藤本 正樹

We have studied the response of large-scale ionospheric convection to substorm expansion onsets on the basis of two weak substorms of 1 May 2001, during which a large part of the dawn cell of the two-cell ionospheric convection pattern was monitored by the Super DARN radars. Ionospheric convection began to enhance first in a localized region of the equatorward part of the dawn cell ∼2 minutes before the expansion onsets of both substorms and then enhanced in the entire dawn cell successively. The enhanced convection persisted throughout their expansion phase, possibly even near the footprint of a plasma sheet region without fast flows observed by Geotail. These observations suggest that ionospheric convection begins to enhance just before substorm expansion onset and then enhances in the entire cell, possibly regardless of the presence of fast earthward flows in the corresponding plasma sheet region of the magnetotail. The global enhancement of ionospheric convection is consistent with that of magnetotail convection, which also begins just before onset. © 2008 by the American Geophysical Union.

Interaction between the solar wind and objects in the solar system varies largely according to the settings, such as the existence of a global intrinsic magnetic field and/or thick atmosphere. The Moon's case is characterized by the absence of both of them. Low energy ion measurements on the lunar orbit is realized more than 30 years after the Apollo period by low energy charged particle analyzers MAP-PACE on board SELENE(KAGUYA). MAP-PACE ion sensors have found that 0.1%similar to 1% of the solar wind protons are reflected back from the Moon instead of being absorbed by the lunar surface. Some of the reflected ions are accelerated above solar wind energy as they are picked-up by the solar wind convection electric field. The proton reflection that we have newly discovered around the Moon should be a universal process that characterizes the environment of an airless body. Citation: Saito, Y., et al. (2008), Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA), Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077.

Earth's magnetosphere is an obstacle to the supersonic solar wind and the bow shock is formed in the front side of it. In ordinary hydrodynamics, the flow decelerated at the shock is diverted around the obstacle symmetrically about the Earth-Sun line, which is indeed observed in the magnetosheath most of the time. Here we show a case under a very low-density solar wind in which duskward flow was observed in the dawnside magnetosheath. A Rankine-Hugoniot test shows that the magnetic effect is crucial for this "wrong flow'' to appear. A full three-dimensional magnetohydrodynamics (MHD) simulation of the situation confirming this interpretation and earlier simulations is also performed. It is illustrated that in addition to the "wrong flow'' feature, various peculiar characteristics appear in the global picture of the MHD flow interaction with the obstacle.

Spacecraft observations indicate that low-frequency (0.006-0.025 Hz) fluctuations of the magnetic field appear a few min prior to a substorm-associated dipolarization. These fluctuations are examined on the basis of the linear magnetohydrodynamic (MHD) fluid theory. We propose a "low-frequency fitting method" for identifying the characteristics of the MHD waves, such as the mode, the wavevector, and the frequency. Using the magnetic field and the ion velocity data, the frequency in the plasma rest frame is obtained by removing the Doppler shift. The fitting method takes the inhomogeneity of the ambient magnetic field into account, so that the parameter which is equivalent to the sum of the field line curvature and the gradient scale length of the ambient magnetic field is obtained as an output. We applied the method to a selected dipolarization event in which Geotail remained in the vicinity of the magnetic equator and also in the same magnetic local time as the onset region of an auroral breakup. The low-frequency fluctuations were detected from ∼4 min before the local dipolarization onset. What has been found are as follows: (1) the parallel magnetic field fluctuations had a strong correlation with the perpendicular ion velocity fluctuations, which indicates that the observed waves were explained as magnetosonic modes. The slow magnetosonic wave was identified ∼3 min before the dipolarization onset, whereas the fast magnetosonic wave was identified ∼1.5 min before the dipolarization onset. This fast wave was estimated to be propagating tailward with a phase velocity of 400 km/s and a period of 70 s in the plasma frame. (2) The perpendicular fluctuations of the magnetic field and the ion velocity were only weakly correlated, which indicates the presence of a mode that cannot be captured by the present method. Using the variance analysis, we show that this mode is likely to be a drift mode, which had almost zero frequency in the plasma frame and was propagating duskward together with the plasma bulk flow. A possible interpretation of the observed waves is briefly discussed with a relevance to previously proposed substorm initiation models. Particularly, the drift wave can be interpreted as a linear stage of the ballooning instability. Copyright 2008 by the American Geophysical Union.

We present in situ observations consistent with the ballooning mode in the vicinity of the magnetic equator at XGSM = -10 to -13 RE prior to substorm-associated dipolarization onsets. The ballooning instability is expected to have a wavevector along the Y direction and to give variation to the curvature of the ambient magnetic field lines. The magnetic field fluctuations appearing in the Bx component are transported by the ambient plasma drift in the Y direction. A discrete frequency band would be identified in time series data if the mode has a discrete wavelength. The ballooning mode of this property was identified at the magnetic equator a few min before dipolarization onsets only when the plasma β was large (20 to 70). Using low-energy ion velocity data, we show that the mode has almost zero frequency in the plasma rest frame so that ωsc ∼ ky · vy, where ωsc is the frequency in the spacecraft frame, and ky and vy are the wavenumber and the ambient plasma flow in the Y direction, respectively. This enables us to estimate the wavelengths of the ballooning mode, which were found to be of the order of the ion Larmor radius. Copyright 2008 by the American Geophysical Union.

We report on Cluster observations of a thin current sheet interval under the presence of a strong vertical bar B(Y)vertical bar during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong vertical bar B(Y)vertical bar in the tail could be associated with a strong IMF vertical bar B(Y)vertical bar, but the large fluctuations in B(Y), not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of B(Y)-dominated magnetic flux and/or a change in B(Y) due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655: 53 UT with a peak current density of 182 nA/m(2), the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be similar to 290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case.

Recently, Keiling et al. (2006) showed that periodic (similar to 90 s) traveling compression regions (TCRs) during a substorm had properties of Pi2 pulsations, prompting them to call this type of periodic TCRs "lobe Pi2". It was further shown that time-delayed ground Pi2 had the same period as the lobe Pi2 located at 16 R-E, and it was concluded that both were remotely driven by periodic, pulsed reconnection in the magnetotail. In the study reported here, we give further evidence for this association by reporting additional periodic TCR events (lobe Pi2s) at 18 R-E all of which occurred in succession during a geomagnetically very quiet, non-substorm period. Each quiet-time periodic TCR event occurred during an interval of small H-bay-like ground disturbance (<40 nT). Such disturbances have previously been identified as poleward boundary intensifications (PBIs). The small H bays were superposed by Pi2s. These ground Pi2s are compared to the TCRs in the tail lobe (Cluster) and both magnetic pulsations and flow variations at 9 R-E inside the plasma sheet (Geotail). The main results of this study are: (1) Further evidence is given that periodic TCRs in the tail lobe at distances of 18 R-E and ground Pi2 are related phenomena. In particular, it is shown that both had the same periodicity and occurred simultaneously (allowing for propagation time delays) strongly suggesting that both had the same periodic source. Since the TCRs were propagating Earthward, this source was located in the outer magnetosphere beyond 18 R-E. (2) The connection of periodic TCRs and ground Pi2 also exists during very quiet geomagnetic conditions with PBIs present in addition to the previous result (Keiling et al., 2006) which showed this connection during substorms. (3) Combining (1) and (2), we conclude that the frequency of PBI-associated Pi2 is controlled in the outer magnetosphere as opposed to the inner magnetosphere. We propose that this mechanism is pulsed reconnection based on previous results which combined modeled results and observations of substorm-related periodic TCRs and ground Pi2. (4) We show that TCRs with small compression ratios (Delta B/B< 1%) can be useful in the study of magnetotail dynamics and we argue that other compressional fluctuations with Delta B/B< 1% (without having all of the characteristic signatures of TCRs) seen in the tail lobe could possibly be related to the same mechanism that generates TCR with Delta B/B> 1% (which are more commonly studied). (5) Finally, it is noted that both quiet time and substorm-related periodic TCRs had remarkably similar periods in spite of the drastically different geomagnetic conditions prevailing during the events which poses the important question of what causes this periodicity under these different conditions.

A detailed analysis of successive tailward flow bursts in the near-Earth magnetotail (X similar to- 19 R-E) plasma sheet is performed on the basis of in-situ multi-point observations by the Cluster spacecraft on 15 September 2001. The tailward flows were detected during a northward IMF interval, 2.5h after a substorm expansion. Each flow burst (V-x <300 km/s) was associated with local auroral activation. Enhancements of the parallel and anti-parallel similar to 1 keV electron flux were detected during the flows. The spacecraft configuration enables to monitor the neutral sheet (B-x approximate to 0) and the level of B-x approximate to 10-15 nT simultaneously, giving a possibility to distinguish between closed plasmoid-like structures and open NFTE-like surges. The data analysis shows NFTE-like structures and localized current filaments embedded into the tailward plasma flow. 3-D shapes of the structures were reconstructed using the four-point magnetic filed measurements and the particle data.

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.

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE (SELenological and ENgineering Explorer) satellite. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low energy ions below 28 keV/q. Each sensor has a hemispherical field of view. Since SELENE is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining a three-dimensional distribution function of electrons and ions. The scientific objectives of PACE are (1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, (2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, (3) to resolve the Moon-solar wind interaction, (4) to resolve the Moon-Earth's magnetosphere interaction, and (5) to observe the Earth's magnetotail.

[1] The location and variability of magnetic reconnection are investigated using simultaneous in situ observations of boundary layer flows at the dayside magnetopause and remote sensing of proton cusp aurora in the ionosphere. Two events when the Geotail spacecraft was at the magnetopause and the Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft was observing cusp proton precipitation are used in this investigation. The directions of high-speed flows observed in the boundary layer by the Geotail spacecraft are compared to predicted directions from the antiparallel reconnection model and from two component reconnection models. The IMAGE cusp proton aurora observations provide additional information on the type of reconnection and on variability in the reconnection rate. For the first event, antiparallel reconnection may be occurring at the magnetopause and there is a long-duration (10 s of minutes) decrease in the proton aurora intensity. For the second event, component reconnection is occurring and variability in the cusp emissions on a timescale of several minutes appears to indicate variability in the reconnection rate.

The small intrinsic magnetic field of Mercury together with its proximity to the Sun makes the Hermean magnetosphere unique in the context of comparative magnetosphere study. The basic framework of the Hermean magnetosphere is believed to be the same as that of Earth. However, there exist various differences which cause new and exciting effects not present at Earth to appear. These new effects may force a substantial correction of our naïve predictions concerning the magnetosphere of Mercury. Here, we outline the predictions based on our experience at Earth and what effects can drastically change this picture. The basic structure of the magnetosphere is likely to be understood by scaling the Earth's case but its dynamic aspect is likely modified significantly by the smallness of the Hermean magnetosphere and the substantial presence of heavy ions coming from the planet's surface. © 2007 Springer Science+Business Media B.V.

Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun's gravitational well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner 10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late 1990s, two missions MESSENGER and BepiColombo - received the go-ahead; MESSENGER is on its way to its first encounter with Mercury in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in detail.

Abstract. To investigate the cold plasma sheet formation under northward IMF, we study the temperature anisotropies of electrons and two-component protons observed by the Geotail spacecraft. The two-component protons, which are occasionally observed in the dusk plasma sheet near the low-latitude boundary, are the result of spatial mixing of the hot protons of the magnetosphere proper and the cold protons from the solar wind. Recent research focusing on the two-component protons reported that the cold proton component at times has a strong anisotropy, and that the sense of the anisotropy depends on the observed locations. Since electrons have been known to possess a strong parallel anisotropy around the low-latitude boundary layer, we compare anisotropies of electrons and protons to find that the strengths of parallel anisotropies of electrons and the cold proton component are in good correlation in the tail flank. The parallel anisotropy of electrons is stronger than that of the cold proton component, which is attributed to selective heating of electrons. We further find that the strengths of the parallel anisotropies in the tail flank depend on the latitudinal angle of the IMF; strong parallel anisotropies occur under strongly northward IMF. We discuss that the Kelvin-Helmholtz vortices, which developed under strongly northward IMF, and the resultant magnetic reconnection therein may lead to the strong parallel anisotropies observed in the tail flank.

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.

Through the effort to obtain clues toward understanding of transport of cold plasma in the near-Earth magnetotail under northward IMF, we find that two-component protons are observed in the midnight plasma sheet (&amp minus 10&gt XGSM&gt &amp minus 30 &gt i&gt RE, |YGSM| &lt 10 RE) under northward IMF by the Geotail spacecraft. Since the two-component protons are frequently observed on the duskside during northward IMF intervals but hardly on the dawnside, those found in the midnight plasma sheet are thought to come from the dusk flank. The cold proton component in the midnight region occasionally has a parallel anisotropy, which resembles that in the tail flank on the duskside. The flows in the plasma sheet with two-component protons were quite stagnant or slightly going dawnward, which supports the idea that the observed two-component protons in the midnight region are of duskside origin. Because the two-component protons in the midnight plasma sheet emerge under strongly northward IMF with the latitudinal angle larger than 45 degrees, and because the lag from the strongly northward IMF to the emergence can be as short as a few hours, we suggest that prompt plasma transport from the dusk to midnight region occurs under strongly northward IMF. We propose that the dawnward flows result from viscous interaction between the high-latitude portion of the plasma sheet and the lobe cell. Another candidate for plasma transport process from the dusk to the midnight region is turbulent flow due to vortical structures of the Kelvin-Helmholtz instability that developed around the dusk low-latitude boundary under strongly northward IMF. In addition, we also suggest that gradual cooling of hot protons under northward IMF is a global phenomenon in the near-Earth magnetotail.

To further our understanding of the solar wind entry across the magnetopause under northward IMF, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortex event on 24 March 1995. We have found that the protons consist of two separate (cold and hot) components in the magnetosphere-like region inside the KH vortical structure. The cold proton component occasionally consisted of counter-streaming beams near the current layer in the KH vortical structure. Low-energy bidirectional electron beams or flat-topped electron distribution functions in the direction along the local magnetic field were apparent on the magnetosphere side of the current layer. We discuss that the bidirectionality of the electrons and the cold proton component implies magnetic reconnection inside the KH vortical structure. In addition, we suggest selective heating of electrons inside the vortical structure via wave-particle interactions. Comparing temperatures in the magnetosphere-like region inside the vortical structure with those in the cold plasma sheet, we show that further heating of both the electrons and the cold proton component is taking place in the cold plasma sheet or on the way from the vortices to the cold plasma sheet.

2020年代の実施を目指して、木星探査計画を日欧共同ミッションで行うという計画がはじまった。木星は、太陽系最大の惑星で、その起源を調べることはすなわち、太陽系形成を制約付ける。計画では、エウロパ周回機、衛星観察オービター（もしくは木星観察オービター）、磁気圏探査衛星の３機構成で木星系を調べるという野心的なものである。ヨーロッパ側はCosmic Visionへの応募を行い、日本側はISAS/JAXAでワーキンググループ設立を本年行った。

The timescale for the formation of cold-dense plasma sheet ions was investigated with an event in which the interplanetary magnetic field ( IMF) was northward for almost one day. The plasma sheet dawn and dusk flanks appear to reach cold dense states ( n > 1 cm(-3); T < 2 keV) within a few hours after IMF northward turning. Closer to the center ( midnight meridian), the ion temperatures reach < 2 keV within a few hours of IMF northward turning, but the ion densities do not reach above 1 cm(-3) for at least similar to 8 hours after IMF northward turning. The connection between solar wind ions and plasma sheet cold-component ions is demonstrated. The plasma sheet dawn flank ions appear to lag the solar wind ions by about 3 hours. This study confirms the previous statistical results: ( a) the densification of the plasma sheet can be attributed to the influx of the cold-component ( magnetosheath/ solar wind origin) ions; and (b) the cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also to the cooling of the hot components. Order of magnitude calculations of the plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn-dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms.

[ 1] Three-dimensional MHD simulations of the Kelvin-Helmholtz instability (KHI) have been performed to investigate its relevance to the magnetotail-flank situation. The effect of the KH-stable lobe region on its growth at the plasma sheet - magnetosheath interface can be a crucial factor. To assess this effect, we study how the KHI grows in an unstable layer of finite thickness ( plasma sheet) sandwiched between stable regions ( north and south lobes). The results show that when the magnetosheath magnetic field is northward, the instability grows vigorously to form a highly rolled-up vortex even when the plasma sheet thickness is smaller than the wavelength of the fastest-growing mode. Furthermore, two rolled-up vortices can coalesce into a larger one as long as the thickness is larger than the wavelength. The coalescence under the tail-flank geometry causes strong stretching of the field lines, leading to the condition under which reconnection could easily be triggered. These findings suggest that when the magnetosheath condition is favorable for the KHI and when the plasma sheet is thicker than a few R-E, the vortices become an important element of the tail-flank dynamics. We also found that in rolled-up vortices, the tailward speed of the plasma sheet plasma exceeds that of the magnetosheath flow in regions where it penetrates toward the magnetosheath. Since this overshoot acceleration is seen only when the vortex is rolled up and is detectable even from single-spacecraft observations, we suggest that it can be used as a marker of the roll-up of vortices.

We have surveyed field-aligned electrons at the lobe/plasma sheet boundary and their association with reconnection in the distant magnetotail where reconnection is quasi-steady and large scale. Asymmetric (in energy) counterstreaming electrons are detected in similar to 30% of the boundary crossings when high-speed flows are present in the plasma sheet. In 98% of the electron beam cases the low-energy electrons are directed toward and the higher-energy electrons directed away from the X-line. The well-organized (by the signs of B-x and V-x) quadrupolar pattern of the electrons indicates that these electrons are associated with reconnection. The low-energy electrons could be the outer part of the Hall current loop, similar to previous reports in the near-Earth region. The mean electron energy in the distant tail is a factor of ten lower than in the near Earth tail. Our observations suggest that the Hall effect can be detected even at large distances from the diffusion region.

We report on the observation of two distinct cold (T i<5 keV), dense (Ni>2 cm&minus;3) ion populations at geosynchronous orbit. A statistical study was performed on measurements from the geosynchronous Los Alamos plasma instruments, for the period 1990&ndash;2004, and complemented by corresponding large-scale plasma sheet data obtained by mapping DMSP observations in the tail. The first population, which has previously been reported in several studies, is observed in the midnight region of geosynchronous orbit. The second population, which has drawn less attention, is detected on the dawn side of geosynchronous orbit. No such cold, dense population is observed on the dusk side of geosynchronous orbit on a frequent basis. The temporal evolution of various plasma parameters as a function of local time shows that the two populations appear at geosynchronous orbit as distinct populations, since the appearance of a midnight population is not usually associated with that of a dawn population, and vice versa. The midnight ion population is typically observed after the IMF has been northward for some time and is convected inward toward geosynchronous orbit after an observed mild southward turning of the average IMF. It is interpreted that the source of the midnight population is the cold, dense plasma sheet (CDPS). The dawn-side cold and dense ion population is associated with previously strong southward IMF and consequently occurs during substantial geomagnetic activity. These events are typically observed around the end of the main phase of the corresponding Dst decrease, down to &minus;50 nT on average. It is unlikely that this dawn population is simply the low-latitude boundary layer (LLBL) moving closer to Earth because (1) no symmetric dusk population is observed and (2) on average a small sunward flow (∼15 km/s) is observed for those events. The cold, dense population at dawn is thus observed during active times (based on Dst, K p and AE indices) in comparison with the midnight case. However, since the dawn population is observed only around the end of the main Dst decrease, it is concluded that this population does not typically contribute to the Dst decrease during the main phase. This population may rather be transported to geosynchronous orbit by means of a compression and convection enhancement in the magnetosphere, with a preferential access from the dawn flank with no apparent counterpart at dusk. DMSP data suggest that a cold and dense plasma source is mainly present at dawn.

Three-dimensional MHD simulations of Kelvin-Helmholtz instability (KHI) have been performed to investigate its relevance to the magnetotail-flank situation. The stabilization effect of the intense lobe magnetic field can be a crucial factor that has not been considered. To assess this effect we have studied how the mode grows in an unstable layer of finite thickness (the plasma sheet) sandwiched between stable regions (the lobe in the two-hemispheres). The condition on the magnetosheath side is set in the favorable range for KHI. Results from the two basic configurations studied here suggest that KHI grows vigorously as long as the unstable layer is thicker than a half of the wavelength. The results imply that the condition on the magnetospheric side for KHI to be an important element of the tail-flank dynamics can be met rather easily under northward IMF. © 2005 COSPAR.

Kelvin-Helmholtz Instability (KHI) is an MHD-scale instability that grows in a velocity shear layer such as the low-latitude boundary layer of the magnetosphere. KHI is driven unstable when a velocity shear is strong enough to overcome the stabilization effect of magnetic field. When the shear is significantly strong, vortices in the nonlinear stage of KHI is so rolled-up as to situate magnetospheric plasma outward of the magnetosheath plasma and vice versa. The big question is if such highly rolled-up vortices contribute significantly to the plasma transport across the boundary and to the filling of the plasma sheet by cool magnetosheath component, which is observed under northward Interplanetary Magnetic Field (IMF) condition. Here we review our recent results from two-fluid simulations of MHD-scale KHI with finite electron inertia taken into account. The results indicate that there is coupling between the MHD-scale dynamics and electron-scale dynamics in the rolled-up stage of the vortices. While the details differ depending on the initial magnetic geometry, the general conclusion is that there is significant modification of the MHD-scale vortex flow pattern via coupling to the micro-physics. The kick-back from the parasitic micro-physics enhances highly the potential for large-scale plasma mixing of the parent MHD-scale vortices, which is prohibited by definition in ideal-MHD. We also review our recent 3-D MHD simulation results indicating that KHI vortex can indeed roll-up in the magnetotail-flank situation despite the strong stabilization by the lobe magnetic field. These results encouraged us to search for evidence of rolled-up vortices in the Cluster formation flying observations. As reviewed in this paper, a nice event was found during northward IMF interval. This interval is when the plasma transport via large scale reconnection becomes less efficient. The finding supports the argument that KHI is playing some role in transporting solar wind into the magnetosphere when the normal mode of transport cannot dominate.

筆頭

We have performed three-dimensional MHD simulations of magnetic reconnection to investigate the structure of a reconnection jet. Our simulation results show that the leading edge of a reconnection jet deforms to put on a significantly complicated structure. In a randomly perturbed environment, smaller scale structure develops rapidly and finally the front of the reconnection jet becomes turbulent. In contrast when the initial perturbation is composed of a single sinusoidal mode, coherent magnetic bubbles are generated. The magnetic bubbles elongate significantly when the wavelength is small giving onset to a secondary instability that may subsequently lead to turbulence. We interpret the deformation of the jet front to be due to excitation of the Rayleigh-Taylor/ballooning-like instability and emphasize the significance of the three-dimensionality in considering the dynamics of a reconnection jet.

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.

On 2001-12-02 Wind crossed the dayside magnetopause (MP) at similar to 15 MLT and traversed the adjacent low-latitude boundary layer (LLBL) over a period of 2 hours. The IMF was steady (northward and dawnward) during the MP/LLBL encounter. Reconnection flows were observed in the MP that were directed 130 degrees away from the magnetosheath flow direction. In contrast, the LLBL flow was aligned with the magnetosheath flow. The counterstreaming field-aligned and anti-field-aligned electrons have different energies and their fluxes are unbalanced in the open MP whereas they are precisely balanced throughout most of the LLBL indicative of a closed LLBL. These observations indicate that reconnection occurs at the low-latitude MP during northward IMF (with a significant By), but low-latitude reconnection is not responsible for the creation of the LLBL. Instead, reconnection appears to be in the process of eroding a pre-existing LLBL that was created either by diffusive entry or by non-simultaneous double-cusp reconnection.

[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 R-E 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 - V-x x B-s, where V-x is the x component of the solar wind velocity and B-s 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.

Statistical properties of low-frequency (0.01-0.1 Hz) electromagnetic waves and their relations to ion distribution functions in "transition regions'' from lobe to the plasma sheet, which include the plasma sheet boundary layer (PSBL) and boundary plasma sheet, are investigated based on 5-year data of the Geotail spacecraft observations in X-GSM = [-31, -15] and \Y-GSM\ &lt; 5 R-E. It is shown that the amplitude of the magnetic field fluctuations increases with increasing plasma beta ( the ratio of the plasma thermal pressure to the magnetic pressure), while the electric field amplitude decreases in high-beta regions. These tendencies are consistent with a decrease of the local Alfven velocity (V-A) in the transition region with increasing beta. The statistical results also indicate that the low-frequency wave power has clear correlation with the energy flux of ion flows parallel to the magnetic field. If 10% of the beam energy is converted to the wave power, the ion beams could be the source of free energy of the large-amplitude electromagnetic waves. The estimated Poynting flux of the waves is distributed in the range from 1.0 x 10(-6) to 5.6 x 10(-2) mW/m(2). The maximum Poynting flux is the same order of the pointing flux of Alfven waves observed by the Polar spacecraft at altitudes of 4-7 R-E, when mapped along converging magnetic field lines to the ionosphere at an altitude of 100 km. The good agreement of the Poynting fluxes is consistent with the idea that the low-frequency electromagnetic waves in the tail PSBL are the source of kinetic Alfven waves in the high-latitude auroral regions. The results of partial moment calculations with data bins selected by careful eye inspection for each 12-s ion data during large-amplitude wave events show that in most of the events, the relative drift speed between cold-core and hot-beam ion components is below 2 V-A, the density ratio of the cold-core to the hot-beam is typically a few tens of percent, and the beam component has a strong temperature anisotropy of T-parallel to/T-perpendicular to similar to 0.44. The linear dispersion analysis using the observed distribution functions suggests the importance of the ion cyclotron anisotropy instability modified by the existence of cold-core ions for the generation of low-frequency large-amplitude electromagnetic waves in the PSBL.

The structure of the plasma sheet under northward IMF is studied by data from the Geotail spacecraft. The plasma sheet is known to become cold and dense (T &lt 2 keV, n &gt 1 cm-3) during extended northward IMF periods. We show that such cold-dense ions (CDIs) appear at itYgsm&gt 10 Re, that is, 10Reoff the tail axis to both flanks. CDIs on the dawnside have higher temperatures and some reach the upper-limit of 2 keV that we use to select CDIs. These CDIs having the highest temperature are distributed at the dawnside plasma sheet inner-edge (R &lt 15 Re) and is connected to the hot-dense ions (HDIs: T &gt 2 keV, n &gt 1 cm-3) in the further inner region. A survey shows that HDIs under nominal solar wind dynamic pressure appear mostly in the dawnside inner-magnetosphere during extended northward IMF intervals. Both results point to the idea that HDIs are the inner-magnetosphere extension of dawnside CDIs, while such a partner to the duskside CDIs cannot be identified. This structure of the plasma sheet suggests that there is significant dawn-dusk asymmetry in heating and transport in the magnetotail under northward IMF. © 2005 Elsevier B.V. All rights reserved.

In a transient reconnection process, the leading edge of the reconnection jet pushes and compresses the plasma standing ahead of it. This causes curved magnetic field lines and a strong pressure gradient to develop at the edge. It implies that the leading edge of the jet could be unstable to the ballooning instability. We studied this situation by three-dimensional MHD simulations and clarified that the interface indeed becomes unstable. The leading edge on the current sheet plane is deformed into a wavy shape and then to a mushroom-like pattern subsequently. The growth rate of the instability is controlled by the wavelength in the current-wise direction with a shorter wavelength mode growing faster. The dispersion curve obtained from a series of simulations is given. © 2005 Elsevier B.V. All rights reserved.

We have studied by MHD simulations the effects of the guide field in three-dimensional magnetic reconnection. The guide field is introduced by adding a constant B-y component B-y0 to the anti-parallel reconnecting component B-x = tanh(z). An ad-hoc anomalous resistive region, which facilitates the study of reconnection in MHD, is assumed to have a finite extent in the gamma direction and this gives rise to a three-dimensional situation. It is shown that the guide field makes the U-shaped reconnected field lines as well as the reconnection jet to be inclined from the z axis. The guide field also acts as an obstacle to the jet and this effect produces a pair of helical streamlines in the jet leading part. Time series data from a virtual spacecraft that encounters or skims the jet leading part are found to show good agreement with the observed FTE signatures on either side of the dayside magnetopause.

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.

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.

We have studied the coupling between the distant and near-Earth plasma sheet during an 8 hour interval on April 1, 1999 when the interplanetary magnetic field was northward and dominated by the By component. During these 8 hours the Geotail spacecraft sampled the near-Earth magnetotail at X-GSM similar to - 25 R-E while the Wind spacecraft was located in the distant magnetotail at X-GSM similar to - 60 R-E. Wind detected long duration (&gt; 8 hours) and likely spatially extended convective high speed flows indicative of continuous magnetic reconnection, whereas only plasma sheet boundary layer beams and some locally generated bursty bulk flows were observed at the Geotail location. Hence the high speed distant tail reconnection flows did not reach the near-Earth plasma sheet at high speed. No apparent signatures of the long duration distant tail reconnection flows in terms of global and auroral geomagnetic activity were observed.

On February 23, 2001, during southward and strongly dawnward IMF, Wind traversed the distant magnetotail near X-GSM = -90 R-E. Wind encountered the mantle in the southern hemisphere and a nearly empty lobe in the northern hemisphere. In the current layer ( plasma sheet), high-speed tailward plasma jets with plasma density intermediate between the mantle and the lobe were detected. The speed of these flows were 96-99% of the Alfven speed measured in the deHoffmann-Teller frame. We interpret these observations as evidence for asymmetric reconnection involving a rotational discontinuity in the distant tail when the density in the two inflow regions are vastly different. This is in contrast to typical symmetric magnetotail reconnection involving two lobes of equal density where the flow acceleration across the slow shock is usually sub-Alfvenic and the outflow density is enhanced relative to the inflow density. Hall effects are also observed in this event.

[1] On the basis of Geotail observations, we investigate the ion and electron behavior in the dayside low-latitude magnetosphere just inside the magnetopause in order to obtain clues for understanding the formation mechanism of the low-latitude boundary layer (LLBL). The dayside region is classified into several categories according to ion energy spectrum characteristics, electron pitch angle anisotropy, the north-south polarity of the interplanetary magnetic field (IMF), and the observed locations. An important category is the ion mixing region, which contains dense ions of the solar wind origin and hot magnetospheric ions simultaneously. We define this class as data samples for which both the number density and the ion flux at &gt;10 keV exceed certain threshold values. Our statistics show that the ion mixing region thus automatically identified is encountered far more often when the IMF having a northward component lasts for about 4 hours or more. The ion mixing region under extended northward IMF is almost always accompanied by field-aligned, bidirectional electrons of a few hundreds eV, which energy is higher than that of typical magnetosheath electrons. The flux of &gt;2 keV electrons, that is, electrons of the magnetospheric origin is significantly reduced in the mixing region as compared to that in the region earthward of the mixing region. These facts suggest that the plasma transport process operating for extended northward IMF periods plays an important role both for heating low-energy (presumably magnetosheath) electrons in field-aligned directions and for escape/cooling of the magnetospheric electrons. The mixing region exhibits a clear dawn-dusk asymmetry in the ion energy spectrum, supporting the idea that the transport/heating process of the entrant solar wind ions is different for different sides of the magnetosphere. On the basis of similarities/differences between these observed signatures and those having been found in the tail-flanks, relationships among the dayside and tail LLBLs under extended northward IMF are discussed.

It has been recognized that during extended periods of the northward interplanetary magnetic field the tail plasma sheet becomes cold and dense, showing a positive density correlation with the solar wind plasma. Recently it has been also recognized that the plasma density integrated along the Z (north-south) direction across the plasma sheet becomes also high during the northward IMF periods, which suggests a fairly high plasma supply rate of ~ 1026 protons/sec amounting nearly 10% of the enhanced supply rate during the southward interplanetary magnetic field periods. While the latter rate is considered to be caused by the efficient dayside magnetopause reconnection, it is not yet known how the plasma transport occurs during the northward interplanetary magnetic field periods. Since the highly evolved LLBL is also observed during such periods, we expect some causal relation between the plasma transport to the plasma sheet and the evolution of the LLBL. We review the key observations and discuss possible physical mechanisms of the plasma transportation.

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.

Geotail observed a fast tailward flow burst (a speed of &gt;600 km/s) with southward B-z in the Earth's magnetotail at a radial distance of 15.5 R-E on March 30, 1995. Ions in this burst consisted of a single-component plasma showing convection motion, and these ions were confined near the neutral sheet. This flow burst was likely associated with a pseudobreakup rather than a major substorm onset. In this event, magnetic reconnection appeared to take place only for the field lines near the neutral sheet. The magnetic reconnection was quenched soon and resulted in a short-lived tailward flow burst embedded in the plasma sheet.