藤本 正樹

Understanding the field aligned current (FAC) system in substorms is one of the central themes of magnetospheric physics. Three-dimensional (3D) magnetohydrodynamic (MHD) simulations of magnetotail reconnection has shown that FAC structures similar to the wedge-current system are indeed reproduced either with or without a dipolar region situated at the earthward end of a current sheet. In this study, however, by noting that the ion inertia length is not negligibly small compared to the current sheet width in the magnetotail, we investigate the FAC generation mechanism in the Hall-MHD system. For an idealized current sheet geometry without the dipolar region, we find that the results obtained to be totally different from the MHD results.

Magnetic reconnection is a process that converts magnetic energy into bi-directional plasma jets; it is believed to be the dominant process by which solar-wind energy enters the Earth's magnetosphere(1,2). This energy is subsequently dissipated by magnetic storms and aurorae(3,4). Previous single-spacecraft observations(5-7) revealed only single jets at the magnetopause-while the existence of a counter-streaming jet was implicitly assumed, no experimental confirmation was available. Here we report in situ two-spacecraft observations of bi-directional jets at the magnetopause, finding evidence for a stable and extended reconnection line; the latter implies substantial entry of the solar wind into the magnetosphere. We conclude that reconnection is determined by large-scale interactions between the solar wind and the magnetosphere, rather than by local conditions at the magnetopause.

We have analyzed in detail a Wind spacecraft crossing of the high-latitude magnetosheath, magnetopause, lobe, and the high- and low-latitude plasma sheet on November 27, 1998. The crossing occurred during the first perigee pass of Wind's (new) high-inclination (45 degrees) petal orbit. Between the lobe and the hot plasma sheet on the duskside, an extended region (x(GSM)similar to -2.8 to -7.8R(E), y(GSM)similar to 7.8 R-E, z(GSM)similar to -11.6 to -4.3 R-E) of mixed low-energy magnetosheath and high-energy plasma sheet ion populations was detected. This region was detected in a region of strong (similar to 40nT) and steady lobe-like magnetic field and ceased to exist when the spacecraft approached the neutral sheet. The mixed ion populations are remarkably similar (in their thermal properties) to the distributions detected in the adjacent magnetosheath and plasma sheet proper, except that the mixed ions are nearly stagnant. As the neutral sheet is approached, the low-energy component of the mixed ions is gradually heated, while the high-energy component is unmodified. Near the neutral sheet the ion distributions are dominated by a single, high-energy population. The electron behavior is significantly different from that of the ions. In the mixed ion region the electrons consist of a single population with energy between those of the magnetosheath and the plasma sheet proper. Electron pitch angle information suggests that the entire mixed region is on closed field lines. However, the large reduction of high-energy plasma sheet electrons in this region may indicate that the field lines threading this regions were once open. The exact path of plasma entry to form the mixed region cannot be discerned with single-point observations, especially since the mixed ions are stagnant. However, it is possible (for this event) to rule out the mantle as a source for the low-energy component of the mixed ions. The similarity between the thermal property of the low-energy component of the mixed ions and the magnetosheath population suggests that the magnetosheath plasma had direct access to this region without significant heating along its path. The presence of nearly unmodified magnetosheath plasma deep inside the magnetosphere, raises questions concerning the processes of plasma entry across the magnetopause. Finally, the mixed region in this event was detected during an extended period of persistent northward and duskward interplanetary magnetic field, which makes this event ideal for model comparisons.

Following the suggestion that the low-latitude boundary layer (LLBL) in the near-Earth magnetotail can be the site of capturing magnetosheath plasma into the plasma sheet during extended northward IMF periods, we have made a case study of the plasma sheet using data from WIND, Geotail and Akebono. On Feb. 9-10, 1995, IMF was northward for more than 24 hours. The plasma sheet at (X-gsm, Y-gsm) = (-30 similar to -15,0 similar to -10)R-e observed by Geotail changed its status from hot-tenuous to cold-dense in this period. The ions in the cold-dense plasma sheet have two characteristic energies (temperatures) just as those in the LLBL. The colder component had a temperature as low as a few hundreds eV. From Akebono observations during the same time interval, the convection pattern in the high-latitude ionosphere was found to show a clear four-cell structure. Although these facts are well-known on statistical basis, the simultaneous observations definitely put a constraint that the plasma transport which produces the cold-dense plasma sheet should be in accord with the four-cell convection pattern. Inspection on the Akebono particle data shows that a few keV ion/soft (< 1 keV) electron precipitation are seen in the sunward convection zone located adjacent to the trapped particle region at invariant latitudes of 75 degrees similar to 80 degrees. Discussion on the formation of the cold-dense plasma sheet, during northward IMF period is made on the basis of these observations. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

3-D hybrid simulations (ion particle, charge neutralizing massless electron fluid) of magnetic reconnection in a thin current sheet (thickness comparable to the relevant ion inertia length) are conducted. In this study, reconnection is initiated by putting localized anomalous resistivity. The thin current sheet, which models the near-Earth plasma sheet just before substorm onsets, is also unstable to the tail K-H instability that kinks the plasma sheet in the cross-tail plane. For smaller resistivity cases, the plasma sheet kink is already well developed by the time explosive reconnection starts. Despite this different circumstance, the temporal development of the reconnection jet is virtually unchanged from a high resistivity case reported previously. The structure of the reconnection jet is modified as it intrudes into the pre-existing kinked plasma sheet, but still, the essence of the ion kinetic response remains unchanged. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

During the anomalously low density solar wind interval of May 1999. GEOTAIL was in the magnetosheath for Ο37 hours after making an inbound crossing of the expanding bow shock at Ο8 RE upstream of its nominal position. Comparison among data sets obtained from GEOTAIL (magnetosheath), WIND (near upstream - bow shock), and ACE (far upstream) reveals several unique features: Firstly, during the interval of 1430-1530 UT on 11 May, we observed both in the solar wind and magnetosheath double-peaked protons with a peak separation of 250-300 km/s, which was close to the local Alfvén velocity during the event. Secondly, we observed extremely strong strahl electrons both in the solar wind and magnetosheath during the interval of 0600-2100 UT on 11 May 1999. We present an overview of the GEOTAIL observations, and discuss their physical significance.

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near-Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search-coil magnetometer and the spherical double-probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi-linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively, The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.

The strahl component of solar wind electrons, which constitutes field-aligned electron heat flux running away from the Sun, is a strong candidate for the origin of the polar rain. We investigate the entry process of the strahl electrons into the distant-tail magnetosphere and discuss topologies of Earth's field lines in this paper. The Geotail satellite has often observed either gradual or abrupt transitions from the magnetosheath electrons to the bidirectional lobe electrons at the magnetopause. In some cases, the strahl flux of the sheath electrons flowing tailward gradually turned near the magnetopause and finally became the earthward flux of the bidirectional lobe electrons. This transition is accompanied by the rotation of the magnetic field direction and indicates the direct entry of the strahl electrons along open field lines. On the basis of the data of 38 magnetopause crossings by Geotail, we investigate variation of density of the strahl (polar rain) electrons and that of ion density near the magnetopause. It is shown that the strahl electrons decrease upon crossing the magnetopause, and the decrease is correlated with that of ions, although, quantitatively, the strahl electrons do not decrease so much as ions. It is suggested that the strahl electrons enter the magnetosphere along open field lines more freely than ions, but their entry is under the influence of charge neutrality with ions. It remains as a problem why the Geotail data do not show the presence of electrons escaping from the magnetosphere along open field lines.

Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the magnetotail at 20-30 R-E for substorm onsets in Geotail observations. These fast tailward flows are occasionally accompanied by a few keV electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnection site can be divided into three regions: the neutral sheet region (near the neutral sheet with the absolute magnitude of B-x of < 5 nT), the boundary region (near the plasma sheet/tail lobe boundary with the absolute magnitude of B-x is near or > 10 nT), and the off-equatorial plasma sheet (the rest). In the neutral sheet region, plasmas are transported with strong convection, and accelerated electrons show nearly isotropic distributions. In the off-equatorial plasma sheet, two ion components coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, high-energy (higher than 10 keV) electrons show streaming away from the reconnection site along the magnetic field line. In the boundary region, ions also show two components: ions with convection toward the neutral sheet and field-aligned ions flowing out of the reconnection region, although acceleration and heating during convection are weak. In the boundary region, high-energy (10 keV) electrons stream away, while medium-energy (3 keV) electrons stream into the reconnection site. Magnetic reconnection usually starts in the premidnight sector of the magnetotail between X-GSM = -20 R-E and X-GSM = -30 R-E prior to an onset signature identified with Pi 2 pulsation on the ground. Magnetic reconnection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase.

Geotail observations of the low-latitude boundary layer (LLBL) in the near-Earth tail flanks are reported. Cold-dense stagnant ions, which are likely to he of the magnetosheath origin, are detected in this region of the magnetosphere. Charge neutrality is maintained by accompanying dense thermal (< 300 eV) electrons presumably also from the magnetosheath. Compared to the magnetosheath component, however, the electrons are anisotropically heated to have enhanced bidirectional flux along the field lines. The enhanced bidirectional flux is well balanced, and this fact, together with the slow convection, suggest the closed topology of the field lines. In addition to these common characteristics, a dawn-dusk asymmetry is observed in data for several keV ions, which is attributed to the dawn-to-dusk cross tail magnetic drift of the plasma sheet ions. We also show a case that strongly suggests that this entry of cold-dense plasma from the magnetosheath via near-Earth tail flanks can be significant at times. In this case, the cold-dense plasma is continuously detected as the spacecraft moves inward from the magnetospheric boundary to deep inside the magnetotail. By referring to the solar wind data showing little dynamic pressure variation during the interval, we interpret the long duration of the cold-dense status as indicative of a large spatial extent of the region: The cold-dense plasma is not spatially restricted to a thin layer attached to the magnetopause (LLBL) but constitutes an entity occupying a substantial part of the magnetotail, which we term as the cold-dense plasma sheet. The continuity of the cold-dense plasma all the way from the boundary region supports the idea that the magnetosheath plasma is directly supplied into the cold-dense plasma sheet through the flank.

A case study of the structure of the low-latitude boundary layer (LLBL) on dawnside (0600-0900 MLT) is reported. It is shown that the LLBL consists of two regions, the sheath-like region and the mixing region. The sheath-like region is where cold ions from the magnetosheath are flowing tailward. Detailed study has shown that this region is produced by reconnection and subsequent draping of open field lines over the dayside magnetosphere [Fujimoto et al., 1997]. The mixing region, on the other hand, is characterized by mixing of cold ions from the magnetosheath with the magnetospheric hot ions. Thermal electrons (< 500 eV) enhanced bidirectionally along field lines are detected to accompany these mixed ions. The balanced bi-directional flux is taken to indicate the closed topology of the field lines. An interesting finding is that the flow direction in this region tends to be sunward. Flow in the mixing region and the adjacent plasma sheet is faster sunward than in the ring current region, suggesting that these two constitute a channel of sunward returning convection in the dayside outer magnetosphere for this particular case. More new information on the mixing region from the present study is the ion distribution function showing a three-component feature and anticorrelation between plasma density and the degree of electron bidirectional heating. These observational facts would offer clues for understanding of the mixing region formation, which still remains open.

A prolonged period of B-y > 0 dominated IMF conditions was monitored by WIND on Dec., 18-19, 1994. Observations on convection over the southern polar cap in the middle of this period (IMF B-z similar to 0) were available from AKEBONO. Detection of the well-known dawn-to-dusk flow observing simultaneously energy-dispersed ions in the cusp strongly indicates that the field lines reconnected at the dayside were azimuthally accelerated. GEOTAIL happened to be in the duskside flank at (X-GSM,Z(GSM)) similar to (-15 similar to -23, -1 similar to -7)R-E. The GEOTAIL data not only prove that this near-tail flank region in the southern hemisphere is filled with azimuthally accelerated flux tubes that are loaded with solar wind plasma, but they also show that Of ions are transported tailward along these field lines. The latter is a, new finding which suggests a route for transporting O+ ions from the ionosphere to the magnetotail.

GEOTAIL observations of the low-latitude boundary layer (LLBL) in the tail-flanks show that they are the region where the cold-dense plasma appears with stagnant flow signatures accompanied by bi-directional thermal electrons (< 300 eV). It is concluded from these facts that the tail-LLBL is the site of capturing the cold-dense plasma of the magnetosheath origin on to the closed field lines of the magnetosphere. There are also cases that strongly suggest that the cold-dense plasma entry from the flanks can be significant to fill a substantial part of the magnetotail. In such cases, the cold-dense plasma is not spatially restricted to a layer attached to the magnetopause (that is, the LLBL), but continues to well inside the magnetotail, constituting the cold-dense plasma sheet. Inspired by the fact that these remarkable cases are found for northward interplanetary magnetic field (IMF), a statistical study on the status of the near-Earth plasma sheet is made. The results show that the plasma sheet becomes significantly colder and denser when the northward IMF continues than during southward IMF periods, and that the cold-dense status appears most prominently near the dawn and dusk flanks. These are consistent with the idea that, during northward IMF periods, the supply of cold-dense ions to the near-Earth tail from the flanks dominates over the hot-tenuous ions transported from the distant tail.

An anomalously large southward Bz (-23 nT) is detected in association with tailward jet (1600 km/s) by GEOTAIL at X(GSM) = -46 R(E). Variations of MHD parameters suggest that the large southward Bz is caused by piling-up of reconnected field lines by the jet against the standing plasma sheet. In addition, the ion distribution at the time of the large southward Bz is found to show a counter streaming ions (CSI) feature. A hybrid simulation of a reconnection process not only reproduces this feature, but also proves that the CSI ions are from both the lobes. Taking this CSI feature as new kinetic evidence that both the lobes are magnetically connected, we firmly conclude that the event is interpreted in terms of near-Earth reconnection.

Hybrid code simulations of the MHD scale Kelvin-Helmholtz (K-H) instability have been conducted. In contrast to the homogeneous background case where the large-scale ion mixing across the shear layer occurs inside the vortex obeying the hydrodynamic similarity law, the ion mixing is found to be reduced when the background magnetic field is inhomogeneous. This stabilization effect is twofold: it is an ion kinetic effect which reduces the saturation vortex size and so the mixing length when the shear layer width and the wavelength of the growing mode are small. The reduction effect still works even when the shear layer width/wavelength are large and a well defined vortical motion as in the fluid model is evident. The mixing is suppressed for this case because the ions on the lower magnetic field (magnetosheath) side cannot enter within the vortex island: The rolled-up magnetopause current sheet block them out electrostatically. The geophysical implication of the present result is that the K-H mixing alone may not be as efficient as to produce the large-scale mixing blob of the low-latitude boundary layer (LLBL) when the magnetic field intensity is highly different across the magnetopause.

The acceleration process of minor heavy ions in the magnetotail reconnection layer is studied by 1D hybrid simulations. In the plasma sheet, different acceleration mechanisms are found to be operative according to the heavy ion mass, not only because of their different inertia but also because of the evolutions of different downstream states. Despite this difference, the ions, including the major protons, are accelerated up to the maximum velocity of twice the lobe Alfven velocity: the energy gain is proportional to the ion mass.

Hybrid code simulations (ion particles, charge neutralizing electron fluid) of the MHD scale transverse Kelvin-Helmholtz (K-H) instability in a uniform plasma have been conducted. On a macroscopic scale the saturation states are found to be similar to the MHD result, even if the initial shear layer half thickness is as thin as twice the ion thermal Larmor radius. However, as opposed to the conventional belief based on the frozen-in concept that a large-scale mixing will be prohibited, an enhanced mixing of ions across the shear layer is found to occur inside the vortex. The temporal/spatial scales of the mixing process are determined by the fluid dynamical parameters and are anomalously quick/large compared to what can result from the mixing due to the finite Larmor radius overlapping at the interface. We propose that the low latitude boundary layer just inside the magnetopause is the mixing layer formed possibly by this mechanism.