ZhenXing Liu

Formation of B2 intermetallic NiAl and FeAl by mechanical alloying

Abstract Nanostructud B2 intermetallic compounds NiAl and FeAl have been prepared by mechanical alloying (MA) the elemental powder mixtures and subsequent heating. The structural evolution during MA was monitored by in situ thermal analysis and X-ray diffraction (XRD). The final products were characterized by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the nanocrystalline intermetallic compound NiAl, which is difficult to disorder by milling, was synthesized directly after an exothermic explosive reaction; whereas FeAl compound was formed after a thermal process of asmilled Fe(Al) solid solution obtained through interdiffusion during MA. The large heat of formation of NiAl compound is the main driving force for the exothermic explosive reaction, and the difference in diffusivity between NiAl system and FeAl system is suggested to be the main cause of the different behaviors of formation between NiAl and FeAl compounds by MA.

Temporal and spatial scales of a high-flux electron disturbance in the cusp region: Cluster observations

Using the Cluster multispacecraft observations, we analyze a long-duration field-aligned high-flux electron disturbance in the cusp region on 30 September 2001. All four Cluster spacecraft observed the same disturbance in which both the upward and downward electrons are observed and the electron flux was 1 order of magnitude higher than usual in the cusp. The temporal scale of the field-aligned electron disturbance was at least 50 min. The spatial scale was about 540 km in the direction along the orbit and at least 1800 km in local time extent in the midaltitude cusp region. It was the longest duration and the largest spatial scale of any field-aligned electron disturbance observed in the polar region up to date, and its observation would not have been possible with a single satellite. Both upward and downward electrons are the main contributors to the field-aligned currents in the electron disturbance. During this electron disturbance, the solar wind dynamic pressure increased, and the interplanetary magnetic field (IMF) kept being southward. It is likely that the field-aligned high-flux electron disturbance with its long temporal and large spatial scales was caused by high dynamic pressure of the solar wind during a permanently southward IMF. This enables us to learn more about electron disturbances in the cusp and is important to understand the physical mechanism, especially for the solar wind-magnetosphere-ionosphere coupling.

Two-and-one-half-dimensional magnetohydrodynamic simulations of the plasma sheet in the presence of oxygen ions: The plasma sheet oscillation and compressional Pc 5 waves

Two-and-one-half-dimensional magnetohydrodynamic simulations of the multicomponent plasma sheet with the velocity curl term in the magnetic equation are represented. The simulation results can be summarized as follows: (1) There is an oscillation of the plasma sheet with the period on the order of 400 s (Pc 5 range); (2) the magnetic equator is a node of the magnetic field disturbance; (3) the magnetic energy integral varies antiphase with the internal energy integral; (4) disturbed waves have a propagating speed on the order of 10 km/s earthward; (5) the abundance of oxygen ions influences amplitude, period, and dissipation of the plasma sheet oscillation. It is suggested that the compressional Pc 5 waves, which are observed in the plasma sheet close to the magnetic equator, may be caused by the plasma sheet oscillation, or may be generated from the resonance of the plasma sheet oscillation with some Pc 5 perturbation waves coming from the outer magnetosphere.

A preliminary exploration of the mechanism for the occurrence of two types of various magnetic structures in the magnetotail

As well known, the magnetic cross-tail component By in the magnetotail is in direct proportion to the interplanetary magnetic field (IMF) Bycomponent. And the polarity of IMF and plasmoid / flux rope Bycomponents do indeed agree. This results indicate that the IMF Bypenetrates plasmoids and the magnetic structures must therefore be three-dimensional. In this note, the dynamical processes of magnetotail in the course of a substorm are studied using a MHD code with two-dimensions and three components on the basis of two types of initial equilibrium solutions of the quiet magnetotail. The numerical results of two cases illustrate various features of time evolution of By component that correspond to two kinds of plasmoid-like structures: one is associated with a flux rope core and the other resembles a “closed loop” plamoid. Therefore, the occurrence of various magnetic structures in the magnetotail might be related to nonsteady driven reconnection with different distributions of the Bycomponent.

Simulation of interplanetary magnetic field By penetration into the magnetotail

Based on our global 3D magnetospheric MHD simulation model, we investigate the phenomena and physical mechanism of the By component of the interplanetary magnetic field (IMF) penetrating into the magnetotail. We find that the dayside reconnected magnetic field lines move to the magnetotail, get added to the lobe fields, and are dragged in the IMF direction. However, the By component in the plasma sheet mainly originates from the tilt and relative slippage of the south and north lobes caused by plasma convection, which results in the original Bz component in the plasma sheet rotating into a By component. Our research also shows that the penetration effect of plasma sheet By from the IMF By during periods of northward IMF is larger than that during periods of southward IMF.

An interpretation of electrostatic density shocks in space plasma

A physical model of electrostatic shocks observed in space plasma is established by deriving the “Sagdeev potential” from the magnetohydrodynamic equations in a cylindrical coordinate system. The results show that the electrostatic density shock and its corresponding solitary electric-field structure can develop from an ion acoustic wave or an ion cyclotron wave if the Mach number and the initial electric field satisfy some conditions. Some features of the shock wave are discussed. The result can be used to interpret the electrostatic shock observed in geospace plasma.

Continuous lobe reconnection in the mid-tail and its relationship to substorms: Cluster observations of continuous lobe reconnection in the mid-magneto tail

When the IMF turns southward, a great amount of magnetic energy is stored in the magnetotail, and the electric field across the magnetotail substantially enhances. As long as magnetic reconnection (MR) in the magnetotail initiates and continues, the magnetic field and plasma in the central plasma sheet are carried away to the near-Earth and down to the tail, the magnetic field and plasma in the lobe region enter the CPS and are involved in MR. We call this process“Continuous Lobe Reconnection (CLR)”. In this paper a detailed analysis of Cluster observation of MR through 2001–2003 is made. Plenty of CLR events are found that led to considerable changes of tail configuration, appearance of BBF, as well as large-scale bubbles in which both plasma temperature and number density substantially decrease. It is shown that in general CLR events last for dozens of minutes and have good correspondence to substorm initiation under the condition of continuous southward IMF.

Simulation of the dawn-dusk magnetosheath asymmetry under quasi-steady states

The dawn-dusk asymmetry of the magnetosheath under quasi-steady states has been studied by using a newly developed 3D MHD magnetosphere simulation model. The results show that the dawn-dusk asymmetry is substantial because of the Parker spiral IMF. It is found that the dawn-dusk magnetosheath thickness asymmetry is the effect of different shock conditions. The plasma density and flux asymmetry are mainly caused by the different thickness of the dawn-dusk magnetosheath, and the magnetic reconnection on the magnetopause has no significant effects. It is also showed that the Plasma Depletion Layer in front of the dayside magnetopause can cause duskward plasma flow, and the total plasma flux on the dusk side will be higher.

Cluster observations of unusually high concentration of energetic O+ carried by flux ropes in the nightside high‐latitude magnetosheath during a storm initial phase

We present measurements from Cluster spacecraft to investigate the energetic singly charged oxygen ions, O+, within the flux ropes in the nightside high-latitude magnetosheath during the initial phase of an intense storm on 24 October 2011. Three magnetic flux ropes were identified by Cluster 4 in the intervals from 20:10 UT to 20:20 UT. Unusually, large number density of energetic O+ ions at energy of tens of keV was detected within these flux ropes. The number density of O+ ions was above 0.1cm(-3) and the maximum value was about 0.25cm(-3), 1 order of magnitude larger than the ambient value (0.01cm(-3)) in the magnetosheath. The O+/H+ ratio is as large as 0.08 within the flux ropes. Enhanced convection electric fields E-y (10mV/m) are associated with the flux rope and the high concentrations of energetic O+. The flux ropes, which are presumably produced by magnetic reconnection at the dayside magnetopause or cusp, are convected at a larger velocity than the tailward velocity of ambient flows in the magnetosheath. These observations together show that abundant energetic O+ ions are carried by the flux ropes toward tail in the nightside magnetosheath. Our observations present new evidence for a chain linking the dayside to the nightside in the global O+ transport process.

Influence of tail-like magnetic field on O+ ion distribution in the Martian magnetosphere

Based on the comparison with the Earth, using the LB magnetic field model, the distribution of O+ ion originating from the ionosphere in the Martian magnetosphere is theoretically studied under different conditions of the tail-like magnetic field. The results show that the tail-like magnetic field has influence on the O+ ion flux in the Martian magnetotail: (i) the O+ ion flux in the Martian tail will increase if the tail-like magnetic field increases; when the tail-like magnetic field increases from 5 nT to 20 nT, the O+ ion flux increases 3 times in the region of 2.8Rm in the Martian tail; and (ii) the O+ ion flux decreases with increasing intrinsic moment; when the intrinsic moment increases about 5 times, the flux decreases to one fourth in the region of 2.8Rm in the Martian tail. According to the data on the O+ ion flux and theoretical result in this paper, the deduced Martian intrinsic moment is about 2 ×1021 Gcm3. This is consistent with the most recent observation by the USA satellite MGS.