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NASA NTRS1994

The heliospheric plasma sheet

High-resolution magnetic field and plasma data gathered by ISEE 3/ICE during several sector boundary crossings are used to investigate the narrow heliospheric current sheet (approximately equal 3 x 10 (exp 3) km to 10 (exp 4) km thick), together with the heliospheric plasma sheet in which it is embedded. The heliospheric plasma sheet region is identified by a significantly enhanced plasma beta caused by density enhancements and diminished magnetic field strength and is about 20 to 30 times the thickness of the current sheet. The thickness of the heliospheric plasma sheet is found to increase exponentially with its average proton density. The heliospheric current sheet is often displaced to one edge or the other of the heliospheric plasma sheet. Further, the point of maximum plasma beta in the plasma sheet, where the magnetic field strength is at a broad local minimum, is not colocated with the heliospheric current sheet. Within the plasma sheet, changes in the magnetic pressure are balanced by corresponding changes in the plasma thermal pressure as expected for a convected solar wind feature. In addition, observations show small pressure differences between the regions upstream and downstream of the plasma sheet, which are interpreted as causing the plasma sheet to move across the spacecraft.

Winterhalter, D.↗

NASA NTRS1977

Some speculations about the origin of the high speed HCN jets in Comet Kohoutek /1973f/

The origin of the high-speed HCN jets observed in Comet Kohoutek (1973f) is considered. It is argued that the occurrence of these jets with Doppler shifts corresponding to velocities of approximately 3-5 km/s with respect to the nucleus cannot be explained in terms of either explosive outbursts of trapped sub-surface volatiles, highly exothermic chemical reactions involving free radicals, or the outward expansion of the cometary atmosphere with additional heating. Instead, acceleration of the precursor ions of HCN by energetic electrons in the ionospheric current sheets or current arcs is suggested to play an important role.

Ip, W.-H.↗

NASA NTRS1983

The magnetosphere of Uranus - Plasma sources, convection, and field configuration

It is suggested by qualitative considerations based on analogy with earth, Jupiter, and Saturn that the magnetosphere of Uranus may lack a plasma source able to produce significant internal currents, internal convection, and associated effects. A class of approximately self-consistent quantitative magnetohydrostatic equilibrium configurations for the case of a pole-on magnetosphere with variable plasma parameters is presently constructed in order to test this hypothesis by means of forthcoming Voyager measurements. The configurations that can be computed for the geometries of the magnetic field and of the tail current sheet, for a given distribution of plasma pressure, have a single, funnel-shaped polar cusp pointing into the solar wind and a cylindrical tail plasma sheet whose currents close within the tail, rather than on the tail magnetopause. Interconnection of interplanetary and magnetospheric fields yields a highly asymmetric tail-field configuration.

Voigt, G.-H.↗

NASA NTRS1990

Solar wind and coronal structure near sunspot minimum - Pioneer and SMM observations from 1985-1987

Changes in solar wind speed and magnetic polarity observed at the Pioneer spacecraft are discussed here in terms of the changing magnetic geometry implied by SMM coronagraph observations over the period 1985-1987. The pattern of recurrent solar wind streams, the long-term average speed, and the sector polarity of the interplanetary magnetic field all changed in a manner suggesting both a temporal variation, and a changing dependence on heliographic latitude. Coronal observations during this epoch show a systematic variation in coronal structure and the magnetic structure imposed on the expanding solar wind. These observations suggest interpretation of the solar wind speed variations in terms of the familiar model where the speed increases with distance from a nearly flat interplanetary current sheet, and where this current sheet becomes aligned with the solar equatorial plane as sunspot minimum approaches, but deviates rapidly from that orientation after minimum.

Mihalov, J. D.↗

NASA NTRS1991

The optical analogy for vector fields

This paper develops the optical analogy for a general vector field. The optical analogy allows the examination of certain aspects of a vector field that are not otherwise readily accessible. In particular, in the cases of a stationary Eulerian flow v of an ideal fluid and a magnetostatic field B, the vectors v and B have surface loci in common with their curls. The intrinsic discontinuities around local maxima in absolute values of v and B take the form of vortex sheets and current sheets, respectively, the former playing a fundamental role in the development of hydrodyamic turbulence and the latter playing a major role in heating the X-ray coronas of stars and galaxies.

Parker, E. N.↗

NASA NTRS2019

Observational Evidence of Magnetic Reconnection in the Terrestrial Bow Shock Transition Region

We report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65°. An ion‐skin‐depth‐scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X‐line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field‐aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.

Wang, Shan↗

NASA NTRS2019

Magnetic Reconnection in a Quasi‐Parallel Shock: Two‐Dimensional Local Particle‐in‐Cell Simulation

Magnetic reconnection in a quasi‐parallel bow shock is investigated with two‐dimensional local particle‐in‐cell simulations. In the shock transition and downstream regions, large amplitude magnetic fluctuations exist, and abundant current sheets form. In some current sheets, reconnection occurs, and ion‐scale and electron‐scale magnetic islands are generated. In electron‐scale island regions, only electron outflow jets are observed, producing a quadrupolar out‐of‐plane magnetic field pattern, while in ion‐scale islands, both ions and electrons are involved and energized in reconnection. Normalized reconnection rates are obtained to be between around 0.1 to 0.2, and particle acceleration signatures are seen in distribution functions.

Bessho, N.↗

NASA NTRS1993

Energy storage and dissipation in the magnetotail during substorms. I - Particle simulations. II - MHD simulations

2D electromagnetic particle simulations are used to investigate the dynamics of the tail during development of substorms under the influence of the pressure in the magnetospheric boundary layer and the dawn-to-dusk electric field. It is shown that pressure pulses result in thinning of the tail current sheet as the magnetic field becomes pinched near the region where the pressure pulse is applied. The pinching leads to the tailward flow of the current sheet plasma and the eventual formation and injection of a plasmoid. Surges in the dawn-to-dusk electric field cause plasma on the flanks to convect into the center of the current sheet, thereby thinning the current sheet. The pressure in the magnetospheric boundary laser is coupled to the dawn-to-dusk electric field through the conductivity of the tail. Changes in the predicted evolution of the magnetosphere during substorms due to changes in the resistivity are investigated under the assumption that MHD theory provides a suitable representation of the global or large-scale evolution of the magnetotail to changes in the solar wind and to reconnection at the dayside magnetopause. It is shown that the overall evolution of the magnetosphere is about the same for three different resistivity distributions with plasmoid formation and ejection in each case.

Winglee, R. M.↗

NASA NTRS1989

A circuit model for filament eruptions and two-ribbon flares

A circuit model is derived for solar filament eruptions and two-ribbon flares. In the model the filament is approximated as a line current and the current sheet as infinitely thin. The model reproduces the slow energy buildup and eruption of the filament and the energy dissipation in a current sheet at the top of postflare loops during the two-ribbon flare. The two circuits considered are that of the filament and its return current and that of the current sheet and its return current. These circuits are inductively coupled, and free energy stored in the filament in the pre-flare phase is found to be transferred to the sheet during the impulsive phase and rapidly dissipated there. In the solutions for the evolution of the filament current sheet system four phases are distinguished: (1) a slow energy buildup, (2) a 'metastable' state, (3) the eruptive phase, and (4) a postflare phase. These phases are described in detail.

Martens, P. C. H.↗

NASA NTRS1995

Understanding the large-scale structure of magnetic fields in the corona and heliosphere

To understand the non-radial nature of the southwestern streamer in the 3 November 1994 eclipse and the near absence of a latitudinal gradient of the interplanetary magnetic field strength seen by Ulysses over the southern pole, we calculate the coronal and inter planetary magnetic field from WSO photospheric field data using both the horizontal current-current sheet model (Zhao and Hoeksema, 1994) and the current sheet-source surface model (Zhao and Hoeksema, 1995). These models account for the effects of large-currents and current sheets in the corona. Comparison of the calculated results with white light, X-ray, and IMF observations show that non-radial coronal streamers and a near-zero latitudinal gradient of the radial component of the IMF may be explained by a large-scale current sheet. To best match the observations, the height of the cusp point (above which current sheets form) differs from one streamer to another. While heliospheric volume currents flowing above the source surface certainly affect the interplanetary field, they have little or no influence on the structure of the corona.

Zhao, X. P.↗

NASA NTRS1972

Magnetic models of solar flares.

A solar flare is interpreted as the explosive release of magnetic energy associated with a current sheet in the solar atmosphere. The release is due to field-line reconnection caused by the tearing-mode instability. Current sheets may be classified into closed and open, depending on whether the adjacent field lines are closed (contained in the sun's atmosphere) or open, extending into interplanetary space. A high-energy event requires an open current sheet. It is shown that certain types of photospheric motion may lead to the formation of current sheets. One of these is associated with high-energy events, another with filaments, and a third with surge flares.

Sturrock, P. A.↗

NASA NTRS1997

Nonguiding Center Motion and Substorm Effects in the Magnetotail

Thick and thin models of the middle magnetotail were developed using a consistent orbit tracing technique. It was found that currents carried near the equator by groups of ions with anisotropic distribution functions are not well approximated by the guiding center expressions. The guiding center equations fail primarily because the calculated pressure tensor is not magnetic field aligned. The pressure tensor becomes field aligned as one moves away from the equator, but here there is a small region in which the guiding center equations remain inadequate because the two perpendicular components of the pressure tensor are unequal. The significance of nonguiding center motion to substorm processes then was examined. One mechanism that may disrupt a thin cross-tail current sheet involves field changes that cause ions to begin following chaotic orbits. The lowest-altitude chaotic region, characterized by an adiabaticity parameter kappa approx. equal to 0.8, is especially important. The average cross-tail particle drift is slow, and we were unable to generate a thin current sheet using such ions. Therefore, any process that tends to create a thin current sheet in a region with kappa approaching 0.8 may cause the cross-tail current to get so low that it becomes insufficient to support the lobes. A different limit may be important in resonant orbit regions of a thin current sheet because particles reach a maximum cross-tail drift velocity. If the number of ions per unit length decreases as the tail is stretched, this part of the plasma sheet also may become unable to carry the cross-tail current needed to support the lobes. Thin sheets are needed for both resonant and chaotic orbit mechanisms because the distribution function must be highly structured. A description of current continuity is included to show how field aligned currents can evolve during the transition from a two-dimensional (2-D) to a 3-D configuration.

Kaufmann, Richard L.↗

DOE OSTI2021

Computational general relativistic force-free electrodynamics: II. Characterization of numerical diffusivity

Scientific codes are an indispensable link between theory and experiment; in (astro-)plasma physics, such numerical tools are one window into the universe’s most extreme flows of energy. The discretization of Maxwell’s equations – needed to make highly magnetized (astro)physical plasma amenable to its numerical modeling – introduces numerical diffusion. It acts as a source of dissipation independent of the system’s physical constituents. Understanding the numerical diffusion of scientific codes is the key to classifying their reliability. It gives specific limits in which the results of numerical experiments are physical. We aim at quantifying and characterizing the numerical diffusion properties of our recently developed numerical tool for the simulation of general relativistic force-free electrodynamics by calibrating and comparing it with other strategies found in the literature. Our code correctly models smooth waves of highly magnetized plasma. We evaluate the limits of general relativistic force-free electrodynamics in the context of current sheets and tearing mode instabilities. We identify that the current parallel to the magnetic field (j ∥ ), in combination with the breakdown of general relativistic force-free electrodynamics across current sheets, impairs the physical modeling of resistive instabilities. We find that at least eight numerical cells per characteristic size of interest (e.g., the wavelength in plasma waves or the transverse width of a current sheet) are needed to find consistency between resistivity of numerical and of physical origins. High-order discretization of the force-free current allows us to provide almost ideal orders of convergence for (smooth) plasma wave dynamics. The physical modeling of resistive layers requires suitable current prescriptions or a sub-grid modeling for the evolution of j ∥ .

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1984

The effects of dipole tilt on the structure of the magnetosphere

The structure of the magnetosphere as a function of dipole tilt was studied by using a global magnetohydrodynamics (MHD) model. We show here that the cusp current sheet in the MHD model is related to the tail current sheet and that the calculated shape and positions of the tail current sheet are consistent with observations. In addition, we discuss the difference between the MHD model and the Chapman-Ferraro type model which manifests as a function of tilt angle.

Wu, C. C.↗

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 NTRS2022

Impact of 3D Structure on Magnetic Reconnection

Results from 2.5D and 3D studies of the onset and development of the tearing instability are presented, using high fidelity resistive MHD simulations. A limited parameter study of the strength of the reconnecting field (or shear angle) was performed. An initially simple 1D equilibrium was used, consisting of a modified force-free current sheet, with periodic boundary conditions in all directions. In all cases, the linear and nonlinear evolution led to a primary current sheet between two large flux ropes. The global reconnection rate during this later stage was analyzed in all simulations. It was found that in 2.5D the primary current sheet fragmented owing to plasmoids, and as expected, the global reconnection rate, calculated using multiple methods, increases with the strength of the reconnecting field owing to a stronger Alfvén speed. In 3D, the presence of interacting oblique modes of the tearing instability complicates the simple 2.5D picture, entangling the magnetic field of the inflow and introducing a negative effect on the reconnection rate. The two competing effects of stronger Alfvén speed and entangling, which both increase with the strength of the reconnecting field, resulted in a decrease in the reconnection rate with increasing reconnecting field. For all simulations, the 3D rates were less than in 2.5D but suggest that as one goes to weak reconnecting field (or strong guide field) the system becomes more 2.5D-like and the 2.5D and 3D rates converge. These results have relevance to situations like nanoflare heating and flare current sheets in the corona.

Lars K S Daldorff↗

NASA NTRS2010

Thrust Stand Measurements of the Conical Theta Pinch FARAD Thruster

It is found that the impulse of a pulsed inductive plasma thruster utilizing preionization is maximized for a particular ratio of the stored energy in the capacitor to the injected propellant mass. The fact that the impulse depends on the ratio of the initial stored energy to injected propellant mass agrees with previous current sheet studies, supporting the idea that a Townsend-like breakdown process strongly influences current sheet formation, and in turn, current sheet formation strongly affects the operational efficiency of the device. The optimum in half cone angle of the inductive coil can be explained in terms of a balance between the direct axial acceleration and the radial pinching contribution to thrust. From the trends in these data we conclude that operation at the correct ratio of capacitor energy to propellant mass is essential for efficient operation of pulsed inductive plasma thrusters employing a preionized propellant.

Hallock, Ashley K.↗

DOE OSTI2021

Thermodynamic Phase Transition in Magnetic Reconnection

By examining the entropy production in fully kinetic simulations of collisional plasmas, it is shown that the transition from collisional Sweet-Parker reconnection to collisionless Hall reconnection may be viewed as a thermodynamic phase transition. The phase transition occurs when the reconnection electric field satisfies E = $E_{D}\sqrt{m{e}/{m}_i}$ , where ${m{e}/{m}_i}$ is the electron-to-ion mass ratio and E D is the Dreicer electric field. This condition applies for all ${m{e}/{m}_i}$, including ${m{e}/{m}_i}$ = 1 , where the Hall regime vanishes and a direct phase transition from the collisional to the kinetic regime occurs. In the limit ${m{e}/{m}_i}$ → 0, this condition is equivalent to there being a critical electron temperature $Te ≈ m_iΩ^{2}_{i}δ^{2}$, where $Ω_i$ is the ion cyclotron frequency and δ is the current sheet half-thickness. The heat capacity of the current sheet changes discontinuously across the phase transition, and a critical power law is identified in an effective heat capacity. A model for the time-dependent evolution of an isolated current sheet in the collisional regime is derived.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗