篠原 育

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.

Recently, quick triggering of magnetic reconnection (QMRT) even in an ion-scale current sheet is found to be possible with the help of the nonlinear evolution of the lower hybrid drift instability (LHDI). The details of the QMRT mechanism are reviewed mostly based on three-dimensional full-particle simulation results of our group. QMRT is mediated by LHDI and its time scale is comparable to the saturation time scale of LHDI. Depending on the initial current sheet thickness, two types of QMRT, so-called Type-I and Type-II QMRT, are demonstrated.

The 'whistler critical Mach number', Mcrit,w is one of the dimensionless parameters that characterizes collisionless shocks. Originally, it was introduced to indicate the critical point above which whistler waves do not propagate upstream. Indeed our analysis of Geotail data at the Earth's bow shock shows intense whistler waves in the sub-critical regime, MA &lt Mcritw but not in the super-critical regime. In this paper, we further report that Mcritw seems to regulate the electron acceleration efficiency at the shocks. At the shock transition layer it is found that the spectral index Γ of electron energy spectra defined by F(E) ∝ E-Γ is distributed between 3.5 and 5.0 in the sub-critical regime, while the hardest energy spectra with Γ = 3-3.5 are detected in the super-critical regime. We discuss a possible relationship between Mcritw and the electron acceleration. Copyright 2006 by the American Geophysical Union.

The details of Quick Magnetic Reconnection Triggering (QMRT) in ion-scale current sheets have been investigated using three-dimensional (3-D) full particle simulations with the mass ratio of 400. QMRT is mediated by coupling to the Lower-Hybrid Drift Instability (LHDI) and its time scale is as quick as the LHDI time scale. Up to the initial half-thickness D of D/lambda(i) = 0.875 (lambda(i): the ion inertial length), QMRT is attained via thin current layer formation at the center of the current sheet in reaction to the LHDI activity (Type-I). At D/lambda(i) = 1, QMRT is still available but is attained by boosting up the tearing instability growth rate via the electron temperature anisotropy T-e perpendicular to/T-e parallel to similar to 1.2 (Type-II). In this case the LHDI activity induces the current density redistribution that does not result in enhanced current density at the center but produces electron perpendicular heating therein.

[1] We examine in detail the rapid decrease in B-z just before dipolarizations observed by the Geotail satellite in the near-Earth plasma sheet at (X-GSM, Y-GSM) = (-8.3 R-E, - 5.1 R-E). The observations were made using high-time-resolution data from a fluxgate magnetometer (16-Hz sample), a search-coil magnetometer ( 128 Hz), and an electric field antenna ( 64 Hz). Two dipolarizations were observed during a short time interval of 2 min. The magnetic Bz component suddenly decreased 2 - 4 s prior to the dipolarization. Characteristic waves with frequencies of 5 - 20 Hz and amplitudes of 1 - 3 mV m(-1) and 5 - 15 nT s(-1) were observed in the electric and magnetic field data at the time of the sudden decreases in B-z. We discuss two possible causes of the sudden decreases in B-z prior to the dipolarizations: ( 1) passage of a field-aligned line current associated with the substorm current wedge and ( 2) explosive growth phase and subsequent disruption of the tail current caused by the observed characteristic field oscillations.

Using high-time-resolution data for the magnetic field (1/16 s), plasma moment (12 s), and electric field (3 s) obtained by Geotail, we have investigated various characteristics of magnetic field fluctuations for 21 events of substorm-associated near-Earth dipolarization at X-GSM = -8 similar to -11 R-E and vertical bar Y(GSM)vertical bar < 5 R-E. The dipolarizations were always accompanied by fluctuations of the magnetic field. The amplitude of the fluctuations tends to be larger for smaller ambient magnetic field. For several events we found that magnetic field elevation angles show rapid decrease prior to the rapid increase (features like explosive growth phase). Compressional fluctuations of magnetic field intensity B show characteristic spikes of a sudden decrease and increase with a timescale of seconds. For quite a few cases the fluctuations are accompanied by enhancements of earthward plasma velocity. The spectral shape of the B fluctuations had a steeper slope (f(-2.7)) at higher frequency range at 1.0-0.2 Hz than at 0.01-0.2 Hz (f(-2.0)) for the events closer to the cross-tail current sheet (smaller ambient-B region). We found that the steeper slope extends to lower frequency range below 0.2 Hz for the events away from the current sheet (larger ambient-B region). This fact may suggest transition from three-dimensional to two-dimensional turbulence from smaller B to larger B region. We also investigated correlation of fluctuations in plasma velocity and magnetic field for waves with periods around 60 s. The correlations were rather weak, suggesting that non-MHD plasma processes are going on. For these waves, the magnetic and plasma pressure variations were clearly antiphase.

We have recently found that a quick triggering of magnetic reconnection in an ion-scale current sheet is possible. For the quick triggering of magnetic reconnection, the lower hybrid drift waves excited at the edges of the current sheet is indispensable. This wave excitation brings about formation of a thin magnetic neutral layer sustained by accelerated electrons, and this thin layer is subject to the quick reconnection. We found that the electron acceleration process is strongly coupled with the non-linear evolution of the lower hybrid drift instability. The inductive electric field, which is generated through the change of the current profile, can efficiently accelerate meandering electrons around the magnetic neutral layer. As a result, electric current in the thin layer is mostly carried by non-adiabatic electrons. The production of non-adiabatic electrons is playing a crucial role in making the quick triggering available.

[1] To understand magnetotail dynamics, it is essential to determine where magnetic reconnection takes place in the near-Earth magnetotail during substorms. The Geotail spacecraft thoroughly surveyed the near-Earth plasma sheet at radial distances of 10-31 RE during the years 1995-2003. Thirty-four clear reconnection events were identified using the criterion of strong electron acceleration. Various solar wind parameters prior to each reconnection event were examined in order to find the factor controlling the location of the magnetic reconnection site in the magnetotail. The same analyses were carried out for fast tailward flow events. The most important factor was determined to be the solar wind energy input, which can be expressed by - Vx × Bs, where Vx is the x component of the solar wind velocity and Bs is the southward component of the interplanetary magnetic field. It is likely that higher efficiency of energy input, rather than the total amount of energy input, primarily controls the location of magnetic reconnection magnetic reconnection takes place closer to the Earth when efficiency of energy input is higher. The effect of solar wind dynamic pressure is minor. The present result suggests that the tail magnetic reconnection location during substorms is controlled by solar cycle variations in the solar wind. Copyright 2005 by the American Geophysical Union.

In order to open the new horizon of research in the plasma universe, SCOPE will perform formation flying multi-scale observations combined with high-time resolution electron detection and will enable data-based study on the key space plasma processes from the cross-scale coupling point of view. Key physics to be studied are magnetic reconnection under various conditions, shocks in space plasma, collisionless plasma mixing at the boundaries, and physics of current sheets embedded in complex magnetic geometries. The SCOPE mission is made up of the 5 spacecraft (s/c) formation put into the equatorial orbit with the apogee at 30Re (Re: earth radius). One of the s/c is a large mother ship which is equipped with a full suite of particle detector including ultra-high sampling cycle electron detector. Among other 4 small s/c one remains near (∼10 km) the mother ship and the s/c-pair will focus on wave-particle interaction utilizing inter-s/c communication. Others are used for wave-particle interaction study when the distance from the mother ship is small (∼100 km) and are used as the plasma monitors at ion-scales when the distance is larger (100-300 0km). There is lively on-going discussion on the SCOPE-M3 collaboration, which would certainly make the coverage over the scales of interest better and thus make the mission success to be attained at an even higher level.

Influence of the guide field on quick magnetic reconnection triggering (QMRT) is investigated by three-dimensional (3-D) full-particle simulations with the mass ratio of 400. Here two runs with different current sheet thickness (D = 0.35 and 0.5, D: the current sheet half thickness normalized by the ion inertial length) are performed with a constant guide field of B-0y = 0.75B(0x) (B-0x: asymptotic magnitude of reconnecting component). QMRT has been found in a null guide-field case of D = 0.5 similar to 0.875 and is quick because reconnection onset is directly coupled to the most quickly growing mode in the system, the lower-hybrid drift instability (LHDI) (I. Shinohara and M. Fujimoto, Quick triggering of magnetic reconnection in an ion-scale current sheet, submitted to Physical Review Letters, 2004, hereinafter referred to as Shinohara and Fujimoto, submitted manuscript, 2004). In contrast D = 0.5 with B-0y = 0.75B(0x) is too thick to be subject to QMRT. Recovery of QMRT is found at D = 0.35, accompanying a single X-line formation with slight delay from the LHDI saturation.

The spacecraft Geotail has observed the Hall current system in the vicinity of the magnetic reconnection site of the near-Earth magnetotail for substorm onsets. In the outermost region near the plasma sheet/tail lobe boundary, field-aligned currents flow out of the magnetic reconnection site. In the adjacent region, just inside the outflowing current layer, field-aligned currents flow into the magnetic reconnection site. Hence, the Hall current circuit forms a thin double-sheet structure near the separatrix layer. Copyright 2003 by the American Geophysical Union.

The suprathermal electrons of ≥20 keV that extend from the hot thermal electron with 2-3 keV temperature are sometimes observed in Earth's 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. Copyright 2001 by the American Geophysical Union.

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 Halt 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.

We have found clear evidence for intensification of the cross-tail current and associated occurrence of magnetic reconnection starting from several minutes before a substorm onset at a distance of similar to 17 Re down the tail. Following a gradual increase in the growth phase, the current density was intensified stepwise at similar to 5 minutes before the onset, and then gradually increased on the average while fluctuating. It is noted that the intensification of the cross-tail current density was due to enhancement of the dawnward electron velocity. The increasing current density accompanied a gradual increase of the tailward flow velocity and small enhancements of the electron and ion temperatures. A few minutes later, the magnetic Bz field dropped down to very small values, and shortly thereafter (around the substorm onset) the rate of change of the tailward velocity was enhanced in association with negative Bz field, significant increasing of the ion and electron temperatures, and decreasing of the plasma density. These signatures can be well understood in terms of temporal evolution of magnetic reconnections associated with local thinning of the current sheet and intensification of the current density. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

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.

In order to clarify the macroscopic magnetic field diffusion process associated with the lower hybrid drift and the drift kink instabilities, a large-scale two-dimensional electromagnetic full particle simulation has been performed. We find a new magnetic diffusion process which is caused by coupling of the lower hybrid and drift kink instabilities. This process can dissipate the cross-tail current much more efficiently than without coupling. Furthermore, we find that electrons are accelerated along the ambient magnetic field and form flat-top distributions at the outside edge of the plasma sheet in the case of oblique propagation. This electron acceleration may result from the wave-particle interaction between the obliquely propagating lower hybrid waves and electrons. ©1999 COSPAR. Published by Elsevier Science Ltd.

A statistical survey of GEOTAIL observations reveals the following. properties of the near-Earth plasma sheet (-15 < X-GSM' < -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 (similar to several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma, sheet through the the magnetotail flanks.

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. Copyright 1997 by the American Geophysical Union.

A new method to estimate the spatial structure of the moving magnetohydrodynamic (MHD) system is presented. We integrate the Faraday's induction equation, [Formula Omitted], assuming that the system is one-dimensional. The spatial gradient direction, along which the integration is made, is determined by minimizing the residue of the integrated Faraday's equation against the observed magnetic field. This new method can be regarded as an extension of the conventional magnetic minimum variance method. Sample data analyses using this new method for magnetotail events are also presented. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

A new method to estimate the spatial structure of the moving magnetohydrodynamic (MHD) system is presented. We integrate the Faraday's induction equation, ∂B→/∂t = -rot E→, assuming that the system is one-dimensional. The spatial gradient direction, along which the integration is made, is determined by minimizing the residue of the integrated Faraday's equation against the observed magnetic field. This new method can be regarded as an extension of the conventional magnetic minimum variance method. Sample data analyses using this new method for magnetotail events are also presented.

Copyright 1996 by the American Geophysical Union. In the lobe/mantle region at ∼159 RE away from the Earth during a geomagnetically disturbed period, we have found the coexistence of three ion species, H+, He++, and O+, streaming tailward with nearly the same flow velocity ∼200-500 km/s. Both H+ and O+ ions are detected almost continuously from near plasma sheet to near magnetopause region. From a positive correlation between the proton density and their velocity component parallel to the magnetic field VH+∥, we conclude that most of protons have come from the solar wind. The existence of He++ further supports this conclusion, which implies the importance of solar wind contribution to the magnetotail. The existence of O+, on the other hand, suggests that the ions of ionospheric origin have mixed with those of solar wind origin. The lack of positive correlation between O+ density and VO+∥ is consistent with the idea that O+; ions have some source mechanism different from that of protons. Simultaneously, curious velocity differences are also observed: VO+∥ appears to be often faster than VH+∥; by ΔV∥ = 20-30 km/s. This observation may provide a key for further discussion.