Evolution Of Current Sheet Research Articles

Experimental study of current sheet evolution in magnetic configurations with null-points and X-lines

A review is presented on experimental research of current sheet formation in 3D magnetic configurations containing null-points and/or singular lines of the X-type. Formation of current sheets is revealed to occur in various 3D configurations, both with and without isolated magnetic null-points, specifically in configurations with X-lines. Local characteristics of 3D magnetic configuration define the parameters of the current sheet, which forms usually a twisted surface with an angular orientation determined by a local value of the transverse magnetic field gradient. A degree of plasma compression into the sheet decreases with a rise of longitudinal magnetic field component, displaying a transition to a behavior of uncompressible plasma. It is established that current sheet formation and magnetic reconnection processes can take place within a limited range of initial conditions, while a gradient of transverse magnetic field is the most important parameter.

Evolution of thin current sheet with a southward interplanetary magnetic field studied by a three‐dimensional electromagnetic particle code

This paper reports the spatial and temporal development of sheet current intensification in the near‐Earth tail using our three‐dimensional global electromagnetic particle simulation with a southward turning interplanetary magnetic field (IMF) in a context of the substorm onset. As the solar wind with the southward IMF advances through the Earth, the near‐Earth tail thins and the sheet current intensifies. The peak of the current density moves toward the Earth. Before its peak becomes maximum the reconnection takes place. The earthward flows are generated by the reconnection, which coincides with the earthward expansion of the intensified sheet current. Because of the pileup of the earthward plasma flows, the dawnward current is generated near the Earth. This dawnward current dissipates rapidly with the sheet current because of the opposite current direction, which coincides with the dipolarization in the near‐Earth tail. This simulation study shows the sequence of the substorm dynamics in the near‐Earth tail, which is similar to the features obtained by the multisatellite observations. The identification of the timing and mechanism of triggering substorm onset requires further studies in conjunction with observations.

Correction to “Hall‐MHD modeling of near‐Earth magnetotail current sheet thinning and evolution” by L. Rastätter et al.

Journal of Geophysical Research: Space PhysicsVolume 105, Issue A5 p. 10787-10787 CorrectionsFree Access Correction to “Hall-MHD modeling of near-Earth magnetotail current sheet thinning and evolution” by L. Rastätter et al. L. Rastätter, L. RastätterSearch for more papers by this authorM. Hesse, M. HesseSearch for more papers by this authorK. Schindler, K. SchindlerSearch for more papers by this author L. Rastätter, L. RastätterSearch for more papers by this authorM. Hesse, M. HesseSearch for more papers by this authorK. Schindler, K. SchindlerSearch for more papers by this author First published: 01 May 2000 https://doi.org/10.1029/2000JA000020AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume105, IssueA51 May 2000Pages 10787-10787 RelatedInformation

Thin current sheet evolution as seen in observations, empirical models and MHD simulations

The inner and midtail current sheet is examined during a substorm on Dec 10, 1996 using multispacecraft observations, empirical models, and global MHD simulations. Both the simulation and empirical magnetic field model show good agreement with observations, which substantially increases the confidence in both methods. Both locate the thin current sheet inner edge at ∼7 RE, and give a scale size of ∼15 RE in tailward and cross‐tail directions. The MHD simulation confirms the empirical model result that the thin current sheet remained stable after a local auroral breakup and indicates that the activity followed from flow channels generated at ∼40 RE independent of the thin current sheet evolution closer to the Earth.

Evolution of current sheets and regularity of ideal incompressible magnetic fluids in 2D

The ideal incompressible magnetohydrodynamic equations in two dimensions are studied analytically and numerically. We focus on a standard scenario of singularity formation where level sets of the magnetic stream function contain a hyperbolic saddle. Analytically we obtain an exponential estimate on the closing of the angle under the assumption that the velocity remains bounded. This result is supported by high-resolution numerical simulations using adaptive structured mesh refinement. © 2000 John Wiley & Sons, Inc.

Evolution of current sheets and regularity of ideal incompressible magnetic fluids in 2D

The ideal incompressible magnetohydrodynamic equations in two dimensions are studied analytically and numerically. We focus on a standard scenario of singularity formation where level sets of the magnetic stream function contain a hyperbolic saddle. Analytically we obtain an exponential estimate on the closing of the angle under the assumption that the velocity remains bounded. This result is supported by high-resolution numerical simulations using adaptive structured mesh refinement. © 2000 John Wiley & Sons, Inc.

Numerical simulation of current sheet creation above real active regions

The current sheet creation above the preflare active region 20 MAY, 1991 has been numerically simulated in 3D MHD approximation. A current sheet development, and plasma ejection along the sheet is shown.

Spatial extent and dynamics of a thin current sheet during the substorm growth phase on December 10, 1996

Multispacecraft observations are analyzed to examine the dynamics of thin current sheet evolution during the early phases of a substorm during the period 0630–0800 UT on December 10, 1996. The ground signatures showed two activations, one latitudinally localized at 0731 UT and another, larger expansion around 0800 UT. A local model consisting of two current sheets (a thin current sheet and a thicker plasma sheet) was developed utilizing two‐spacecraft magnetic field measurements within the magnetotail. Extension of the thin current sheet model to the inner magnetotail shows that during most of the growth phase, the thin current sheet intensified simultaneously and at the same rate in the mid‐magnetotail (∼30 RE) and near geosynchronous orbit. These results show that a thin current sheet extended from the near‐geostationary region to the midtail, and that the entire current sheet responded to the driving in a coherent way. Furthermore, timings suggest that the onset‐associated disturbances were observable in the midtail before the ground or geosynchronous signatures.

Hall‐MHD modeling of near‐Earth magnetotail current sheet thinning and evolution

The formation of thin current sheets is a common feature of the late substorm growth phase in the magnetotail of the Earth. The location and the structure of the evolving thin current sheet determine the onset and the dynamic phase of magnetic substorms. To determine the formation and further evolution of thin current sheets in the near‐Earth magnetotail, we employ a two‐fluid model of electrons and ions, the Hall‐MHD. We start from a two‐dimensional tail‐equilibrium model and apply a plasma inflow from the lobes to simulate the presubstorm loading process of the tail. Our results confirm recent 2.5‐dimensional particle and hybrid simulations which have shown thin current sheet formation, with the majority of the new current supported by the electrons. The Hall‐MHD simulations are extended to a current sheet thinning model with three‐dimensional driving conditions and different magnetic field boundary conditions. We investigated the changes to the tail magnetic and current structure brought about by the cross‐tail magnetic field components generated by Hall electric fields. For all these cases, the additional current is carried by the electrons, with very small effects of magnetic field and driving boundary conditions.

Plasma dynamics with electron-inertia effects

Some aspects of plasma dynamics with electron-inertia effects are considered. The behaviour of (i) one-dimensional steady current sheets, (ii) two-dimensional steady reconnection models, (iii) one-dimensional current-sheet evolution, (iv) linear tearing-mode evolution and (v) nonlinear magnetic-island evolution in this regime is investigated. Stability of the two-dimensional plasma in this regime is then discussed, and necessary conditions for linear stability in the Liapunov sense for steady states are then given.

Reconnection in adjoining coronal helmet streamers

Complex magnetic and plasma structures observed in the coronal streamer belt (Crooker et al., 1993; Woo 1994) might arise from the instabilities and evolution of multiple current sheets formed by adjoining coronal helmet streamers. Previously we examined the static triple current sheet (TCS), and found that three linearly unstable modes exist, two of which are potentially observable by the LASCO instrument onboard SOHO (Dahlburg and Karpen 1995). Here we investigate the variations created in this model by the inclusion of wake flows, which have been observed in coronal streamers (see Figure 1). Our principal finding is that the structure of the modes is changed significantly by the Alfvénic and sub-Alfvénic wake flow, while their growth rates are not.

Anomalous Electric Fields Inside a Dense Plasma of a Current Sheet

AbstractFor the first time there have been analyzed profiles of electric‐field‐sensitive na spectral lines of the Li‐like impurity ions CIV, NV, OVI emitted by a hot dense plasma of a plane current sheet. Theoretical profiles of these lines have been calculated with an allowance for Anomalous Electric Fields (AEF). By comparing the experimental and theoretical profiles of the lines it has been established for the first time that in the plane current sheet there were developed AEF of rms strengths F0 ≈ (100–120) kV/cm in the time interval from τ = −0.2 μs to τ = 0 (the time τ was counted from the instant of the maximum intensity of the spectral line OVI 5291 Å). These results have been backed up by a similar analysis of the experimental profiles of the spectral line HeII 4686 Å of the primary gas of the discharge. The AEF reached these strengths at the same instant of time, τ = −0.2 μs, when electron and ion temperatures reached their maxima: Te ≫ 100 eV, Ti ≈ 300 eV. The instant τ = −0.2 μs corresponds to a transition from the metastable stage to the explosive phase of the current sheet evolution. At this time the ratio of the AEF energy density to the thermal energy density of the plasma was § ≡ F20/[8π/NeTe + NiTi)] ≈ 10−3, which exceeded a similar ratio for the thermal wave field by three orders of magnitude.

Evolution and statistics of current sheets in coronal magnetic loops

Evolution and statistics of current sheets in coronal magnetic loops

The coalescence instability and the development of current sheets in two-dimensional magnetohydrodynamics

A class of two-dimensional force-free equilibria are shown to be subject to an ideal linear instability. This instability is similar to the coalescence instability for chains of magnetic islands. Semianalytic methods are used to find second magnetic equilibrium which is accessible from the first and has lower magnetic energy. This equilibrium has a discontinuous magnetic field and current sheets at these discontinuities. Numerical simulations indicate that the nonlinear evolution of the instability allows the plasma to relax to this discontinuous equilibrium.

The thermal and plasma-physical evolution of laminar current sheets formed in the solar atmosphere by emerging flux

view Abstract Citations (6) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Thermal and Plasma-physical Evolution of Laminar Current Sheets Formed in the Solar Atmosphere by Emerging Flux Larosa, T. N. Abstract A time-dependent analysis of emerging flux is carried out, and the time evolution of both the current sheet energetics and the plasma state is calculated. This evolution is determined in two different regimes. In the first case the width of the current sheet is assumed to be independent of the sheet thermodynamics and is fixed by the initial conditions. In the second, the width of the current sheet is a function of the resistivity and is allowed to decrease to its minimum given by the electron gyroradius. In both cases the resistivity is computed according to the marginal stability hypothesis. In each case the thermodynamic evolution is found to be quite rapid, with the temperature increasing from 10,000 to 1,000,000 K in a second or less. In contrast to previous studies, it is found that the resistivity is not significantly enhanced by the current-driven plasma wave turbulence. It is concluded that a laminar current sheet cannot be responsible for the activity associated with emerging flux. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171716 Bibcode: 1992ApJ...396..289L Keywords: Current Sheets; High Temperature Plasmas; Magnetic Flux; Solar Activity Effects; Solar Atmosphere; Solar Flares; Electron Energy; Ion Temperature; Time Dependence; Solar Physics; MAGNETOHYDRODYNAMICS: MHD; SUN: FLARES; SUN: MAGNETIC FIELDS full text sources ADS |

Current sheet formation in a sheared force-free-magnetic field

This paper presents the results of a study showing how continuous shearing motion of magnetic footpoints in a tenuous, infinitely conducting plasma can lead to the development of current sheets, despite the absence of such sheets or even of neutral points in the initial state. The calculations discussed here verify the earlier suggestion by Low and Wolfson (1988) that extended current sheets should form due to the shearing of a force-free quadrupolar magnetic field. More generally, this work augments earlier studies suggesting that the appearance of discontinuities - current sheets - may be a necessary consequence of the topological invariance imposed on the magnetic field geometry of an ideal MHD system by virtue of its infinite conductivity. In the context of solar physics, the work shows how the gradual and continuous motion of magnetic footpoints at the solar photosphere may lead to the buildup of magnetic energy that can then be released explosively when finite conductivity effects become important and lead to the rapid dissipation of current sheets. Such energy release may be important in solar flares, coronal mass ejections, and other eruptive events.

Evolution of current sheets near a hyperbolic magnetic neutral point

An exact solution for the magnetohydrodynamic equations for a time-dependent, two-dimensional flow of an incompressible plasma of finite conductivity in a hyperbolic magnetic field is given. This solution admits a nonzero initial current density, and predicts a sequence of events associated with the evolution of a current sheet in a plasma subjected to an initial hyperbolic magnetic field configuration. This is confirmed by laboratory experiments. Next, the calculation is extended to include the density variations in the plasma. The corresponding solution predicts that the current sheet at the neutral point evolves more rapidly when the plasma density builds up locally around the magnetic neutral point.

Evolution of current sheets following the onset of enhanced resistivity

An important aspect of pre-flare current sheets in the solar atmosphere is the sudden enhancement of the effective electrical resistivity in the sheet due to the onset of a plasma micro-instability. Numerical and analytical solutions to the isothermal MHD equations are here presented that describe the evolution of a current sheet subsequent to such an enhancement in the resistivity. The solutions show that, if the initial width of the current sheet is less than the acoustic-diffusion length obtained by dividing the resistivity by the sound speed, then isomagnetic shocks are formed. These shocks propagate outward from the the centre of the current sheet and are transformed into fast-mode magneto-acoustic waves when they reach the edges of the current sheet. The fast-mode waves thus formed continue to propagate outward beyond the confines of the current sheet. In contrast to a previous study by Cheng, the present solutions demonstrate that flow speeds several times greater than the local fast-mode wave speed can be produced if the plasma beta parameter and the initial sheet width are sufficiently small. The results may be relevant to the triggering of a solar flare, as in the emerging flux model of flares.

Eruptive solar magnetic fields

This paper considers the quasi-steady evolution of solar magnetic fields in response to gradual photospheric changes. Special interest is taken in the threshold of a sudden eruption in the solar atmosphere. The formal model of an evolving, force-free field dependent on two Cartesian coordinates has been treated previously, and we extend it to a field which is not force free but in static equilibrium with plasma pressure and gravity. The basic physics is illustrated by the evolution of a loop-shaped electric current sheet enclosing a potential bipolar field with footpoints rooted in the photosphere. A free-boundary problem is posed and solved for the equilibrium configuration of the current sheet in a hydrostatically supported isothermal atmosphere. As the footpoints move appart to spread a constant photospheric magnetic flux over a larger region, the equilibria available extend the field to increasingly great heights. Two basic behaviors are possible, depending on the ratio of the total magnetic flux to an equivalent flux constructed dimensionally from the pressure difference across the current sheet and the density scale height. For a small, total magnetic flux, nonequilibrium can set in with the appearance of a marginally stable equilibriu, as demonstrated previously for the frece-free fields. Formore » a total magnetic flux exceeding a certain critical value, the field lines rise high enough for gravity to play a significant role. The sequence of equilibria in this case suggests that nonequilibrium can set in with the opening of the field lines by magnetic buoyancy. This eruption can also take place with a prominence filament and may be the origin of the white light coronal transient.« less

Preflare conditions, changes and events

Prefiare conditions, changes and events are loosely categorized as distinct, evolutionary or statistical. Distinct preflare phenomena are those for which direct physical associations with flares are implied. Also, they are not known to occur in a like manner during the absence of flares. These include the early stage of filament eruptions within active centers, preflare vortical structures, some transient X-ray emitting features, 5303 A accelerating coronal arches, and increases in circular polarization at cm wavelengths. Evolutionary preflare changes are considered to be any long-term effect that may be related to the flare build-up even though the same changes may occur in the absence of flares. This category covers the development of current sheets or strongly sheared magnetic fields, evolving magnetic features, emerging flux regions, the development of satellite fields around sunspots, the evolution of reverse polarity field configurations, the merging of adjacent active centers, sunspot motions and the development of velocity patterns. Statistical preflare changes logically include both distinct and evolutionary preflare changes. However, in addition, there are preflare conditions and events that are not necessarily linked to the flare in either a direct physical or indirect evolutionary way. Such parameters or events that may only be statistically significant are certain magnetic field properties, the brightness of active centers at various wavelengths, the previous occurrence of flares and subflares, increased turbulence in filaments and certain radio events.