Mechanisms that generate the field-aligned current (FAC) systems in the magnetosphere-ionosphere coupling scheme by virtue of the solar wind-magnetosphere interaction are investigated with a three-dimensional magnetohydrodynamic (MHD) simulation. As a simulation scheme, the finite volume total variation diminishing (TVD) scheme on an unstructured grid system is employed for precise calculations of the ionospheric region. In the ionosphere, the divergence of the Pedersen and Hall currents is matched with FAC, mainly assuming uniform conductivity. The present calculation reproduces the traditional region 1 and 2 currents in the polar ionosphere, for both the northward and southward interplanetary magnetic fields (IMFs). The calculated magnitude of the region 1 current becomes large on the dayside, in agreement with observational results. For the northward IMF, NBZ currents that dominate the entire polar cap are obtained, with a maximum on the dayside. This current is totally absent in the southward IMF result. Corresponding to the FACs, the northward IMF results in multicell convection in the polar ionosphere, and the southward IMF results in two-cell convection. On the evening side, the calculated region 1 currents flow almost along the field lines away from the Earth toward the magnetospheric low-latitude boundary layer (LLBL), then flow up the magnetopause across the field lines to high latitudes. The region 1 currents in the morning side are similar but opposite in direction. In the noon-midnight meridian (xz) plane, the main part of the region 1 current passes the tailward side of the cusp in the magnetosphere. The region 1 current converges to a very narrow region in the noon-midnight meridian (xz) plane when the IMF is northward, whereas it passes the noon-midnight meridian (xz) plane diverging to wide regions in the x direction when the IMF is southward. These differences are attributed to the efficient current-driving effect (J · E 0). In the evening magnetosphere, the NBZ current that flows into the dayside ionosphere passes the low-latitude side of the NBZ current that flows into the nightside ionosphere, then it turns aside to the outward (+y) direction and turns back before reaching the dayside ionosphere. Consequently, the dayside NBZ current flows from the low-latitude side of the lobe, while the nightside NBZ current flows from the high-latitude side of the lobe. Calculation assuming nonuniform ionospheric conductivity results in a wedge-current-like structure in the evening side. This result indicates that the current generated in the ionosphere cannot be ignored in the magnetosphere-ionosphere current systems.

Generation mechanisms for magnetosphere-ionosphere current systems deduced from a three-dimensional MHD simulation of the solar wind-magnetosphere-ionosphere coupling processes

MOCCT: A numerical technique for astrophysical MHD

Magnetic reconnection can lead to the formation of observed boundary layers at the dayside magnetopause and in the nightside plasma sheet of the earth's magnetosphere. In this paper, the structure of these reconnection layers is studied by solving the one-dimensional Riemann problem for the evolution of a current sheet. Analytical method, resistive MHD simulations, and hybrid simulations are used. Based on the ideal MHD formulation, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the dayside reconnection layer. Fast expansion waves are also present in the solution of the Riemann problem, but they quickly propagate out of the reconnection layer. Our study provides a coherent picture for the transition from the reconnection layer with two slow shocks in Petschek's model to the reconnection layer with a rotational discontinuity and a slow expansion wave in Levy et al's model. In the resistive MHD simulations, the rotational discontinuities are replaced by intermediate shocks or time-dependent intermediate shocks. In the hybrid simulations, the time-dependent intermediate shock quickly evolves to a steady rotational discontinuity, and the contact discontinuity does not exist. The magnetotail reconnection layer consists of two slow shocks. Hybrid simulations of slow shocks indicate that there exists a critical number,M
c, such that for slow shocks with an intermediate Mach numberM
I≥M
c, a large-amplitude rotational wavetrain is present in the downstream region. For slow shocks withM
I<M
c, the downstream wavetrain does not exist. Chaotic ion orbits in the downstream wave provide an efficient mechanism for ion heating and wave damping and explain the existence of the critical numberM
c in slow shocks.

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Structure of Reconnection Layers in the Magnetosphere

[1] One-dimensional (1-D) full particle simulations of almost perpendicular supercritical collisionless shocks are presented. The ratio of electron plasma frequency ωpe to gyrofrequency Ωce, the ion to electron mass ratio, and the ion and electron β (β = plasma to magnetic field pressure) have been varied. Due to the accumulation of specularly reflected ions upstream of the shock, ramp shocks can reform on timescales of the gyroperiod in the ramp magnetic field. Self-reformation is not a low ωpe/Ωce process but occurs also in (ωpe/Ωce)2 ≫ 1, low β simulations. Self-reformation also occurs in low ion β runs with an ion to electron mass ratio mi/me = 1840. However, in the realistic mass ratio runs, an electromagnetic instability is excited in the foot of the shock, and the shock profile is considerably changed compared to lower mass ratio runs. Linear analysis based on three-fluid theory (incident ions, reflected ions, and electrons) indicates that the instability is a modified two-stream instability between the decelerated solar wind electrons and the solar wind ions on the whistler mode branch. In the reforming low ion β shocks, part of the potential drop occurs at times across the foot, and part of the potential (∼40%) occurs over a few (∼4) electron inertial lengths in the steepened up ramp. Self-reformation is a low ion β process and disappears for a Mach 4.5 shock at/or above βi ≈ 0.4. It is argued that the ion thermal velocity has to be an order of magnitude smaller than the shock velocity in order for reformation to occur. Since according to these simulations only part of the potential drop occurs for relatively short times over a few electron inertial lengths, it is concluded that coherent shock surfing is not an efficient acceleration mechanism for pickup ions at the low β heliospheric termination shock.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002JA009515

Quasi-perpendicular shocks: Length scale of the cross-shock potential, shock reformation, and implication for shock surfing

Two‐dimensional electromagnetic particle simulations evidence a self‐reformation of the shock front for a collisionless supercritical magnetosonic shock propagating at angle θ0 around 90°, where θ0 is the angle between the normal to the shock front and the upstream magnetostatic field. This self‐reformation is due to reflected ions which accumulate in front of the shock and is observed (i) in both electric and magnetic components, (ii) for both resistive and nonresistive two‐dimensional shocks, and (iii) over a cyclic time period equal to the mean ion gyroperiod measured downstream in the overshoot; resistive effects may be self‐consistently included or excluded for θ0≂90° according to a judicious choice of the upstream magnetostatic field orientation. The self‐reformation leads to a nonstationary behavior of the shock; however, present results show evidence that the shock becomes stationary for θ less than a critical value θr, below which the self‐reformation disappears. Present results are compared to p...

Nonstationarity of a two-dimensional quasiperpendicular supercritical collisionless shock by self-reformation

The interaction between the solar wind and the earth's magnetosphere has been studied by using a time-dependent three-dimensional MHD model in which the IMF pointed in several directions between dawnward and southward. When the IMF is dawnward, the dayside cusp and the tail lobes shift toward the morningside in the northern magnetosphere. The plasma sheet rotates toward the north on the dawnside of the tail and toward the south on the duskside. For an increasing southward IMF component, the plasma sheet becomes thinner and subsequently wavy because of patchy or localized tail reconnection. At the same time, the tail field-aligned currents have a filamentary layered structure. When projected onto the northern polar cap, the filamentary field-aligned currents are located in the same area as the region 1 currents, with a pattern similar to that associated with auroral surges. Magnetic reconnection also occurs on the dayside magnetopause for southward IMF.

An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the Earth's magnetosphere during southward interplanetary magnetic field

A three-dimensional MHD simulation code is used to model the magnetospheric configuration when the IMF has both a northward B(z) component and a B(y) component in the east-west direction. Projections of the plasma pressure, the field-aligned velocity, the field-aligned vorticity, and the field-aligned current along the magnetic field lines into the northern ionosphere are shown and discussed. Cross-sectional patterns of these parameters are shown. The results demonstrate that the B(y) component of the IMF strongly influences the plasma sheet configuration and the magnetospheric convection pattern.

An MHD simulation of By ‐dependent magnetospheric convection and field‐aligned currents during northward IMF

Linear and nonlinear electron damping of the whistler precursor wave train to low Mach number quasi-perpendicular oblique shocks is studied using a one-dimensional electromagnetic plasma simulation code with particle electrons and ions. In some parameter regimes, electrons are observed to trap along the magnetic field lines in the potential of the whistler precursor wave train. This trapping can lead to significant electron heating in front of the shock for low beta(e). Use of a 64-processor hypercube concurrent computer has enabled long runs using realistic mass ratios in the full particle in-cell code and thus simulate shock parameter regimes and phenomena not previously studied numerically.

Numerical studies of electron dynamics in oblique quasi-perpendicular collisionless shock waves

A computer simulation study of the excitation of lower hybrid waves by external sources and the associated plasma heating is presented. A plasma slab with a nonuniform density profile is modeled with a two and one‐half dimensional electrostatic particle code. Both a finite‐length electrostatic antenna and a single wave exciter are considered. The frequency is chosen so that a wave conversion layer exists in the plasma. It is found that a narrow sheath is formed at the plasma edge and the dominant effect is surface electron heating within that region. Ponderomotive force effects as well as the nonlinear generation of harmonics and subharmonics also occur. A low amplitude lower hybrid wave is observed to be excited by the sources and propatates to the wave conversion layer where it is absorbed.

J. M. Dawson

Papers

An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the Earth's magnetosphere during southward interplanetary magnetic field

An MHD simulation of By ‐dependent magnetospheric convection and field‐aligned currents during northward IMF

Numerical studies of electron dynamics in oblique quasi-perpendicular collisionless shock waves

Simulation of lower‐hybrid heating in a nonuniform plasma slab

Simulation of lower-hybrid heating in a nonuniform plasma slab