I. Shinohara

Geotail observations of the Hall current system: Evidence of magnetic reconnection in the magnetotail

In a two-fluid picture of magnetic reconnection, inflow electrons flow with the magnetic field line to the diffusion region, whereas inflow ions cannot reach the diffusion region and rest around a distance of the ion inertial length. The relative motion of electrons and ions results in electric currents, that is, the Hall currents. The Hall current system produces a quadrupole structure in the cross-tail component of the magnetic field near the magnetic reconnection region. Furthermore, this relative motion forms the electric field, whose direction is toward the equatorial plane (midplane). We have investigated the plasma and magnetic field structure near the magnetic reconnection region in the magnetotail with the Geotail spacecraft. We commonly observed inflowing low-energy (less than 5 keV) electrons in the outermost layer of the plasma sheet in magnetic reconnection events, where accelerated ions and electrons flow away from the magnetic reconnection region. These electrons can carry currents to form part of the Hall current system. The observed east-west variations in the magnetic field are consistent with the quadrupole structure produced by the Hall current system. We also noted that inflowing ions have consistently a dawnward motion, almost perpendicular to the magnetic field. These ions indicate the presence of the electric field toward the equatorial plane. The present observations demonstrate the ion-electron decoupling processes for magnetic reconnection in the magnetotail.

Solar wind control of density and temperature in the near-Earth plasma sheet: WIND/GEOTAIL collaboration

A statistical survey of GEOTAIL observations reveals the following properties of the near-Earth plasma sheet (−15 < XGSM′ < −50 Re): During the periods when the northward IMF dominates, (1) the plasma sheet becomes significantly cold and dense, (2) the best correlations between the plasma sheet and the IMF parameters occur when the latter quantities are averaged over 9−4+3 hours prior to the plasma sheet observations, and (3) temperatures diminish and densities increase near the dawn and dusk flanks of the plasma sheet. We suggest that during prolonged northward IMF periods (∼ several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma sheet through the the magnetotail flanks.

Suprathermal electron acceleration in magnetic reconnection

The suprathermal electrons of ≥20 keV that extend from the hot thermal electron with 2–3 keV temperature are sometimes observed in Earths magnetosphere in association with reconnection. We study the origin of the hot and suprathermal electrons in terms of the kinetic magnetic reconnection process by using the two-dimensional particle-in-cell simulation. We find that the hot and suprathermal electrons can be formed in the nonlinear evolution of a large-scale magnetic reconnection. The electrons are, at the first stage, accelerated in the elongated, thin, X-type current sheet. Next the preheated/accelerated electrons are transported to the stronger magnetic field region produced by piling up of magnetic field lines due to colliding of the fast reconnection outflow with the preexisting plasma. In this region they are further accelerated owing to the ∇B drift and the curvature drift. The mirror force of the reconnecting magnetic fields, the effective pitch angle scattering that occurs when the Larmor radius is comparable to the magnetic field line curvature radius, and the broadband waves excited by the Hall electric current are the other important agents to control the particle acceleration.

Low‐frequency electromagnetic turbulence observed near the substorm onset site

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near-Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search-coil magnetometer and the spherical double-probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi-linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively. The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.

Electron flat-top distributions around the magnetic reconnection region

[1] Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat-top-type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole-like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat-top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross-tail current density is sometimes very small. These results indicate that the flat-top-type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat-top distributions, strong field-aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off-equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat-top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high-energy end, and it is found that they are not correlated with the increase of the higher-energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat-top-type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Observations of earthward streaming electrons at the trailing boundary of a plasmoid

In this paper, we report on highly asymmetric spectrum of electrons observed at the boundary of postplasmoid plasma sheet (PPPS). The data were obtained when GEOTAIL made repeated crossings of the boundary after encountering with a tailward flowing plasmoid at XGSM ∼ −46RE. In the spectrum, electrons in 0.1–1 keV energy range are seen to flow earthward along the lobe-like field lines into the PPPS counter to more energetic components (ions and electrons) leaking from the PPPS. In the boundary region, earthward streaming electrons are observed even when the energetic leaking components almost disappear, which makes us characterize this region more generally by tailward flowing field aligned currents (FAC). By referring to hybrid code (ion particles, massless electron fluid) results, we propose that the earthward flowing electrons sustain the FAC away from the X-line (tailward if a spacecraft is located tailward of the X-line), which is a part of a Hall current loop built-up in the course of magnetic reconnection.

Kinetic effects on the Kelvin–Helmholtz instability in ion-to-magnetohydrodynamic scale transverse velocity shear layers: Particle simulations

Ion-to-magnetohydrodynamic scale physics of the transverse velocity shear layer and associated Kelvin-Helmholtz instability (KHI) in a homogeneous, collisionless plasma are investigated by means of full particle simulations. The shear layer is broadened to reach a kinetic equilibrium when its initial thickness is close to the gyrodiameter of ions crossing the layer, namely, of ion-kinetic scale. The broadened thickness is larger in B⋅Ω<0 case than in B⋅Ω>0 case, where Ω is the vorticity at the layer. This is because the convective electric field, which points out of (into) the layer for B⋅Ω<0 (B⋅Ω>0), extends (reduces) the gyrodiameters. Since the kinetic equilibrium is established before the KHI onset, the KHI growth rate depends on the broadened thickness. In the saturation phase of the KHI, the ion vortex flow is strengthened (weakened) for B⋅Ω<0 (B⋅Ω>0), due to ion centrifugal drift along the rotational plasma flow. In ion inertial scale vortices, this drift effect is crucial in altering the ion vortex size. These results indicate that the KHI at Mercury-like ion-scale magnetospheric boundaries could show clear dawn-dusk asymmetries in both its linear and nonlinear growth.

Slow shock downstream structure in the magnetotail

In this paper we discuss that the two different plasma regions exist inside the plasma sheet during the magnetic reconnection in the magnetotail. The inner plasma sheet consists of a weak magnetic field and an isotropic plasma, while the outer plasma sheet contains the intermediate magnetic field intensity between the lobe and the inner plasma sheet and an anisotropic plasma. The plasma temperature parallel to the magnetic field is larger than the perpendicular temperature in the outer plasma sheet. On the basis of Geotail data analysis, we find that two different discontinuities are formed in the magnetotail: One is the slow-mode shock, and the other is the contact discontinuity. The slow shock downstream contains a contact discontinuity which separates the slow shock heated plasmas from the isotropic plasmas in the plasma sheet.

Full electromagnetic Vlasov code simulation of the Kelvin-Helmholtz instability

Recent advancement in numerical techniques for Vlasov simulations and their application to cross-scale coupling in the plasma universe are discussed. Magnetohydrodynamic (MHD) simulations are now widely used for numerical modeling of global and macroscopic phenomena. In the framework of the MHD approximation, however, diffusion coefficients such as resistivity and adiabatic index are given from empirical models. Thus there are recent attempts to understand first-principle kinetic processes in macroscopic phenomena, such as magnetic reconnection and the Kelvin–Helmholtz (KH) instability via full kinetic particle-in-cell and Vlasov codes. In the present study, a benchmark test for a new four-dimensional full electromagnetic Vlasov code is performed. First, the computational speed of the Vlasov code is measured and a linear performance scaling is obtained on a massively parallel supercomputer with more than 12 000 cores. Second, a first-principle Vlasov simulation of the KH instability is performed in order ...

Merging of magnetic islands as an efficient accelerator of electrons

In a thin elongated current sheet, it is likely that more than one X-line forms and thus multiple magnetic islands are produced. The islands are then subject to merging. By simulating such a case with a two-dimensional full-particle code, we show that a merger forming a large island produces the most energetic electron population in the system. By setting the lateral extent of the simulation box to be as large as ∼ 100 ion inertial lengths, we introduce many (16) small islands in the initial thin current sheet ( ∼ 1 ion inertial length thickness). Merging of these islands proceeds to leave only two islands in the system. Then, strong electron acceleration is seen upon the final merger that produces the single island in the large simulation box. The most energetic electrons in the system are accelerated at the merging line. The merging line acceleration dominates because the reverse-reconnection facilitating the final merger is in such a strongly driven manner that the associated electric field is an order of magnitude larger than those available upon normal reconnection. Combining the results from additional runs enables us to obtain a scaling law, which suggests a non-negligible role played by merging lines in the observed electron acceleration phenomena.