Magnetic Field Line Reconnection Research Articles

Reconnection of magnetic field lines in viscous conducting fluids

In the formulation of steady hydromagnetic flows of a viscous conducting fluid, continuous solutions in similarity form are obtained which exhibit the topology of field line reconnection without the inclusion of discontinuity fronts. The properties of the flow support the assertion that magnetic merging can be mainly controlled by distant boundary conditions.

Laboratory solar flare experiments

Several laboratory experiments on magnetic field line reconnection are briefly reviewed. Emphasis is placed on the double inverse pinch device (DIPD) in which magnetic flux is built up during a quiescent reconnection phase and then abruptly transferred during an impulsive reconnection phase. Scaling estimates show that this impulsive phase corresponds to a solar release of 1030 ergs in 102 seconds with the production of GeV potentials. The trigger for the impulsive “flare” is a conduction mode instability (ion-acoustic) which abruptly changes the resistance of the neutral point region when the reconnection current density reaches a critical value.

Magnetic field-line reconnection in a highly-conducting incompressible fluid: properties of the diffusion region

The properties of the field and flow near a two-dimensional X-type magnetic neutral line are considered for an incompressible, inviscid, conducting fluid in the steady state. In this region the magnetic field is not ‘frozen’ into the plasma, and finite conductivity effects must be considered. In the region surrounding the X line, it has hitherto been assumed that the field lines to lowest order form systems of hyperbolae (the field components increasing linearly with distance from the null), while the fluid flow lines form systems of rectangular hyperbolae with fluid flow perpendicular to the field on the axes of symmetry of the field lines. The assumption of such a configuration was based upon a somewhat cursary consideration of the governing equations, a detailed examination never having been carried out. We present the results of such an examination, which reveal that the above configuration is not a valid solution of the MHD equations, and that, for the case studied, the field to lowest order must form a neutral sheet. The properties of the field and flow parallel to the plane of symmetry are also considered. It is found that non-uniform fields and flows may exist within the diffusion region, in support of recent convection-region studies. This work shows clearly the need for more detailed and careful consideration of such systems.

Effect of obstacles on the rate of reconnection of magnetic field lines

Further results of a laboratory magnetic field line reconnection experiment are presented. In particular, it is found that the reconnection rate can be slowed by placing solid obstacles to impede the outflow of plasma from an x-type magnetic neutral point. Without the obstacles the reconnection rate is faster and more impulsive. The fastest reconnection event has strong similarities to solar flares and geomagnetic substorms. It is suggested that more stationary features of solar activity such as prominences may be the result of reconnection slowed by obstacles such as the photosphere.

Is there an electrostatic field tangential to the dayside magnetopause and neutral line?

The question of possible energy dissipation that might be associated with the dayside magnetopause current requires consideration of the tangential component of the electric field. If the total current of 9 × 106 amperes flowed through the average magnetospheric dawn‐dusk potential difference of 6 × 104 volts the power dissipation would be 5 × 1011 watts. We might expect this energy to be carried away by energized particles, perhaps as increased thermal or flow energy at or near the magnetopause boundary layer or mantle. Satellite observations are available, but they do not provide evidence of such energization. This failure leads us to conclude that the magnetopause current does not flow through the magnetospheric potential difference, and that the magnetopause is approximately an equipotential surface. Because of the necessary continuity of the tangential component of the electric field, this conclusion implies that there is no steady state electric field along the neutral or X‐line that must be located near the dayside magnetopause in an open magnetospheric model (which is clearly supported by other evidence). Such an electric field is an essential part of the theory of merging or reconnection of magnetic field lines, and it is therefore concluded that the merging model for the electric field is not correct. This conclusion could be avoided if some evidence for the implied energy dissipation were found.

The reconnection of magnetic field lines in the solar corona

Skylab XUV coronal spectroheliograms and photospheric magnetograms are compared. This comparison shows that, as new bipolar magnetic fields emerge through the solar surface into the corona, the new coronal fields interact with the old ones in a manner that suggests the reconnection of the field lines.

Possible acceleration of charged particles through the reconnection of magnetic field lines in interplanetary space

Prominent intensity spikes in the flux of protons and alphas with less than 0.5 MeV per charge have been observed in the region several hours behind an interplanetary shock front. The small spatial scale of these events and the high anisotropy of the particle flux suggest local acceleration. The spectra of the particles, which are cut off at equal energy per charge, suggest acceleration through an electric field. We examine the possibility that these events have their origin in active magnetic neutral sheets in the shocked solar wind.

On the Reconnection of Magnetic Field Lines in Compressible Conducting Fluids

A previous study by Yeh and Axford of the reconnection of magnetic field lines in an incompressible fluid is extended to a compressible fluid. From hydromagnetic flows in similarity form, it is found that expansion takes place before the reconnection and compression after the reconnection. The pressure gradient in the radial direction causes the field lines to bend into separatrices in the incoming flows. The kinks on the reconnected field lines propagate out as slow-mode hydromagnetic shocks in the outgoing flows. In the expansion phase of the merging the magnetic energy is piled up around the separatrices, and then in the compression phase the energy is converted from magnetic into kinetic and thermal.

Evidence of a diffuse magnetopause boundary

Observational evidence is presented to show that the magnetosphere-magnetopause boundary was diffuse during the Pioneer 8 magnetopause traversal on December 14 and 15, 1967. This boundary was characterized by proton fluxes of significantly lower intensity than those observed in the magnetosheath. In addition, the angles of flow associated with the fluxes at the boundary were more diverse than those observed in the magnetosheath. Proton energy spectra are presented to indicate the change in the spectral shape at the boundary. The observational evidence is consistent with calculations by Axford and Dryer of a viscous magnetopause boundary layer, which is a function of an anomalous nonclassical kinematic viscosity. The observational evidence presented for the diffuse magnetosphere boundary may also be consistent with Dungey's model for reconnection of magnetic field lines and an open magnetosphere.

SWEET'S MECHANISM FOR THE DESTRUCTION OF MAGNETIC FLUX

Sweet's mechanism is one of the more efficient ways of converting magnetic energy into other forms by the reconnection of magnetic field lines in a narrow current sheet or boundary layer. This paper presents a more quantitative analysis of the mechanism than hitherto. In particular, it applies the Pohlhausen approximation to the two-dimensional magnetohydrodynamic boundary-layer equations in order to calculate variations along the boundary layer in the boundary-layer thickness 8 and in the components along the boundary layer of magnetic field B and fluid velocity V. For an incompressible fluid forms are assumed for the profiles of magnetic field and fluid velocity across the layer and the set of resulting ordinary differential equations is integrated numerically. B decreases monotonically to zero along the layer while 8 tends to infinity and V first increases to a maximum and then decreases to zero. About fifty per cent of the energy input goes into ohmic dissipation and thirty-five per cent into kinetic energy of fluid motion. For compressible flow neglect of the density outside the boundary layer compared with that inside leads to an approximate expression for the variation of the magnetic field across the layer; the y-component increases linearly in an inner region and is constant in an outer region, with a sharp transition between the two. The Pohlhausen approximation then shows that 8 decreases to zero and V tends to infinity as B decreases to zero.

A Model of the Geomagnetic Cavity

A heuristic model of the geomagnetic cavity is introduced. The model includes a unipolar induction current JU, that is driven by the solar wind. The current is generated by the solar wind plasma flowing in the magnetopause boundary layer. This current closes in the ionosphere. The electromagnetic forces associated with JU drive the magnetospheric convection. Even though the solar wind plasma at the magnetopause is basically collisionless, the plasma penetrates more deeply into the geomagnetic cavity than it would if the magnetopause were an ideal Chapman‐Ferraro sheath because the boundary layer flow is non‐adiabatic in the regions where JU is nonzero. Downstream from the earth the resulting expanded boundary layer forms the plasma sheet and the geomagnetic tail. Since the generation of JU does not depend on the existence of an interplanetary magnetic field, the geomagnetic cavity is closed when the interplanetary field is zero. The tangential drag is developed in the part of the boundary layer that lies forward of the midnight‐meridian intersection of the leading edge of the neutral sheet. The tail is produced by the inflation of the downstream part of the modified cavity that is formed by the superposition of the geomagnetic field and the magnetic field of JU. Transient phenomena produced by changes in JU include substorm expansions and injections of ring current particles which are initiated by decreases in JU. Changes in JU are caused by changes in the solar wind velocity (i.e., by changes in the unipolar emf) and by changes in the effective resistance to the flow of JU. If there is a critical value of |JU| above which the current is unstable, then there is a corresponding critical magnitude of the unipolar emf and therefore of the solar wind velocity. Such an instability in JU would account for the positive correlation between solar wind velocity and Kp. When the interplanetary magnetic field is greater than zero, shear stresses are set up between the solar wind plasma in the magnetopause boundary layer and that in the magnetosheath. The unipolar induction, i.e., the unipolar emf, and the resulting tangential drag then depend on the direction and magnitude of the interplanetary field as well as on the velocity of the solar wind and the electrical (Pedersen) resistivity of the magnetospheric plasma. The many attractive features of the model, especially the simplicity of the tangential drag mechanism, suggest that unipolar induction is a competitive alternative to magnetic field line reconnection and the other mechanisms that have been proposed to account for the tangential drag on the cavity and the various transient cavity phenomena.

Magnetic field perturbations in the magnetotail associated with polar magnetic substorms

Using data from the IMP-A and IMP-B satellites, it is shown that when a polar magnetic substorm onset is observed at the earth's surface, there is a pronounced effect in the magnetotail at approximately the same time. This magnetotail perturbation appears as a temporary reduction in the tail field reflecting primarily a reduction in the component of the field pointing along the sun–earth line parallel to the neutral sheet. Large perturbations in the component of the magnetic field perpendicular to the ecliptic plane which are observed around 24RE suggest that reconnection of magnetic field lines is the physical mechanism responsible for the observed effects. It is suggested that substorms act as a safety valve limiting the size of the magnetotail by periodically bleeding magnetic flux from the tail into the inner dipole configuration.

Tubular Pinch and Tearing Instability

Theoretical considerations and previous experimental findings with fast tubular (cylindrical sheet) pinch discharges are reviewed briefly. More recent work with magnetic probes, as well as streak and Kerr cell photography, corroborates that the sheet-current configuration can be sufficiently stable to survive a fair number of pinch oscillations. It is shown, however, that under conditions of high compression the current layer is likely to break up into a set of individual channels because of a resistive instability. The process involves tearing and reconnection of magnetic field lines. Wavelengths and growth rates seem to be in agreement with rough theoretical predictions. The current channels form separate linear pinches, which under certain circumstances disintegrate subsequently by a secondary instability. A generally turbulent behavior results. Tearing is not prevented by superposition of a strong axial magnetic field, again in agreement with theory, although a slight retardation is noted. When deuterium is used in the discharge, neutrons are emitted. Presumably because of the slow rate of reconnection in most of our studies, the neutrons appear mainly during the secondary instability, rather than at the time of tearing. These findings are of interest both to astrophysics and to controlled-fusion research.

Iron (III) reduction in solutions of K 3[Fe(C 2O 4)] in ultrasonic field : Margulis, M.A., Mal'tsev, A.N., Vestnik Moskovskogo Universiteta, Khimiya No 6 (November–December 1968) pp 44–48 (In Russian) (1224)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Theory of the acoustic coagulation of aerosols: Mednikov, E.P., Doklady Akademii Nauk SSR, Vol 193, No 2 (1968) pp 382–385 (In Russian) (1227)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Results of an investigation of the kinetics of ultrasonic crystallisation of potassium bichromate on a heat transfer surface: Sergeeva, K.Y., Soviet Physics—Acoustics, Vol 14, No 2 (October–December 1968) pp 253–256 (1234)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Calculation of the anharmonic damping of Rayleigh surface modes: Maradudin, A.A., Mills, D.L., Physical Review, Vol 173, No 3 (15 September 1968) pp 881–898 (1223)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Electrical and optical properties of Cds-MnS single crystals: Ikeda, M., Itoh, K., Sato, H., Journal of the Physical Society of Japan, Vol 25, No 2 (August 1968) pp 455–460 (1213)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Experimental and theoretical investigation of an opticoacoustic receiver of centimeter electromagnetic waves: Rudin, V.L., Optika i Speltrosk Vol 24, No 2 (February 1968) pp 272–276 (In Russian) (English translation in Optics and Spectra, Vol 24, No 2 (February 1968) pp 139–141) (1232)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.

Generation of ultrasound in the Gunn effect: Kastal'skii, A.A., Fiziki Tekniki Poluprovodnikov, Vol 2, No 8 (August 1968) pp 1187–1189 (In Russian) (1220)

Experimental findings of the last decade shed new light on the physics of the magnetotail plasma system, e.g. investigation of very thin current sheets, and motivated a number of theoretical studies. An example of such theoretical investigations is the analytical 1D collisionless self-consistent model of thin current sheet [Zelenyi L.M., Malova, H.V., Popov, V.Yu., Delcourt, D., Sharma, A.S. 2004. Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy. Nonlinear Processes in Geophysics 11, 1–9]. The principal characteristic of this model is its anisotropy (typical features of thin current sheet). Because anisotropic current sheets have larger free energy than Harris’ ones, we decided to revisit the classical problem of their stability. The linear tearing instability, as the most natural mechanism for the spontaneous reconnection of magnetic field lines, is investigated in the frame of our anisotropic model. We search for regions in the parameter space of the current sheet where the system could become unstable. The main region of instability is concentrated near Bn∼0.1B0 and its width depends on the parameters of the system. This result conforms with an earlier work by Galeev and Zeleny [1976. Tearing instability in plasma configurations. Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 70(6), 2133–2151(in Russian)], which suggested that tearing mode could develop in a limited region of Bn values (however, these gaps were erroneously ascribed to Harris equilibrium). Therefore, the current investigation of the stability of anisotropic current sheets in the magnetotail could help to extend the previous results about the stability of Harris-like current sheets with magnetized electrons, and have implications for the physics of substorm initiation.