Collisionless reconnection in the magnetotail

Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component (B_z) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection.     

PAMELA observational capabilities of Jovian electrons

PAMELA is a satellite-borne experiment that has been launched on June 15th, 2006. It is designed to make long duration measurements of cosmic radiation over an extended energy range. Specifically, PAMELA is able to measure the cosmic ray antiproton and positron spectra over the largest energy range ever achieved and will search for antinuclei with unprecedented sensitivity. Furthermore, it will measure the light nuclear component of cosmic rays and investigate phenomena connected with solar and earth physics. The apparatus consists of: a time of flight system, a magnetic spectrometer, an electromagnetic imaging calorimeter, a shower tail catcher scintillator, a neutron detector and an anticoincidence system. In this work a study of the PAMELA capabilities to detect electrons is presented. The Jovian magnetosphere is a powerful accelerator of electrons up to several tens of MeV as observed at first by Pioneer 10 spacecraft (1973). The propagation of Jovian electrons to Earth is affected by modulation due to Corotating Interaction Regions (CIR). Their flux at Earth is, moreover, modulated because every 13 months Earth and Jupiter are aligned along the average direction of the Parker spiral of the Interplanetary Magnetic Field.PAMELA will be able to measure the high energy tail of the Jovian electrons in the energy range from 50 up to 130 MeV. Moreover, it will be possible to extract the Jovian component reaccelerated at the solar wind termination shock (above 130 MeV up to 2 GeV) from the galactic flux.     

Understanding magnetotail current sheet meso-scale structures using MHD simulations

Recent Cluster observations have strongly supported the existence of meso-scale structure in the magnetotail current sheet. In our study, a magnetohydrodynamic simulation event study exhibited current sheet behavior comparable to that seen in the Cluster observations. Geotail and DoubleStar observations also show that the simulation is providing a realistic representation of the magnetosphere during the period of interest; that is, when the current sheet evidently becomes bifurcated. The magnetohydrodynamic simulation allows us to place the local observations into a global contest. It shows that the observations can be explained in terms of localized reconnection tailward of the Cluster location and the formation of a flux rope nearby. The simulation also features wave-like structure across the current sheet.     

Study of waves in the magnetotail region with cluster and DSP

The study of the neutral sheet is of fundamental importance in understanding the dynamics of the Earth’s magnetosphere. From the earliest observation of the magnetotail, it has been found that the neutral sheet frequently appears to be in motion due to changing solar wind conditions and geomagnetic activity. Multiple crossings of the neutral sheet by spacecraft have been attributed to a flapping motion of the neutral sheet in the north–south direction, a wavy profile either along the magnetotail or the dawn–dusk direction. Cluster observations have revealed that the flapping motions of the Earth’s magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. This flapping motion is shown here to propagate at an angle of ∼45° with x_GSM. A possible assumption that the flapping could be created by a wake travelling away from a fast flow in the current sheet is rejected. Other waves in the magnetotail are found in the ULF range. One conjunction event between Cluster and DoubleStar TC1 is presented where all spacecraft show ULF wave activity at a period of approximately 5 min during fast Earthward flow. These waves are shown to be Kelvin–Helmholtz waves on the boundaries of the flow channel. Calculations show that the conversion of flow energy into magnetic energy through the Kelvin–Helmholtz instability can contribute to a significant part of flow breaking between Cluster and DoubleStar TC1.     

Electron acoustic solitary waves in the Earth’s magnetotail region

Satellite observations have revealed solitary potential structures in the Earth’s magnetotail region. These structures have both positive (compressive) and negative (rarefactive) electrostatic potentials. In this paper we study the electron-acoustic solitary waves (EASWs) in an unmagnetized plasma consisting of cold plasma electrons and isothermal ions with two different temperatures. Using the reductive perturbation method, the nonlinear evolution of such structures is studied. The numerical computations are performed to study the role of two temperature ions in the generation of EASWs. In this case, the model supports the existence of both positive and negative electrostatic potentials with bipolar pulses. The electric field associated with these positive and negative solitary structures are numerically computed. The present study could be useful to construe the compressive and rarefactive electric field bipolar pulses associated with the BEN type emissions in the magnetospheric regions where the electron beams are not present.     

Flux rope passage at the Moon while in the terrestrial magnetotail

The Moon spends 20% of its orbit within the terrestrial magnetosphere. During this time it experiences a dynamic plasma environment, including high-speed streams, flux ropes and a flux of heavy ions from ionospheric outflows. 3D multi-fluid simulations of the Moon within the magnetosphere during a substorm shows that a highly variable plasma flow can develop in the vicinity of the Moon due to the passage of a flux rope. The transit of a flux rope past the Moon potentially leads to a plasma wake that is mis-aligned from the optical wake by nearly 30°$30°$. This will have implications when determining the range of space weathering and surface charging the lunar surface experiences.     

Imprints of small-scale nonadiabatic particle dynamics on large-scale properties of dynamical magnetotail equilibria

We extend our large-scale kinetic (LSK) simulation of the magnetotail by including the global electrostatic effects generated by the field-aligned motion of electrons. Differences in electron and ion dynamics result in significant electrostatic fields near the current sheet (especially near X-lines) and in the auroral zone. In addition, E_ƒ and E alter the ion precipitation profile and affect particle loss from the system through the flanks and downtail. This work provides a basis for including transverse electron currents in our calculations.     

Evaluation of substorm models with Cluster observations of plasma flow reversal in the magnetotail

We evaluate two prevailing substorm models with an event of plasma flow reversal from tailward to Earthward detected by Cluster at the downstream distance of ∼19 R_E in the magnetotail during a substorm on August 22, 2001. We use the unique capability of Cluster measurements in determining gradients to examine the associated current density, Lorentz force, and current dissipation/dynamo term. In association with plasma flow reversal, it is found that (1) there was no clear quadrupole magnetic perturbation signature, (2) the x-component of the Lorentz force did not change sign, (3) the y-component of the product of the current density and the electric field was occasionally negative indicative of a dynamo effect, and (4) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source. These observations are consistent with the near-Earth initiation model for substorms with multiple current disruption sites moving progressively tailward near the late stage of substorm expansion.     

Ion dynamics associated with Alfven wave in the near-Earth magnetotail: Two-dimensional global hybrid simulation

Ion dynamics in the near-Earth magnetotail region is examined during periods of fast Earthward flow with a two-dimensional (2-D) global-scale hybrid simulation. The simulation shows that shear Alfven waves are generated at x ∼ −10R_E, where the strong earthward flow is arrested by the dipole field, and propagate along field lines from the equator to both southern and northern polar ionosphere. Non-gyrotropic ion velocity distributions occur where the large-amplitude Alfven waves are dominant. The simulation indicates that the Alfven waves are generated by interaction of the fast earthward flow with the stationary near-Earth plasma. Beam ions are found to be pitch-angle scattered and trapped in the wave field, leading to the non-gyrotropic ion distributions in the high-latitude plasma sheet boundary. In addition, significant particle heating and acceleration are found to occur behind the dipolarization front due to the effect of wave turbulence.     

Particle acceleration by stochastic fluctuations and dawn-dusk electric field in the Earth’s magnetotail

This work is devoted to investigate the interaction between protons and stochastic time-dependent electromagnetic fields generated by oscillating clouds of finite size, randomly positioned in the x–y  -plane. The geometry of the system is two-dimensional and, beside the time-dependent electromagnetic fluctuations, a steady-state, dawn-dusk electric field, _Ey$E_y$, has been added along the y-direction. The simultaneous presence of the stochastic time-dependent fluctuations and of the constant electric field component in the same system gives rise to two types of acceleration mechanisms operating on test particles: a second order Fermi-like process and a direct acceleration. By performing a parametric study, we extensively study the contribution of the two processes to proton acceleration. The energy values reached by test particles in this simple model are in good agreement with those observed in the Earth’s magnetotail region. Possible applications to the problem of particle acceleration in the terrestrial magnetosphere are widely discussed, and guidelines for future works are drawn.     

Vertical distribution of NH3 in the upper Jovian atmosphere from IUE observations

The first unambiguous identification of ammonia in the upper atmosphere of Jupiter has been obtained from the observation of individual NH₃ bands in an IUE high resolution spectrum in the 2100–2400 Å spectral range. The variation with wavelength of the strengths of these NH₃ bands implies that the NH₃ abundance has to be strongly reduced by photolysis in the upper jovian atmosphere. Preliminary analysis by means of scattering models shows that the ammonia mixing ratio cannot be constant with altitude. The mixing ratio NH₃/H₂ ranges from 5 10^?8 to 5 10^?7 at the 250 mb pressure level, and decreases as P or P² toward higher altitudes.     

Detection of the high energy component of Jovian electrons in Low Earth Orbit with the PAMELA experiment

The PAMELA experiment is devoted to the study of cosmic rays in Low Earth Orbit with an apparatus optimized to perform a precise determination of the galactic antimatter component of c.r. It is constituted by a number of detectors built around a permanent magnet spectrometer. PAMELA was launched in space on June 15th 2006 on board the Russian Resurs-DK1 satellite for a mission duration of 3 years. The characteristics of the detectors, the long lifetime and the orbit of the satellite, will allow to address several aspects of cosmic-ray physics. In this work we discuss the observational capabilities of PAMELA to detect the electron component above 50 MeV. The magnetic spectrometer allows a detailed measurement of the energy spectrum of electrons of galactic and Jovian origin. Long term measurements and correlations with Earth–Jupiter 13 months synodic period will allow to separate these two contributions and to measure the primary electron Jovian component, dominant in the 50–70 MeV energy range. With this technique it will also be possible to study the contribution to the electron spectrum of Jovian e⁻ reaccelerated up to 2 GeV at the Solar Wind Termination Shock.     

Dynamics of the plasma sheet in the magnetotail: Interrelation of turbulent flows and thin current sheet structures

Dynamics of the magnetotail involves elementary processes of magnetic field merging (reconnection layer formation) occurring on medium spatial scales. Every such process features two different stages, a fast one and a subsequent slower one. The corresponding short time scale _T1$T_1$ is associated with disturbances propagating in the tail lobes. The longer time scale _T2$T_2$ is associated with plasma motions in the plasma sheet. A disturbance appearing in the magnetotail on the time scale _T1$T_1$ results in a loss of equilibrium in the plasma sheet. By means of theoretical argument and numerical simulation, it is shown that the relaxation process which follows on the time scale _T2$T_2$, produces extremely thin embedded current sheets, along with generation of fast plasma flows. The process provides an effective mechanism for transformation of magnetic energy accumulated in the magnetotail, into energy of plasma flows. The fast flows may drive turbulent motions on shorter spatial scales. In their turn, those motions can locally produce very thin current sheets; after that, nonlinear tearing process leads to generation of neutral lines, and reconnection. The latter produces new fast disturbances on the time scale _T1$T_1$ closing the feedback loop.     

Jovian periodicities (∼10 h, ∼40 min) on Ulysses’ Distant Jupiter Encounter observations around the Halloween CIR events

We analyzed data from four different instruments (HI-SCALE, URAP, SWOOPS, VHM/FGM) onboard Ulysses spacecraft (s/c) and we searched for possible evidence of Jovian emissions when the s/c approached Jupiter during the times of Halloween events (closest time approach/position to Jupiter: February 5, 2004/R = 1683 R_J,θ = ∼49°). In particular, we analyzed extensively the low energy ion measurements obtained by the HI-SCALE experiment in order to examine whether low energy ion/electron emissions show a symmetry, and whether they are observed at north high latitudes upstream from the jovian bow shock, as is known to occur in the region upstream from the south bow shock as well ( Marhavilas et al., 2001). We studied the period from October 2003 to March 2004, as Ulysses moved at distances 0.8–1.2 AU from the planet at north Jovicentric latitudes <75°, and we present here an example of characteristic Jovian periodicities in the measurements around a CIR observed by Ulysses on days ∼348–349/2003 (R = 1894 R_J,θ = 72°). We show that Ulysses observed low energy ion (∼0.055–4.7 MeV) and electron (>∼40 keV) flux and/or spectral modulation with the Jupiter rotation period (∼10 h) as well as variations with the same period in solar wind parameters, radio and magnetic field directional data. In addition, characteristic strong ∼40 min periodic variations were found superimposed on the ∼10 h ion spectral modulation. Both the ∼10 h and ∼40 min ion periodicities in HI-SCALE measurements were present in several cases during the whole period examined (October 2003 to March 2004) and were found to be more evident during some special conditions, for instance during enhanced fluxes around the start (forward shock) and the end (reverse shock) of CIRs. We infer that the Jovian magnetosphere was triggered by the impact of the CIRs, after the Halloween events, and it was (a) a principal source of forward and reverse shock-associated ion flux structures and (b) the cause of generation of ∼10 h quasi-periodic magnetic field and plasma modulation observed by Ulysses at those times.     

我们向何处去？

从亚里士多德开始,西方的哲学家、科学家就普遍认为生物界存在着一个从低级到高级的阶梯。当拉马克构建第一个成系统的进化假说时,他对这个自然界阶梯的观念全盘接收,只不过把它改造成了动态的。生物进化被视为一个从低级到高级的方向性过程,人类就位于这个过程的巅峰。这种进化观念影响深远,直到现在,中文仍然习惯使用“进化”一词,而不用更为恰当的“演化”,     

The exosat mission

Exosat, the European X-ray Observatory, was placed in orbit on 26 May 1983. The spacecraft, stabilized axes in three to a few arc second, carries four instruments, two one-metre focal length imaging telescopes, a large area proportional counter array and a gas scintillation proportional counter spectrometer. The salient features of the instrumentation, the sensitivities achieved in orbit and the status after the first year of orbital operation are described. Three specific observations, VO332+53, SCO X–1 and M.83 are discussed to demonstrate the power of the EXOSAT instrumentation and the operational flexibility of the spacecraft and ground system.     

灾难之最

自然界一旦发威,人类瞬间就会变得手无缚鸡之力,只能对其俯首称臣.龙卷风、海啸、地震和火山爆发都能在顷刻之间夺去成千上万的生命,而最终的死亡人数通常永远都不得而知.     

The tenma mission

Tenma, the second X-ray astronomy satellite of Japan launched in February 1983, is outlined. The main instrument of Tenma is a large-area gas scintillation proportional counter array. Some of the highlights of the results thus far obtained are briefly discussed.