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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Radiative and reconnection instabilities - Filaments and flares

The way in which the initial development of solar filament radiative cooling and the magnetic reconnection of a solar flare can occur in the center of a field-shear layer is demonstrated. Since the present treatment unites these two mechanisms, it indicates the common as well as the disparate features they possess. Unstable radiation serves to increase the Coulomb resistivity at the X-point, so that the reconnection is not self-quenching. The surprising dominance of the magnetic component of the perturbation in the midwavelength range indicates the need to examine the nonlinear saturation of the energy transport of the radiative mode, taking the accompanying magnetic reconnection and potential-energy release into account, for comparison with observations of filaments as well as for clues to the character of the preflare state.

Van Hoven, G.↗
NASA NTRS1984

Microstructure of magnetic reconnection in earth's magnetotail

The structure of heated electron reconnection events associated with magnetic substorm events in the earth's magnetotail is examined using IMP 8 spacecraft and ground-based magnetometer, plasma analyzer and spectroscopic data. Plasma, magnetic field and energetic particle data for five events are presented. Reconnection is shown to occur in two phases: preheating and heating. In preheating, lasting about 5 min, a strong tailward plasma flow appears and ends with electron heating. A 1-2 min heating phase starts with electron heating and ends with plasma sheet drop out and/or decay of the electron temperature to pre-event levels. The heating pulse is always connected with a Bx reversal at 30 earth radii tailward, where the reconnection occurs.

Bieber, J. W.↗
NASA NTRS1984

Numerical simulation of reconnection in an emerging magnetic flux region

Numerical solutions in two dimensions are presented for the resistive MHD equations of an initial boundary value problem, simulating reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. This numerical model displays four main phases, which are interpreted in terms of (1) a slowly evolving quasi-steady phase during which most of the magnetic flux emerges, with approximate equilibrium between magnetic and pressure forces; (2) an impulsive phase in which either the reconnection or continuing emergence of the first phase comes to disrupt the equilibrium, and extensive acceleration occurs as the high pressure region within the emerging region drives the fluid upwards and outwards; (3) a second quasi-steady phase; and (4) a potential-static phase in which continuing reconnection, ohmic dissipation, and fluid transport through boundaries depletes the system of all currents and flows.

Forbes, T. G.↗
NASA NTRS1984

Fast spontaneous reconnection by the resistively coupled radiative instability

The thermal and tearing instabilities give rise to the development of filaments and flares in sheared magnetic fields. The coexistence of these physical mechanisms in the case when Coulomb resistivity couples the energy evolution to the plasma dynamics is investigated, and it is found that the analog of the thermal mode, which still develops on the radiative time scale, involves significant magnetic-field reconnection. When compared on the basis of equal magnitudes of nonlinear terms, the fast radiative mode provides some 30 percent of the reconnected magnetic flux associated with the much slower (less than 0.01 in growth rate for solar coronal conditions) tearing mode. This finding opens the possibility for a more rapid initiation of magnetic reconnection, with resulting energy release, than had previously been thought feasible.

Steinolfson, R. S.↗
NASA NTRS1984

Magnetic reconnection in comets

Today many of the traditionally puzzling phenomena in the cometary plasma-tail environment can plausibly be linked to magnetic reconnection occurring in several regions of a comet (Niedner and Brandt, 1978 and 1980). The turn-on of these various reconnection sites appears to follow a cyclic pattern in which the plasma-tail disconnection event is the primary feature, and the periodic sector structure of the solar wind is the external driver. The purpose of this review is to discuss these different classes of cometary activity, to state the justifications for linking them to reconnection, to discuss proposed alternate (nonreconnection) models, and to suggest future tests of the hypotheses presented.

Niedner, M. B., Jr.↗
NASA NTRS1984

Magnetic field reconnection at the magnetopause - An overview

The basic qualitative and quantitative aspects of reconnection in its magnetopause setting are summarized. First, the basic morphological and dynamic features of asymmetric reconnection are examined with emphasis on the important role played by the rotational discontinuity in these geometries. Second, the structure and other properties of rotational discontinuities are discussed. Third, the manner in which individual particles are energized or de-energized during their interaction with current layers in general, and rotational discontinuities in particular, is examined. Finally, the question of nonsteady, localized reconnection and its relation to flux transfer events is discussed and a qualitative model is proposed to describe these phenomena.

Sonnerup, B. U. O.↗
NASA NTRS1985

Laboratory experiments on reconnection in current sheets

Insights obtained from laboratory experiments concerning the reconnection of current sheets are discussed. Consideration is given to: the nonlocal characteristics of reconnection; the build-up of pinch current sheets; and impulsive flux transfer events. The application of the experimental results to present models of current sheet reconnection during solar flares is also discussed.

Bratenahl, A.↗
NASA NTRS1986

Acceleration of charged particles in magnetic reconnection Solar flares, the magnetosphere, and solar wind

A possible source of free energy available for accelerating charged particles is conversion of magnetic energy to particle energy in reconnecting magnetic fields. Recent simulations using test particles suggests that reconnection may efficiently accelerate particles to the maximum energies that are observed in several astrophysical contexts. A simple analytic formula is used in conjunction with the simulation results to predict the maximum energy achievable in a particular plasma environment with the result that in solar flares reconnection is capable of accelerating particles to several GeV. In magnetospheric substorms the predicted maximum can reach several hundred keV, and near magnetic sector crossings in the solar wind the maximum energy can approach 100 keV.

Goldstein, M. L.↗
NASA NTRS1986

A reconnection mechanism for coronal condensations in two-ribbon flares

The series of self-consistent, two-dimensional, MHD calculations used to explore the reconnection dynamics implicit in the two-ribbon flare model of Kopp and Pneumann (1976) indicate that there exists, in addition to the standard, slow mode MHD shocks generated by reconnection, a standing, fast mode MHD shock. Due to thermal conduction, the slow shocks generate an evaporative upflow of chromospheric plasma into the reconnection region; an analysis of these effects of radiative and conductive cooling suggests that at least some of this evaporated plasma will undergo thermal condensation in passing through the fast shock.

Forbes, T. G.↗
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 NTRS1988

Magnetic reconnection models of flares

The most feasible energy source for solar and stellar flares is the energy stored in coronal magnetic fields. To convert a significant fraction of this energy into heat and kinetic energy in a short time requires rapid change in the topology of the magnetic fields, and hence, rapid reconnection of field lines. Recent numerical and analytical models of solar flares suggest that the magnetic energy released by reconnection drives chromospheric ablation in the flare ribbons. Simple theoretical arguments based on compressible reconnection theory predict that the temperature of the ablated plasma should be about 1.03 x 10 to the 6th B exp 0.62 K where B is the coronal magnetic field strength in Gauss.

Forbes, T. G.↗
NASA NTRS1990

Three-dimensional kinematic reconnection in the presence of field nulls and closed field lines

The present investigation of three-dimensional reconnection of magnetic fields with nulls and of fields with closed lines gives attention to the geometry of the former, with a view to their gamma-line and Sigma-surface structures. The geometric structures of configurations with a pair of type A and B nulls permit reconnection across the null-null lines; these are the field lines which join the two nulls. Also noted is the case of magnetostatic reconnection, in which the magnetic field is time-independent and the electrostatic potential is constant along field lines.

Lau, Yun-Tung↗
NASA NTRS1990

Three-dimensional driven reconnection in magnetic loops

A numerical study of reconnection in a three-dimensional axially bounded magnetic flux tube is presented. An axial magnetic field is embedded in a conducting fluid, between conducting end plates. An applied flow in the end plates causes distortion and stretching of the magnetic field in the flux tube, and the creation of current sheets. After the current sheets form, the reconnection rate is fast; with a merging rate M of about 1. The reconnection mechanism resembles flux pileup in two dimensions. It is also similar to the nonlinear behavior of a subcritically excited current-driven MHD mode. The numerical model is relevant to a mechanism thought to be important for heating the solar corona, as well as producing solar flares.

Strauss, H. R.↗
NASA NTRS1990

Magnetic reconnection in collisionless plasmas - Prescribed fields

The structure of the dissipation region during magnetic reconnection in collisionless plasma is investigated by examining a prescribed two-dimensional magnetic x line configuration with an imposed inductive electric field E(y). The calculations represent an extension of recent MHD simulations of steady state reconnection (Biskamp, 1986; Lee and Fu, 1986) to the collisionless kinetic regime. It is shown that the structure of the x line reconnection configuration depends on only two parameters: a normalized inductive field and a parameter R which represents the opening angle of the magnetic x lines.

Burkhart, G. 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 NTRS1992

Magnetic reconnection in incompressible fluids

The paper investigates the dynamical relaxation of a disturbed X-type magnetic neutral point in a periodic geometry, with an ignorable coordinate, for an incompressible fluid. It is found that the properties of the current sheet cannot be understood in terms of steady state reconnection theory or more recent linear dynamical solutions. Accordingly, a new scaling law for magnetic reconnection is presented, consistent with fast energy dissipation (i.e., the dissipation rate at current maximum is approximately independent of magnetic diffusivity (eta)). The flux annihilation rate, however, scales at eta exp 1/4, faster than the Sweet-Parker rate of sq rt eta but asymptotically much slower than the dissipation rate. These results suggest a flux pile-up regime in which the bulk of the free magnetic energy is released as heat rather than as kinetic energy of mass motion. The implications of our results for reconnection in the solar atmosphere and interior are discussed.

Deluca, Edward E.↗
NASA NTRS1992

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Particle simulations (including both ions and electrons) of the driven collisionless magnetic reconnection process at the earth's dayside magnetopause were carried out using a specially developed two-and-one-half-dimensional electromagnetic particle code with a driven inflow boundary and an open outflow boundary. The simulations were used to examine many features associated with the dayside magnetic reconnection process, including the acceleration and heating of particles and the generation of energetic particles and particle heat flux. In addition, the fluctuating electromagnetic field associated with a driven collisionless magnetic reconnection was examined and compared with the satellite observations of flux transfer events at the day-side magnetopause.

Ding, D. Q.↗
NASA NTRS1992

The variation of reconnection rate at the dayside magnetopause and cusp ion precipitation

A method is presented which allows estimation of the variation of the rate of magnetic reconnection at the dayside magnetopause. This is achieved using observations of the cusp particle precipitation made by low-altitude polar-orbiting spacecraft. In this paper we apply the technique to a previously published example of a cusp intersection by the DMSP F7 satellite. It is shown that the cusp signature in this case was produced by three separate bursts of reconnection which were of the order of 10 min apart, each lasting roughly 1 min. This is similar to the variation of reconnection rate which is required to explain typical flux transfer event signatures at the magnetopause.

Lockwood, M.↗