Three-dimensional Current System Research Articles

Coordinated layout planning of ocean current turbines and collector system considering spatial velocity distribution

Ocean current generation has become a popular form of supplying electric energy by using fluid kinetic energy. For ocean current, the key difference from other planning can be embodied in the turbine layout planning region, which needs to be considered as a three-dimensional area. Considering that, this paper investigates the three-dimensional ocean current turbines and collector system coordinated layout planning model. First, a velocity-depth curve of interest current area is determined by fitting historical velocity data. Then, a three-dimensional ocean current wake model is derived through the integral form of momentum conservation to analyze the influence of the wake effect on the velocity of the three-dimensional distributed turbine array. Finally, a two-level three-dimensional location and capacity planning model is established which involve three-dimensional topology constraints, with the upper-level goal of minimizing the whole system cost, and the lower-level goal of minimizing the cost of power collection topology. The accuracy and effectiveness of the proposed model are investigated through the case study on the current data of the Gulf of Mexico.

Dipolarization Flux Bundles

Localized fast plasma flows in the geomagnetic tail (bursty bulk flows, BBF, and dipolarizing flux bundles, DFB, having smaller spatial scale) which have been observed in recent years onboard CLUSTER and THEMIS spacecraft, are closely related to processes producing thin current sheets observed on the same satellites. This follows from our previous theoretical analysis and numerical simulation of processes leading to generation of thin current sheets. The theory also makes it possible to explain essential deviations in the behavior of DFB from predictions of the MHD simulation carried out in recent years by other authors. In the framework of the two-fluid model, such deviations can be understood taking into account the ion inertial drift current in a localized three-dimensional current system of DFB.

A self-consistent synthesis description of magnetosphere-ionosphere coupling and scale-dependent auroral process using shear Alfvén wave

[1] In order to correctly describe the dynamical behavior of the magnetosphere-ionosphere (MI) coupling system and the scale-dependent auroral process, we develop a synthesis formulation that combines the process of (1) the inverse Walen separation of MHD disturbance into parallel- and antiparallel-propagating shear Alfven wave to the ambient magnetic field, (2) the shear Alfven wave reflection process including (3) the scale-dependent electrostatic coupling process through the linearized Knight relation, (4) two-layer ionosphere model, and (5) dynamic conductance variations. A novel procedure that applies the inverse Walen relation to the incompressional MHD disturbances at the inner boundary of the MHD region enables to extract the component of the shear Alfven wave incident to the ionosphere. The extracted incident electric field supplies an electromotive force for the generation of the MI coupling system, and the reflected electric field is generated such that it totally satisfies the synthesis MI-coupling equation. A three-dimensional ionospheric current system is represented by a two-layer model in which the Pedersen and the Hall current are confined in the separated layers, which are connected by field-aligned currents driven by the linear current-voltage relation between two layers. Hence, our scheme possibly reproduces two types of the scale-dependent MI-decoupling process of the perpendicular potential structure: due to the parallel potential drop at the auroral acceleration region and the other due to the parallel potential differences inside the ionosphere. Our newly formulation may be well suited for description of scale-dependent auroral process and mesoscale ionospheric electrodynamics interlocked with the dynamical development of magnetospheric processes.

The evolution of fast reconnection in a three-dimensional current sheet system

By showing the details of the numerical procedure, global dynamics of the current sheet system is systematically studied by three-dimensional magnetohydrodynamic simulations in the parameter range where the numerical resistivity is much smaller than the physical resistivity. In the absence of resistivity, initiated by a reconnection disturbance, current sheet thinning drastically occurs because of the sheet pinch, leading to extreme increase in current density around the X neutral point. For the uniform resistivity model, the drastic current increase is suppressed by the magnetic diffusion (reconnection), but the reconnection jet cannot be accelerated effectively, so that any fast reconnection mechanism cannot evolve; for the smaller resistivity, the current density at the X point becomes larger. Once current-driven anomalous resistivities build up, both the reconnection flow and the anomalous resistivity simultaneously grow to enhance each other, eventually giving rise to the Alfvénic fast reconnection jet. However, if the current sheet width is smaller than three times its thickness, the fast reconnection mechanism cannot be realized even in the presence of anomalous resistivity. Hence, only when a thin current sheet of sufficiently large scale is formed in space plasmas, the fast reconnection mechanism is likely to evolve drastically, leading to distinct plasma processes responsible for flares and substorms.

Three-dimensional current systems generated by plasmas colliding in a background magnetoplasma

Results are presented from an experiment in which two plasmas, initially far denser than a background magnetoplasma, collide as they move across the magnetic field. The dense plasmas are formed when laser beams, nearly orthogonal to the background magnetic field, strike two targets. The merging plasmas are observed to carry large diamagnetic currents. A reconnection event is triggered by the collision and the electric field induced in this event generates a field-aligned current, which is the first step in the development of a fully three-dimensional current system. After several ion gyroperiods, the current systems become those of shear Alfvén waves. As local currents move, small reconnection “flares” occur at many locations throughout the volume, but they do not seem to affect the overall system dynamics. The data clearly show that the induced electric field is carried though the system by shear Alfvén waves. The wave electric fields as well as local magnetic helicity are discussed.

Discussion paper: Partial ring current and polarization jet

[1] We analyze the asymmetric part of the ring current (RC), which appears during the main phase of a magnetic storm and is due to strong precipitation of RC ions into the ionosphere. The precipitation is caused by the pitch angle scattering by ion cyclotron waves and is localized in the evening sector inside the storm time plasmaspheric bulge. The asymmetric (partial) RC together with the precipitation decreases the electric shielding of the inner magnetosphere by the region 2 current system and forms an additional threedimensional current system at subauroral latitudes. Using a simplified analytical model, we show that such a current system can be a good candidate for explaining the origin of the polarization jet (subauroral ion drift, PJ/SAID), In particular, spatial-temporal characteristics, ionospheric ion velocities, and electric field values for this current system correspond to those in the PJ.

A 3-D current system of an ionospheric dynamo including a skin effect in an anisotropic plasma

Abstract The numerical two-mode electromagnetic model of an ionospheric dynamo is constructed taking into account a skin effect of a field in an anisotropic plasma, i.e. a non-potential electric field of Sq-variations in the ionosphere. It is shown that a three-dimensional current system generated by the dynamo action of winds is located at altitudes less than about 200 km basically. It does not depend on an ionosphere condition in upper layers and weaker field-aligned currents between the northern and southern hemispheres practically, and an almost complete closed current circuit is generated in the dynamo layer.

Substorm behavior of the auroral electrojet indices

Abstract. The behavior of the auroral electrojet indices AU and AL during classical substorms is investigated by the use of global auroral images. A superposition of the 12 AE stations onto global auroral images and identification of the AL and AU contributing stations enable an understanding of the temporal as well as spatial behavior of the indices with respect to the substorm coordinate system and timeframe. Based on this simple technique it was found that at substorm onset the AL contributing station makes a characteristic jump from a location near the dawn terminator to the onset region, typically bypassing one or more AE stations. During the expansion phase this station typically lies at the poleward edge of the surge region. This is the location of the intense substorm current wedge electrojet in the semiempirical self-consistent substorm model of the three-dimensional current system by Gjerloev and Hoffman (2002). This current wedge is fed primarily pre-midnight by an imbalance of the Region 0 and Region 1 field-aligned currents, not from the dawnside westward electrojet. Then during the early recovery phase the AL contributing station jumps back to the dawn sector. The defining AU station does not show any similar systematic behavior. We also find that the dawn side westward electrojet seems to be unaffected by the introduction of the substorm current wedge. According to our model, much of this current is closed to the magnetosphere as it approaches midnight from dawn. Based on the characteristics of the AL station jumps, the behavior of the dawn-side electrojet, and the understanding of the three-dimensional substorm current system from our model, we provide additional experimental evidence for, and an understanding of, the concept of the two component westward electrojet, as suggested by Kamide and Kokubun (1996).

Derivation of currents and diamagnetic effects from global plasma pressure distributions obtained by IMAGE/HENA

The global ion distributions extracted from ENA images measured by IMAGE/HENA can be used to calculate the plasma pressure. In principle, knowing the plasma-pressure distribution and the magnetic field, one can also calculate the three-dimensional current system that is driven by the pressure gradient. The currents depend on the pressure gradient and the magnetic-field strength. Assuming current continuity ( ∇· J =0 ), we can use Euler potentials for J itself to calculate the current distribution. The method is applied to some pressure distributions measured by IMAGE/HENA and the resulting field-aligned currents (FACs) are compared to the global FAC maps derived from irridium magnetometer data.

IMAGE/HENA: pressure and current distributions during the 1 October 2002 storm

Global energetic neutral atom images (ENA) of the Earth’s ring current can be used to derive the global ion and pressure distribution by applying a constrained linear inversion technique. The ENA images are obtained in the 10–60 keV range by the high energy neutral atom (HENA) onboard the IMAGE spacecraft. We present ion and pressure distributions from the 1 October 2002 storm main phase. The typical storm main phase displays a pressure peak around midnight at L=3−4. We use this pressure distribution to compute the three-dimensional electrical current system. The computed current system is then compared to the region 2 current system obtained by the magnetometers on board the Indium spacecraft.

Transient production of F-region irregularities associated with TCV passage

Abstract. Transient production of F-region plasma irregularities due to traveling convection vortices (TCVs) was investigated using the Super Dual Auroral Radar Network (SuperDARN) combined with ground magnetometer networks and the POLAR ultraviolet imager. We selected two large-amplitude (100–200 nT) TCV events that occurred on 22 May 1996 and 24 July 1996. It is found that the TCV-associated HF backscatter arises in blobs with spatial scale of a few hundreds km. They traveled following tailward bulk motion of the TCV across the three fields-of-view of the SuperDARN HF radars in the prenoon sector. The spectra in the blobs showed unidirectional Doppler velocities of typically 400–600 m/s, with flow directions away from the radar. These unidirectional velocities correspond to the poleward and/or eastward convective flow near the leading edge of upward field-aligned current. The backscatter blobs overlapped the poleward and westward part of the TCV-related transient aurora. It is likely that the transient backscatter blobs are produced by the three-dimensional gradient drift instabilities in the three-dimensional current system of the TCV. In this case, nonlinear rapid evolution of irregularities would occur in the upward field-aligned current region. The spectral width of the backscatter blob is typically distributed between 50 and 300 m/s, but sometimes it is over 400 m/s. This suggests that the temporal broad spectra over 400 m/s are produced by Pc1–2 bursts, while the background spectral width of 50–300 m/s are produced by the velocity gradient structure of convection vortices themselves.Key words. Ionosphere (Electric fields and currents; Ionospheric irregularities; Plasma convection)

Afternoon mid-latitude current system and low-latitude geomagnetic field asymmetry during geomagnetic storms

Abstract. For four geomagnetic storms of middle intensity the relationship between the low-latitude magnetic field asymmetry using ASY indices and the intensity of the auroral eastward and westward electrojet was considered. It was asked whether there exists a connection between ASY and the eastward electrojet. To answer this question equivalent current systems were estimated in mid-latitudes. It was found that the observations obviously show no correlative relationship between the low-latitude magnetic-field asymmetry and the eastward electrojet, whereas one exists between ASY and the westward electrojet. To explain the generally accepted common three-dimensional current system between the partial ring current and the eastward electrojet, a condensor model of the three-dimensional current system was developed. It could be shown that the short periodic variations of the partial ring current are shielded by the condensor and cannot influence the eastward-electrojet current.

Afternoon mid-latitude current system and low-latitude geomagnetic field asymmetry during geomagnetic storms

For four geomagnetic storms of middle intensity the relationship between the low-latitude magnetic field asymmetry using ASY indices and the intensity of the auroral eastward and westward electrojet was considered. It was asked whether there exists a connection between ASY and the eastward electrojet. To answer this question equivalent current systems were estimated in mid-latitudes. It was found that the observations obviously show no correlative relationship between the low-latitude magnetic-field asymmetry and the eastward electrojet, whereas one exists between ASY and the westward electrojet. To explain the generally accepted common three-dimensional current system between the partial ring current and the eastward electrojet, a condensor model of the three-dimensional current system was developed. It could be shown that the short periodic variations of the partial ring current are shielded by the condensor and cannot influence the eastward-electrojet current.

Ionospheric Electrodynamics Using Magnetic Apex Coordinates.

The equations of ionospheric electrodynamics are developed for a geomagnetic field of general configuration, with specific application to coordinate systems based on Magnetic Apex Coordinates. Two related coordinate systems are proposed: Modified Apex Coordinates, appropriate for calculations involving electric fields and magnetic-field-aligned currents; and Quasi-Dipole Coordinates, appropriate for calculations involving height-integrated ionospheric currents. Distortions of the geomagnetic field from a dipole cause modifications to the equations of electrodynamics, with distortion factors exceeding 50% at some geographical locations. Under the assumption of equipotential geomagnetic-field lines, it is shown how the field-line-integrated electrodynamic equations can be expressed in two dimensions in magnetic latitude and longitude, and how the height-integrated and field-aligned current densities can be calculated. Expressions are derived for the simplified calculation of magnetic perturbations above and below the ionosphere associated with the three-dimensional current system. It is shown how the base vectors for the Modified Apex coordinate system can be applied to map electric fields, plasma-drift velocities, magnetic perturbations, and Poynting fluxes along the geomagnetic field to other altitudes, automatically taking into account changes in magnitude and direction of these vector quantities along the field line. Similarly, it is shown how Quasi-Dipole coordinates are useful for expressing horizontal ionospheric currents, equivalent currents, and ground-level magnetic perturbations. A computer code is made available for efficient calculation of the various coordinates, base vectors, and related quantities described in this article.

DP- Y-field-aligned currents and intensity of the BYIMF penetrated into the magnetotail

The three-dimensional global current system produced by currents deflected towards the Earth from the magnetopause is considered. This system is supposed to be divided into two parts: day- and nightside branches. In the frame of the magnetic field line reconnection model, the current intensity and the magnetic field ( Y-component) produced by the latter in the plasma sheet was estimated. It is found that the ratio of magnetic field intensities in the plasma sheet and in the solar wind equals 0.5, which agrees with experimental data.

Inference of magnetospheric and ionospheric electrical properties from the decay of geomagnetic activity

The three-dimensional current system characteristic of directly driven magnetospheric substorm activity that couples the magnetosphere to the ionosphere can be represented by an LCR circuit where the source of the emf, the capacitance, and the resistance are in parallel. An earlier study (G. Rostoker and F. Pascal. Can. J. Phys. 68, 74 (1990).) has shown that the impulse response of the magnetosphere can be represented by the response of such an LCR circuit to a sudden increase in the crosstail potential drop ε, where the circuit parameters, (ε, L, C, R) are all permitted to vary with time as the current in the circuit asymptotically approaches its new level. In this paper we examine the response of the directly driven system to sudden northward turnings of the interplanetary magnetic field. We find that the directly driven westward electrojet in the dawn sector decays in an approximately exponential fashion following the northward turning, and that the decay time is nearly linearly dependent on the value of the auroral electrojet (AE) at the time of the northward turning. We use the relationship between the characteristic time constant of the decay and the magnitude of AE at the time of the northward turning to develop quantitative expressions describing the magnitudes of the electrical bulk parameters, L, C, and R as a function of the level of magnetospheric activity.

Ground magnetic signatures of multiple field‐aligned current sheets in flux transfer events

The ground magnetic signature of a three‐dimensional ionospheric current system with multiple field‐aligned current sheets is calculated under the assumption of uniform ionospheric conductance. Such a current system and the accompanying spiky electric fields are consistent with the model of flux transfer events proposed by Kan (1988). The magnetic signature of the proposed flux transfer event current system is dominated by the H component with a distribution pattern highly localized under the multiple current sheets. The Z component is weaker and less confined. If the upward current density is significantly greater than the downward current density in each pair of flux transfer event field‐aligned current sheets, the negative Z component perturbation spreads over a much wider area than the positive Z component. If the magnitudes of the current density are reversed, the opposite is true. The D component is 30 nT or less which is probably too small to be detectable.

Field-aligned currents in the undisturbed polar ionosphere

Field-aligned currents, FAC’s, which couple ionospheric currents at high latitudes with magnetospheric currents have become an essential cornerstone to our understanding of plasma dynamics in the polar region and in the earth’s magnetosphere. Initial investigators of polar electrodynamics including the aurora were unable to distinguish between the ground magnetic signatures of a purely two-dimensional current and those from a three-dimensional current system, ergo many scientists ignored the possible existence of these vertical currents. However, data from magnetometers and electrostatic analyzers flown on low-altitude, polar-orbiting satellites proved beyond any reasonable doubt that field-aligned currents existed, and that different ionospheric regions were coupled to different magnetospheric regions which were dominated by different electrodynamic processes, e.g., magnetospheric convection electric fields, magnetospheric substorms and parallel electric fields. Therefore, to define the “undisturbed” polar ionosphere and its structure and dynamics, one needs to consider these electrodynamic processes, to select times for analysis when they are not strongly active and to remember that the polar ionosphere may be disturbed when the equatorial, mid-latitude and sub-auroral ionospheres are not.

Inferring field-aligned current systems and other ionospheric quantities from ground-based arrays: A review

This paper aims at discussing new techniques for the use of ground-based magnetometers to determine the three-dimensional current system that links electrically the magnetosphere and the ionosphere. These “remote sensing” techniques are designed to deduce electrodynamic features at high latitudes. In the updated schemes, several different types of measurements can be incorporated: electric fields from radars and polar-orbiting satellites, conductivities from radars, and the auroral distribution from satellites, as well as magnetic perturbations at the ground at satellite heights.

Effects of Non-radial Geomagnetic Field Lines on Evaluating Ionospheric Fields and Three-dimensional Current System from Ground Magnetic Data

Effects of Non-radial Geomagnetic Field Lines on Evaluating Ionospheric Fields and Three-dimensional Current System from Ground Magnetic Data