Daniel W. Swift

Simulation of pressure pulses in the bow shock and magnetosheath driven by variations in interplanetary magnetic field direction

Two-dimensional (2-D) hybrid simulations are carried out to study the effects of the variation in the interplanetary magnetic field (IMF) direction on the bow shock, magnetosheath, and magnetosphere. A curvilinear coordinate system is used in the simulation. The 2-D simulation is also compared with our one-dimensional simulation results. It is found that pressure pulses are generated as a result of the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD). First, a structure consisting of a rotational discontinuity and two slow shocks are present downstream of the bow shock after the BS/RD interaction. The magnetic field and plasma density are anticorrelated in this structure. The dynamic pressure increases in the structure, leading to a pressure pulse in the magnetosheath. Second, a pressure pulse associated with reflected ions at the bow shock may be generated in the foreshock when the IMF changes its direction, especially when a local quasi-parallel bow shock becomes a quasi-perpendicular shock. The magnetic field, plasma density, and dynamic pressure are positively correlated in the upstream pressure pulse. This pressure pulse convects through and interacts with the bow shock, producing a pressure pulse in the downstream region. The downstream pressure pulses propagate to the magnetopause. The amplitude of the downstream pressure pulses can be up to 100% of the background magnetosheath value. It is suggested that the pressure pulses impinging on the magnetopause may lead to the magnetic impulse events observed in the high-latitude ionosphere.

Use of a hybrid code to model the Earth's magnetosphere

This paper presents a demonstration of the use of a hybrid code to model the Earths magnetosphere on a global scale. A hybrid code calculates the interaction of particle ions and a massless electron fluid with the magnetic field. The demonstration is a simulation of the noon meridian plane of the magnetosphere. Innovative features of the code include a numerically generated curvilinear coordinate system, approximation of the cold ionospheric plasma as an MHD fluid that coexists with the kinetic plasma and subcycling of the magnetic field update to the particle push. These innovations allow the code to accommodate disparate time and distance scales. New results include particle acceleration in the cusp and nearly field aligned currents linking the cusp and polar ionosphere. It is pointed out that such a code is needed help resolve many of the outstanding questions regarding substorm dynamics and perhaps form the basis of a numerical space weather prediction scheme.

Possible consequences of the asymmetric development of the ring current belt

Abstract The consequences of an asymmetric injection of protons into the magnetosphere are investigated. A guiding center Vlasov equation is solved by a perturbation method with the solution being given in terms of an electric potential. The electric potential is determined by the condition that the drift currents plus the ionosphere currents be divergenceless. The effect of the Earths rotation also is included. The electric potential and ionsopheric currents are seen to be critically dependent upon the ring current particle energy and ionospheric conductivity. The computations indicate substantial transfer of charge between the magnetosphere and ionosphere in auroral zone locations. The computations suggest that the ring current may have an important role in auroral processes.

A three-dimensional hybrid code simulation of the December 1984 solar wind AMPTE release

A three-dimensional hybrid code has been developed to study the interaction between small dense plasma clouds and an ambient plasma. The primary advantage of this code is a seamless interface between kinetic particles (plasma cloud) and an MHD fluid (ambient plasma). This interface provides momentum coupling between two distinct ion populations. As a preliminary test of our code, we have simulated the first 3 minutes of the December 1984 AMPTE artificial comet. The results show good agreement with observations of the magnetic field distribution as well as the lateral motion of the comet head. Examples of other applications for this code include comets, coronal mass ejections, Ios plasma torus, and plasma injection experiments.

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Basic plasma processes associated with driven collisionless magnetic reconnection at the Earths dayside magnetopause are studied on the basis of particle simulations. A two-and-one-half-dimensional (2½-D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force-free condition for the electrostatic potential. The major findings are as follows: (1) the simulations exhibit both quasi-steady single X line reconnection and intermittent multiple X line reconnection (MXR); the MXR process is characterized by repeated formation and convection of magnetic islands; (2) particle acceleration in the reconnection process results in a power law energy spectrum of ƒ(E) ∼ E−4 for energetic ions with E > 40 keV and energetic electrons with E > 3 keV, where E is the particle energy; particles are accelerated to high energy near magnetic O line regions as particles are trapped within magnetic islands; (3) field-aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed; typical power spectra of fluctuating magnetic and electric fields are found to be PB ∼ f−3.6 and PE ∼ f−1.8, respectively, where f is the wave frequency; (4) when applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two-regime flux transfer events (FTEs); and (5) an intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. Simulation results are applied to the dayside magnetopause to provide an explanation for some features associated with dayside magnetic reconnection and FTEs.

Mechanisms for the discrete aurora — A review

The V-shock is identified as the primary mechanism for the acceleration of electrons responsible for the discrete aurora. A brief review of the evidence supporting the V-shock model is given, including the dynamics of auroral striations, anomalous motion of barium plasma at high altitudes and in-situ observations of large electric fields. The V-shock is a nonlinear, n = 0 ion cyclotron mode soliton, Doppler shifted to zero frequency. The V-shock is also shown to be a generalization of the one-dimensional double layer model, which is an ion acoustic soliton Doppler shifted to zero frequency. The essential difference between the double layer theory and the theory for the oblique, current-driven, laminar electrostatic shock is that the plasma dielectric constant in directions perpendicular to the magnetic field is c2/Va/2, where Va is the Alfvén velocity; but the plasma dielectric constant parallel to the magnetic field is unity. Otherwise, in the limit that the shock thickness perpendicular to the magnetic field is much larger than an ion gyroradius, the equations describing the double layer and the oblique shock are the same. The V-shock, while accounting for the acceleration of auroral electrons, requires an energy source and mechanism for generating large potential differences perpendicular to the magnetic field. An energy source is the earthward streaming protons coming from the distant magnetospheric tail. It is shown how these protons can be energized by the cross-tail electric field, which is the tailward extension of the polar cap dawn-to-dusk electric field. The local, large cross-field potential differences associated with the V-shock are seen to be the result of a non-linear, E × B drift turbulent cascade which transfers energy from small- to large-scale sizes. Energy at the smallest scale sizes comes from the kinetic energy in the ion cyclotron motion of the earthward streaming protons, which are unstable against the zero-frequency flute-mode instability. The review points out the gaps in our understanding of the mechanism of the diffuse aurora and the mechanism of the auroral substorm.

Imaging the Earth's magnetosphere

Abstract The ability to image the Earths magnetosphere would constitute a major advance in our ability to formulate a comprehensive, time-dependent model of the magnetosphere and of substonn processes. Imaging technology is rapidly developing to the point where useful optical images could be made with sufficient spatial and temporal resolution for viewing substorm processes. We identify resonant scattering solar emissions near 834 A by O + as the most promising means of viewing magnetospheric plasma and tracing processes within the magnetosphere. Simulations of images at 834 A observed from the Moons orbit, which include detail on the radiation belts and the plasma sheet, are presented and analyzed. They show that changes in the global structure are expected to be visible through changes in the brightness and structure of features which can be identified on the image. The analysis also shows that, given expected performance of detectors and reflectors now under development for use in e.u.v., useful images could be made in 15–30 min with 1000 pixels. This will give time resolution adequate for observation of structures which could change in the period of a typlcal magnetospheric substorm. In addition to the further development of optical devices, high resolution spectra of solar radiation are needed to make more accurate calculations of the scattering due to O + at 834 A.

Substorm onset viewed by a two-dimensional, global-scale hybrid code

Abstract A two-dimensional hybrid code is used to simulate processes related to the onset of the substorm expansive phase that take place in the midnight meridian plane of the magnetosphere. The simulation domain extends from the Earths ionosphere to 35 Earth radii in the antisunward direction and to 11 Earth radii along the polar axes. Simulation runs were made both with and without a driving dawn-to-dusk electric field imposed at the boundaries. In the driven cases momentum exchange between the plasma being accelerated earthward in the neutral sheet and the magnetic field results in a stretching of the magnetic field lines and thinning of the plasma sheet. In the undriven cases, the induction electric field from the collapsing tail field also accelerates plasma sheet plasma earthward. In both cases a pair of field-aligned currents develops connecting the inner edge of the plasma sheet to the auroral ionosphere, with the downward current equatorward of the upward current. The breaking of the inward plasma flow drives these currents. Next follows the apparent dipolarization. “Dipolarization” is carried outward by plasma rebounding off the dipole field. Ion–ion two streaming instabilities are excited behind the expanding dipolarization front. The effect of these instabilities is propagated earthward along magnetic field lines by shear Alfven waves. These are seen as multiple field-aligned current filaments above the auroral ionosphere. We take the upward field-aligned current filaments as proxies for auroral arcs. Results of the simulations indicate substorm effects to be a consequence of the near-Earth breaking fast earthward flows, which develop in the plasma sheet.

Further possible consequences of the asymmetric development of the ring current belt—effect of variations in ionospheric conductivity

Abstract Numerical calculations of the magnetospheric electrostatic field and ionospheric current field have been made assuming that the auroral zone ionosphere is represented by a ring of enhanced conductivity centered on the geomagnetic pole. These calculations are compared with similar calculations assuming that the ionosphere has a uniform conductivity. For certain ring current particle energies, the computation shows that the ring of enhanced ionospheric conductivity results in a near reversal and intensification of the electrostatic field. Intense ionospheric currents are driven along segments of the ionospheric ring. These calculations suggest that the asymmetric ring current may be the responsible agent for driving the auroral electrojet. The results of the calculations are used to formulate a model of the magnetic substorm associated with the auroral breakup.

Momentum transfer in the combined release and radiation effects satellite plasma injection experiments: The role of parallel electric fields

Optical observations of the combined release and radiation effects satellite (CRRES) plasma injection experiments have revealed signatures of the coupling processes between the injected and ambient plasmas. The elongation of the ion cloud along the geomagnetic field indicated the presence of parallel electric fields. A dense core of ions polarized and E×B drifted along the satellite trajectory for 6–10 s. In an effort to understand the momentum coupling between the two plasma populations, a three-dimensional hybrid code is used. The simulation, which neglected electron inertia, showed a very efficient transfer of momentum via Alfven waves and the skidding ion core was stopped within 2 s. To reconcile the difference between the observed and simulated skidding times, we propose that the cloud must have been decoupled from the ambient plasma via parallel electric fields. The parallel fields may be associated with inertial Alfven waves propagating in filamentary current layers at the edges of the ion cloud, o...