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High-speed jets (HSJs) occur frequently in Earth's magnetosheath downstream of the quasiparallel bow shock. They have great impacts on the magnetosheath and the magnetosphere. Using two-dimensional global hybrid simulations, we investigate the formation and evolution of HSJs with different solar wind conditions. When the quasi-parallel shock is formed, HSJs begin to appear in the quasi-parallel magnetosheath. Some elongated HSJs formed at the quasi-parallel bow shock extend toward the quasi-perpendicular magnetosheath along with the background magnetosheath flow. As the elongated HSJs moves in the magnetosheath, filamentary structures of the velocity, ion density, and temperature occur in the magnetosheath. The filamentary structures are the traces of HSJs moving in the magnetosheath. Moreover, the Kelvin-Helmholtz instability can be excited at HSJs, which causes meandering of HSJs. When the interplanetary magnetic field is aligned to the solar wind velocity, the large-scale HSJs with a parallel (perpendicular) scale size of about 2.5 R-E (0.3 R-E) are formed at the magnetosheath where the theta(Bn) is approaching zero. Then, some HSJs converge, leading to the formation of even larger HSJs with a parallel (perpendicular) scale size of 5 R-E (0.6 R-E).

Magnetopause flux ropes (FRs) play a crucial role in the transport of energy and plasma from the solar wind to the Earth's magnetosphere. Once formed by multiple X-line reconnection, the FRs move poleward, during which they can coalesce with each other through a re-reconnection process. The poleward moving FRs eventually coalesce into the cusp regions, through another re-reconnection process. In this paper, using three-dimensional (3-D) global hybrid simulations, we study the two re-reconnection processes of magnetopause FRs by examining the topological changes of magnetic field lines. These re-reconnection processes are essentially 3-D. Two FRs coalesce with each other and form two new FRs (instead of one new FR in the two-dimensional regime). When FRs move close to the cusp region, their field lines can reconnect with the cusp field lines so that the FRs can break into two shorter ones. The two re-reconnection processes increase the plasma energy and the magnetic flux connected to the Earth, which favors particle and energy transport toward the Earth's magnetosphere.

Electromagnetic ion cyclotron (EMIC) waves are one commonly observed plasma waves in the Earth's inner magnetosphere and play a crucial role in particle dynamics in the radiation belt and ring current. EMIC waves are excited by a proton temperature anisotropy and generally have a left-handed polarization, however satellite observations have usually reported the existence of linearly polarized EMIC waves in the inner magnetosphere. In this paper, we employ a two-dimensional (2D) hybrid code in a dipole field (gcPIC-hybrid) to simulate the propagation of EMIC waves from the equatorial source region. We track one single EMIC wave packet and analyze how its properties evolve along its trajectory. In diagnosing the wave normal angle (WNA) of the packet, we propose a novel method called Wave Front Shape Identification (WFSI). The ellipticity can also been calculated after we know the WNA. By comparing the ellipticity calculated from the linear theory and the ellipticity diagnosed from the simulation, we conclude that in a proton-electron plasma, EMIC waves would turn from a left-handed polarization to a linear polarization solely due to the propagation effect when the waves propagate toward higher latitudes and become oblique. We also find that the peak frequency of the wave packet (the wave mode with the maximum amplitude) decreases when propagating toward higher latitudes, which is due to different growth and damping behavior of different modes.

Flux ropes are ubiquitous at Earth's magnetopause and play important roles in energy transport between the solar wind and Earth's magnetosphere. In this study, structure and coalescence of the magnetopause flux ropes formed by multiple X line reconnection in cases with different southward interplanetary magnetic field (IMF) clock angles are investigated by using three-dimensional global hybrid simulations. As the IMF clock angle decreases from 180 degrees, the axial direction of the flux ropes becomes tilted relative to the equatorial plane, the length of the flux ropes gradually increases, and core field within flux ropes is formed by the increase in the guide field. The flux ropes are formed mostly near the subsolar point and then move poleward toward cusps. The flux ropes can eventually enter the cusps, during which their helical structure collapses, their core field weakens gradually, and their axial length decreases. When the IMF clock angle is large (i.e., the IMF is predominantly southward), the flux ropes can coalesce and form new ones with larger diameter. The coalescence between flux ropes can occur both near the subsolar point when they are newly formed and away from the subsolar point (e.g., in the southern hemisphere) when they move toward cusps. However, when the IMF clock angle is small (

Whistler mode chorus waves in the Earth's inner magnetosphere are usually composed of discrete elements, and each element can be characterized by the following properties: the amplitude, the duration, the frequency span, and the frequency chirping rate. Using a one-dimensional (1-D) particle-in-cell (PIC) simulation code, we study the dependence of these properties of a rising-tone chorus on the number densityn(heq)/n(c0)and temperature anisotropyA(T)of energetic electrons at the magnetic equator. The whistler waves are first excited around the magnetic equator by anisotropic energetic electrons and then develop into a rising-tone chorus when they leave away from the equator. During the propagation toward the pole, the rising-tone chorus with nearly constant frequency span first enhances and then decays. Its frequency chirping rate declines in the early stage and then gradually increases. Meanwhile, the chorus duration is quite the opposite due to propagation effect. Over a suitable range ofn(heq)/n(c0)to generate rising-tone chorus, the frequency chirping rate of the excited rising-tone chorus first increases and then saturates, while its saturated amplitude, duration, and frequency span have a rising tendency with the increasingn(heq)/n(c0). As forA(T), the frequency chirping rate of the generated rising-tone chorus is increasing with the increase ofA(T)that is consistent with prediction of nonlinear theory, while the duration is just the opposite. Our simulation study can give a further understanding of the generation and propagation of rising-tone chorus waves.

Rising-tone chorus waves have already been successfully produced in a mirror magnetic field with the use of one- and two-dimensional particle-in-cell (PIC) simulations. However, in reality, the background magnetic field in the inner Earth's magnetosphere is a dipole magnetic field, unlike symmetric mirror fields. In this paper, with the two-dimensional (2-D) general curvilinear PIC (gcPIC) code, we investigate the generation of rising-tone chorus waves in the dipole magnetic field configuration. The plasma consists of three components: immobile ions, cold background, and hot electrons. In order to save computational resource, the topology of the magnetic field is roughly equal to that at L = 0.6 R-E, although the plasma parameters corresponding to those at L = 6 R-E (R-E is the Earth's radius) are used. Whistler mode waves are first excited around the magnetic equator by the hot electrons with a temperature anisotropy. The excited whistler mode waves propagate almost parallel and antiparallel to the background magnetic field in their source region, which is limited at vertical bar lambda vertical bar

Recently, the generation of rising-tone chorus has been implemented with one-dimensional (1-D) particle-in-cell (PIC) simulations in an inhomogeneous background magnetic field, where both the propagation of waves and motion of electrons are simply forced to be parallel to the background magnetic field. In this paper, we have developed a two-dimensional (2-D) general curvilinear PIC simulation code and successfully reproduced rising-tone chorus waves excited from an anisotropic electron distribution in a 2-D mirror field. Our simulation results show that whistler waves are mainly generated around the magnetic equator and continuously gain growth during their propagation toward higher-latitude regions. The rising-tone chorus waves are observed off the magnetic equator, which propagate quasi-parallel to the background magnetic field with the wave normal angle smaller than 25 degrees. Due to the propagating effect, the wave normal angle of chorus waves is increasing during their propagation toward higher-latitude regions along an enough curved field line. The chirping rate of chorus waves is found to be larger along a field line with a smaller curvature.

In this paper, we perform a 1-D particle-in-cell (PIC) simulation model consisting of three species, cold electrons, cold ions, and energetic ion ring, to investigate spectral structures of magnetosonic waves excited by ring distribution protons in the Earth's magnetosphere, and dynamics of charged particles during the excitation of magnetosonic waves. As the wave normal angle decreases, the spectral range of excited magnetosonic waves becomes broader with upper frequency limit extending beyond the lower hybrid resonant frequency, and the discrete spectra tends to merge into a continuous one. This dependence on wave normal angle is consistent with the linear theory. The effects of magnetosonic waves on the background cold plasma populations also vary with wave normal angle. For exactly perpendicular magnetosonic waves (parallel wave number k(parallel to) = 0), there is no energization in the parallel direction for both background cold protons and electrons due to the negligible fluctuating electric field component in the parallel direction. In contrast, the perpendicular energization of background plasmas is rather significant, where cold protons follow unmagnetized motion while cold electrons follow drift motion due to wave electric fields. For magnetosonic waves with a finite k(parallel to), there exists a nonnegligible parallel fluctuating electric field, leading to a significant and rapid energization in the parallel direction for cold electrons. These cold electrons can also be efficiently energized in the perpendicular direction due to the interaction with the magnetosonic wave fields in the perpendicular direction. However, cold protons can be only heated in the perpendicular direction, which is likely caused by the higher-order resonances with magnetosonic waves. The potential impacts of magnetosonic waves on the energization of the background cold plasmas in the Earth's inner magnetosphere are also discussed in this paper.

Magnetotail reconnection and related phenomena (e.g., flux ropes, dipolarizing flux bundles, flow bursts, and particle injections) occur more frequently on the duskside than on the dawnside. Because this asymmetry can directly result in dawn-dusk asymmetric space weather effects, uncovering its physical origin is important for better understanding, modeling, and prediction of the space weather phenomena. However, the cause of this pervasive asymmetry is unclear. Using three-dimensional global hybrid simulations, we demonstrate that the Hall physics in the magnetotail current sheet is responsible for the asymmetry. The current sheet thins progressively under enhanced global convection; when its thickness reaches ion kinetic scales, some ions are decoupled from the magnetized electrons (the Hall effect). The resultant Hall electric field E-z is directed toward the neutral plane. The Hall effect is stronger (grows faster) on the duskside; i.e., more ions become unmagnetized there and do not comove with the magnetized dawnward E-z x B-x drifting electrons, thus creating a larger additional cross-tail current intensity j(y) (in addition to the diamagnetic current) on the duskside, compared to the dawnside. The stronger Hall effect strength on the duskside is controlled by the higher ion temperature, thinner current sheet, and smaller normal magnetic field B-z there. These asymmetric current sheet properties are in turn controlled by two competing processes that correspond to the Hall effect: (1) the dawnward E x B drift of the magnetic flux and magnetized ions and electrons and (2) the transient motion of the unmagnetized ions which do not execute E x B drift.

著者人数: 18名, Accepted: 2012-06-19, 資料番号: SA1004256000