening-side plasmasphere and wave observation statistics.     

The generation of baroclinic Rossby waves by stationary and and translating currents part 1. Stationary current forcing

Abstract

This paper investigates the generation of linear, baroclinic Rossby waves by an imposed current distribution, in a reduced gravity ocean, both with and without an eastern coast. A zonal current is impulsively applied and maintained along the northern edge of the domain of solution. Using Green's function techniques, analytical solutions are found, and these are evaluated for small times. Numerical solutions are obtained for larger times. The upper layer depth field consists of a transient response, due to the sudden application of the current. Maintenance of the current causes a response which is singular along the line of imposed non-zero h _y. The interior field decays with time (this is shown asymptotically). The parameters used are appropriate for the mid-latitude North Pacific, and the results are relevant to sudden transport changes in the North Pacific Current.     

Numerical integration of a quasi-geostrophic atmospheric model with an asymmetric zonal current

Summary A two-level quasi-geostrophic model on a -plane, with an initial state characterized by an asymmetric zonal current was integrated numerically for a period of 14 days. Various field variables were computed and compared with the corresponding distributions obtained in two experiments with the same model and with initial states characterized by a constant zonal flow and by a flow varying sinusoidally with latitude.The research in this report has been sponsored wholly by the Air Force Cambridge Research Laboratories under Grant AF EOAR 63-108 through the European Office of Aerospace Research (OAR), US Air Force.     

Modeling flows in permeable pressure systems and channels

The study is focused on the velocity and pressure distributions in flows in pipeline systems, as well as hydrological parameters of open flow examined on the basis of Saint Venant equations with allowance made for lateral inflow and the equations of motion of incompressible fluid in perforated or porous pipes. Some analytical solutions are given for the formulated boundary problems, which can be applied to the calculation of operation regimes of sprinkling machines of the “Fregat” or “Valey” type.     

Effects of supplemental viscous damping on inelastic seismic response of asymmetric systems

This paper investigates the effects of supplemental viscous damping on the seismic response of one‐storey, asymmetric‐plan systems responding in the inelastic range of behaviour. It was found that addition of the supplemental damping reduces not only deformation demand but also ductility and hysteretic energy dissipation demands on lateral load resisting elements during earthquake loading. However, the level of reduction strongly depends on the plan‐wise distribution of supplemental damping. Nearly optimal reduction in demands on the outermost flexible‐side element, an element generally considered to be the most critical element, was realized when damping was distributed unevenly in the system plan such that the damping eccentricity was equal in magnitude but opposite in algebraic sign to the structural eccentricity of the system. These results are similar to those noted previously for linear elastic systems, indicating that supplemental damping is also effective for systems expected to respond in the inelastic range. Copyright © 2001 John Wiley & Sons, Ltd.     

Hard X-ray generation in the turbulent plasma of solar flares

The influence of scattering of accelerated electrons in the turbulent plasma on the transformation of their distribution function is studied. The turbulence is connected with the emergence of magnetic inhomogeneities and ion-sound mode. The level of ion-sound turbulence is specified by the ratio W ^s/nk _B T _e = 10^?3, while the value of magnetic fluctuations is δB/B = 10^–3. Different initial angular distributions of the function of accelerated-electron source are regarded: from isotropic to narrow directional distributions. For the chosen energy-density values of the ion-sound turbulence and the level of magnetic fluctuations, it is shown that both types of turbulence lead to a qualitative change in the hard X-ray brightness along the loop, moreover their influence was found to be different. Models with magnetic fluctuations and the ion sound can be distinguished not only by the difference in the hard X-ray distribution along the loop but also by the photon spectrum.     

On transmission of pressure to ocean bottom for generation of microseisms

Summary Problem of transmission of pressure due to surface waves, to the ocean bottom has been discussed. It has been shown that instability in storm troubled surface area really helps in producing microseisms, for then there is facility for pressure being transmitted to the bottom. Considering non-uniform rate of change of sound velocity inside the ocean, the condition derived for transmission of pressure to the bottom is that waves of at least 700 ft long wavelengths should be produced by the storm.