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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1983

Hall current effect on tearing mode instability

From a linear 2-D eigenmode analysis, it is found that the Hall current effect on collisional tearing mode instability becomes important for the thin magnetic reversal layer whose width is comparable to the ion inertia length; Hall currents produce a three-dimensional field structure and increase the reconnection (growth) rate. Since the magnetaic reversal layer widths both in the magnetopause and in the magnetotail are reported to become as thin as the ion inertial length (several hundred km) when the reconnection process is supposed to occur, the Hall current effect may explain the appearance of the dawn-dusk component of the magnetic field in the magnetotail reconnection region.

Terasawa, T.↗
NASA NTRS1993

Low-frequency mobility response functions for the central plasma sheet with application to tearing modes

Consideration is given to the effect of constant cross-tail magnetic field By on the collisionless conductivity produced by chaotic scattering and stochastic diffusion of particles in the current sheet for a parabolic geometry. It is shown that the correlation time scales as (By/Bz)-squared, and from this strong By scaling a strong tendency toward stabilization of the linear tearing modes with increasing values of By is inferred. This effect of increased dawn-dusk mobility is particularly dramatic when electrons are introduced in the calculation, and is in agreement with the results of kinetic particle simulations. The collisionless conductivity is expressed in terms of the ensemble-averaged power spectrum of the single particle trajectories, which makes it possible to calculate directly the linear conductivity instead of deriving it from the calculation of the irreversible heating rates.

Hernandez, J.↗
NASA NTRS1984

Nonlinear evolution of the resistive tearing mode

The nonlinear behavior of the tearing instability is investigated with the aid of numerical solutions of the resistive incompressible magnetohydrodynamic equations. This study is directed toward solar applications, such as flares. Simulations were carried out for values of the Lundquist number S from 100 to 1,000,000 and wavelength parameter alpha from 0.042 to 0.5. The influence of linear conditions, such as the constancy of psi, on nonlinear growth characteristics is investigated. For high S and low alpha, secondary-flow vortices, opposite in direction to the linear vortices, were found to generate a new magnetic island centered at the initial X point. The nonlinear spatial distributions of the physical variables differ greatly from the linear behavior, and growth slows considerably once nonlinear effects become important.

Steinolfson, R. S.↗
NASA NTRS1983

Energetics and the resistive tearing mode - Effects of Joule heating and radiation

The contribution of energy flux to the dynamics of magnetic field reconnection is analytically studied in order to determine the influence of Joule heating and radiation on the linear development of the tearing instability in slab geometry. A temperature-dependent Coulomb-like resistivity is used to provide the coupling between the dynamics and the energy equation. Analytical expressions are derived for the growth rates utilizing constant-psi and long-wavelength approximations. The solutions indicate the occurrence of several modes in addition to the usual tearing mode, several of which have relatively slow, complex growth rates. At large values of the magnetic Reynolds number, there are at least two modes with purely exponential growth when the radiative loss decreases with increasing temperature. If the radiation is neglected, the Joule heating alone also results in two modes with real, positive growth at large S. Below a particular value of S, all the modes are generally stabilized.

Steinolfson, R. S.↗
NASA NTRS1995

Investigation of spherical tearing mode

The purpose of this research was to better understand tearing and reconnection in genuinely three-dimensional configurations. We have identified an equilibrium model that should contain the required features. Three papers have been written and a fourth is in preparation. They are listed in the bibliography.

Greene, John M.↗
NASA NTRS1983

The growth of the tearing mode - Boundary and scaling effects

A numerical model of resistive magnetic tearing is developed in order to verify and relate the results of the principal approximations used in analytic analyses and to investigate the solutions and their growth-rate scalings over a large range of primary parameters which include parametric values applicable to the solar atmosphere. The computations cover the linear behavior for a variety of boundary conditions, emphasizing effects which differentiate magnetic tearing in astrophysical situations from that in laboratory devices. Eigenfunction profiles for long and short wavelengths are computed and the applicability of the 'constant psi' approximation is investigated. The growth rate is computed for values of the magnetic Reynolds number up to a trillion and of the dimensionless wavelength parameter down to 0.001. The analysis predicts significant effects due to differing values of the magnetic Reynolds number.

Steinolfson, R. S.↗
NASA NTRS1992

Topological variation in the magnetic field line at the dayside magnetopause

This paper uses normal mode analysis to examine topological changes in magnetic field lines at the magnetopause. For a modified Harris sheet, the normal modes include a symmetric solution (tearing mode) and an antisymmetric solution (twisting mode). Both modes form flux ropes. The tearing mode in the presence of shear flow is studied. By modeling the external region plasma with drift kinetic formalism, the growth rate of the tearing mode is found to be reduced by shear flow. On the other hand, the shear flow generates a Kelvin-Helmholtz instability which makes the magnetic field perturbation a twisting mode structure. When magnetopause parameters are used, it is difficult to induce a tearing mode in the presence of shear flow, though a shear flow twisting mode did grow under these conditions.

Wang, Zhi↗
NASA NTRS1988

Turbulent reconnection

The reconnection rate in the presence of tearing mode turbulence is calculated. Tearing modes can produce turbulence in three-dimensional sheared magnetic fields. They produce a turbulent anomalous electron viscosity or hyperresistivity. Using quasi-linear and direct interaction approximations, the scaling of the hyperresistivity with magnetic perturbation strength and other parameters is estimated. This yields a reconnection rate, or inflow Mach number M, scaling as delta B, the ratio of rms magnetic fluctuations to mean magnetic field. This can greatly exceed the Sweet-Parker rate. The rate of heating of the current sheet is also large and can be many orders of magnitude larger than in a Sweet-Parker current sheet. Hyperresistivity produced by tearing mode turbulence may explain both fast reconnection and the heating of the solar corona.

Strauss, H. R.↗
NASA NTRS1994

Simulation of magnetic field line stochasticity at the magnetopause

We have conducted a three-dimensional particle simulation to study the magnetic field line stochasticity at the magnetopause current layer. Our results show that the magnetic field lines become stochastic due to the overlap of the destabilized multiple tearing mode islands, which agrees with the percolation model suggested by Galeev et al. (1986). After the field lines become stochastic, these tearing modes grow even 2-3 times faster than in the linear stage and saturate at an amplitude level 3-4 times bigger than the single tearing mode without mode-mode coupling. The field line stochasticity also causes a strong particle diffusion across the current layer. The diffusion coefficient reaches to 10(exp 9) sq m/s for typical magnetopause parameters. Associated with the particle diffusion, the current layer becomes broader in width. As a result, the magnetic energy is dissipated into particle energy by heating parallel to the local magnetic field. The particle energy increases by 60%, while the magnetic helicity, which has always been regarded as a good invariant, changes by 20%.

Wang, Zhi↗
NASA NTRS1991

Does ion tearing exist?

Theoretical models of reconnection in the geomagnetic tail and its relationship to the collisionless tearing mode are examined analytically. The focus is on attempts to remove the stabilizing effects of electron compressibility in the quasi-neutral sheet. It is shown that this goal can be achieved by introducing spatial diffusion of electrons across the magnetic-flux surfaces, but not by wave-turbulent pitch-angle scattering (Coroniti, 1980) or nonadiabatic stochastic first-invariant diffusion (Buechner and Zelenyi, 1988). Hence the ion tearing mode does not exist, but the electron tearing mode does.

Pellat, R.↗
NASA NTRS1991

A numerical simulation of magnetic reconnection and radiative cooling in line-tied current sheets

Radiative MHD equations are used for an optically thin plasma to carry out a numerical experiment related to the formation of 'postflare' loops. The numerical experiment starts with a current sheet that is in mechanical and thermal equilibrium but is unstable to both tearing-mode and thermal-condensation instabilities. The current sheet is line-tied at one end to a photospheric-like boundary and evolves asymmetrically. The effects of thermal conduction, resistivity variation, and gravity are ignored. In general, reconnection in the nonlinear stage of the tearing-mode instability can strongly affect the onset of condensations unless the radiative-cooling time scale is much smaller than the tearing-mode time scale. When the ambient plasma is less than 0.2, the reconnection enters a regime where the outflow from the reconnection region is supermagnetosonic with respect to the fast-mode wave speed. In the supermagnetosonic regime the most rapidly condensing regions occur downstream of a fast-mode shock that forms where the outflow impinges on closed loops attached to the photospheric-like boundary. A similar shock-induced condensation might occur during the formation of 'postflare' loops.

Forbes, T. G.↗
NASA NTRS1990

Collisionless tearing instability in magnetotail plasmas

The problem of the linear stability of collisionless tearing modes in the earth's magnetotail is revisited. It is found that the collisionless tearing mode is linearly unstable with wavelengths of the order of 10 R(E). It is shown that an important feature neglected in earlier theories is a nonzero equilibrium B(y)-field. The physics of the instability is elucidated in the context of a simple slab model and a sheared parabolic model which is representative of the magnetotail in which all three components of the magnetic field are nonzero. The dispersion equation for the instability is obtained by a boundary-layer analysis. The implications of the theory for recent observations on current disruption and diversion during substorms is discussed.

Wang, Xiaogang↗
NASA NTRS1994

On the energy principle and ion tearing in the magnetotail

We re-examine the use of the energy principle as applied to the tearing instability in the magnetotail. We demonstrate that when a magnetic field component normal to the current sheet is present, electron pitch-angle diffusion (PAD) either by micro-turbulence or by chaotic orbits cannot remove the strong stabilization of the tearing mode caused by electron compressibility. We find that our conclusions are in agreement with those of Pellat et al. (1991), who argued on the basis of canonical P(sub y) conservation that the stabilization of the ion tearing mode cannot be removed by the introduction of PAD. Our results are at variance with those of Kuznetsova and Zelenyi (1991), who argued that the application of the energy principle used by Pellat et al. (1991) is incorrect, and that tearing is in fact unstable in the limit of strong PAD. We show that the disagreement between these two studies can be traced to an incorrect orbit evaluation first introduced by Coroniti (1980) and subnsequently used by Kuznetsova and Zelenyi (1991).

Brittnacher, M.↗
NASA NTRS1992

Is the earth's magnetotail balloon unstable?

In the past, the onset of magnetospheric substorms has been attributed to the plasma tearing mode instabilities. This paper investigates the ideal MHD ballooning instability of the near-and middle-tail magnetosphere region, as a first step toward determining whether it could trigger the tearing mode, by using the energy principle to investigate whether standard 2D tail models with the 'hard' ionospheric boundary condition are unstable to ballooning instability. Numerical results are presented for compressible ballooning modes that are symmetric about the center of the current sheet. It is shown that, for such a hard boundary condition, no reasonable magnetotail configuration exists that would be unstable to compressible symmetric ballooning but stable against interchange.

Lee, D.-Y.↗
NASA NTRS1984

Magnetotail energy storage and the variability of the magnetotail current sheet

The antiparallel field configuration in the earth magnetotail is a prime location to search for the important astrophysical process known as magnetic reconnection. The magnetic evidence that reconnection occurs in the earth's magnetotail is that the energy of the tail invariably decreases at the time of global substorm onsets, and that an increased number of closed field lines are seen in the near-earth magnetotail after substorms. This increased northward flux is often convecting earthward from the expected site of the X line after the substorm onset, while southward flux is often convecting tailward. Prior to a substorm, energy accumulates in the tail as the field assumes a more taillike configuration with a smaller field component across the equatorial plane. The high-beta plasma sheet becomes thin and has been observed to be at least as thin as a few thousand km near substorm onset times. The small Bn component and thin plasma sheet are the conditions which favor the onset of the tearing-mode instability, which is thought to lead to neutral-line formation and the reconnection associated with substorm onset. Intervals of weak and highly variable fields in the plasma sheet provide evidence for the tearing-mode instability.

Fairfield, D. H.↗
NASA NTRS1987

Kinetic aspects of tail dynamics - Theory and simulation

Kinetic theories relevant to the geomagnetic tail are reviewed. The topics discussed include kinetic instabilities, simulations, and current-sheet particle acceleration. Tearing mode and reconnection theories are emphasized. Kinetic treatment is appropriate for these topics since the tail plasma is collisionless. Fluid calculations are appropriate when stochastic processes dominate and for studies where long wavelengths are important. However, fluid treatments of tearing modes and reconnection require a finite resistivity in the diffusion region. Thus, although 'anomalous resistivity' can be guessed or in some cases calculated, ideally the kinetic treatment is often to be preferred. Particle motion and acceleration in the current sheet can give rise to beam-like distributions in the plasma-sheet boundary layer. Studies of current-sheet particle motion have also been used as the basis for 'kinetic' tail equilibrium models. Furthermore, quite recently current-sheet particle motion is used directly in Coroniti's explosive tail reconnection model. The 'inertial conductivity' from the equilibrium models provides the 'dissipation' necessary for reconnection.

Speiser, T. W.↗
NASA NTRS1983

Transient brightenings of interconnecting loops. III Interpretation

Two alternative interpretations of the sudden X-ray brightenings observed in loops that interconnect active regions are presented. A fast tearing mode may be excited in those newly formed interconnecting loops within which sufficient magnetic free energy is stored to drive the mode. Alternatively, anomalous Joule heating driven by an inductive electric field parallel to the magnetic field, varying on a time scale of order of a minute, may cause the brightenings. It is suggested that it is plausible that the fast tearing mode may be the cause of brightenings in the young newly formed interconnecting loops, whereas the anomalous Joule heating might occur in old loop connections when an exteral disturbance propagates through them.

Spicer, D. S.↗