藤本 正樹

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 >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.

This paper reports a large southward magnetic field of -23.5 nT in the Earth's magnetotail at a radial distance of 46 RE, observed with Geotail on January 10, 1995. The large southward field was part of a bipolar Bz signature of a plasmoid in association with a well-isolated substorm under steady solar wind conditions. Magnetic field and plasma observations are analyzed and compared with results from a hybrid simulation for magnetic reconnection. The large southward field is produced by the piling up of field lines due to colliding fast tailward flowing plasmas. This study implies that the piling-up mechanism plays an important role in the evolution of plasmoids traveling in the magnetotail. Copyright 1998 by the American Geophysical Union.

Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the raagnetotail at 20-30 RE 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 Bx of &lt 5 nT), the boundary region (near the plasma sheet/tail lobe boundary with the absolute magnitude of Bx is near or &gt 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 XGSM = -20 RE and XGSM = -30 RE 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. Copyright 1998 by the American Geophysical Union.

Geotail observations of the low-latitude boundary layer (LLBL) in the tail- flanks show that it is where the cold-dense ions appear with stagnant flow signatures accompanied by bi-directional thermal electrons (&lt 300 eV). It is concluded from these findings 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 suggesting that the cold- dense plasma entry from the flanks can be significant to fill a substantial part of the magnetotail. In such cases, which are detected mostly during northward IMF intervals, the cold-dense plasma is not spatially restricted to a layer attached to the magnetopause (LLBL) but continues to well inside the magnetotail, constituting the cold-dense plasma sheet. The continuity of the cold-dense plasma all the way from the magnetospheric boundary strongly supports the idea that the magnetosheath ions are directly supplied into the cold-dense plasma sheet from the flank. This cold-dense plasma in the near-Earth region shows significant contrast with the nominal hot-tenuous plasma presumably transported from the distant tail. A statistical study showing significant control on the near-Earth plasma sheet status by the IMF Bz component supports the idea that the cold-dense ion supply to the near-Earth tail from the flanks dominates over the hot-tenuous ion transport from the distant tail during northward IMF periods. We suggest that the formation processes of the plasma sheet differ according to the IMF Bz component

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.

The Akebono satellite has often observed the drop-off of polar rain flux near the polar cap boundary, The energy of the cutoff frequently showed decreasing trend with decreasing latitude, In this paper, we propose a model in which the drop-off is explained by disruption of the earthward polar rain flux at the X line in the tail, This model also predicts the behavior of the polar rain in the distant tail: The polar rain flux, usually bidirectional in the tail lobe, should become unidirectional near the plasma sheet boundary, as it is directed tailward (earthward) on the Earth (tail) side of the X line. Such a unidirectional polar rain layer has been newly detected by the Geotail satellite at X(GSM) = -40R(E) to -200R(E) recently, Modeling this new feature of the polar rain is the main purpose of this study, The proposed model also predicts a relation between the direction of the polar rain flux in this layer and the direction of ion bulk flow in the adjacent plasma sheet, The Geotail data show a good agreement with this prediction, Being convinced that the model is reasonable, the position of the X line relative to the satellite has been derived, It is shown by the Geotail survey over a wide range in GSM X that the neutral line formation occurs most frequently at X(GSM) = -50R(E) to -150R(E).

A monoenergetic drop-off of ions around 10 keV, which we term as "ion drop-off band" (IDB) in this paper, has been observed by Akebono. The IDB is identified as a sharp and deep dip at about 10 keV in ion spectra, which is usually observed at latitudes below the discrete auroral region over several degrees or more. As the ion motion of this energy in this inner part of the magnetosphere is basically described by the adiabatic theory, we have numerically traced the ion drift trajectories. From the results, it is proposed that the lower-energy boundary of the drop-off demarcates the open/closed character of the drift orbits, only below which continuous supply from the magnetotail is present. This model explains the energy, local time, and latitudinal extent of IDE as well as the formation of its poleward edge very well. Copyright 1997 by the American Geophysical Union.

We report on auroral-like electrons detected frequently in the plasma sheet. These electrons are characterized by their highly collimated bi-directional features along the field lines that are extended up to keV energy ranges. In this paper, we concentrate on samples obtained in the dawn-flank plasma sheet. Despite their detected locations in the proximity of the interface to the low-latitude boundary layer (LLBL), the energy spectra and the highly collimated features show striking contrast with the well-known bi-directional thermal electrons in the LLBL. The distribution function shape suggests that these electrons are related with auroral energization processes. It is reported that the electrons are frequently detected accompanying fluctuating flows of a few hundreds km/s. Implications of this observation to the auroral energization processes in the dawn/prenoon sectors are discussed. © 1997 Published by Elsevier Science Ltd.

A case study of the low-latitude boundary layer (LLBL) on the dawnside (7-9 MLT) is reported. As in previous studies, the LLBL structure is well organized if it is taken to consist of two parts, the outer-LLBL and the inner-LLBL. The inner-LLBL is where the mixing of the magnetosheath and the magnetospheric plasma is taking place on closed field lines. The outer-LLBL is where magnetosheath-like plasma is flowing tailward. Detailed analysis of particle signatures, together with the information that IMF B-y was the dominant component directed persistently dawnward for this interval, suggests an interpretation that the outer-LLBL is on reconnected open field lines. The positions of the reconnection sites relative to the spacecraft, and the dynamics of the flux tubes subsequent to reconnection to form the observed outer-LLBL, are also discussed. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

This letter reports on leakage ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) as observed by Geotail at the dayside magnetopause. A reconnection of the interplanetary magnetic field (IMF) with magnetospheric field lines is shown to be responsible for the leakage. Based on analysis of a variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions, we propose a "velocity filter effect" model with a finite source region to explain this variation. The location of the reconnection region is subsequently calculated to be 2.2RE north of the geomagnetic equatorial plane under the proposed velocity filter effect model.

GEOTAIL data from the low-latitude flanks of the magnetotail at -30RE &lt XGSM &lt -15RE are analyzed. An example presented in this paper, which is representative of a group of the cases at the dawnside, shows that the region is characterized by highly varying magnetic field and the appearance of cold (&lt 1 keV) and dense (&gt 0.5/cc) plasma. While some of these cold-dense plasma are detected to flow tailward at 300-500 km/s, others are found to flow only slowly tailward or even sunward sometimes. Electrons of &gt 100 eV are seen to be depleted in the tailward flowing part, suggesting that these data are taken when the satellite was located on open field lines. When the sunward flows are detected, the denisty tends to show an intermediate value, and thermal electrons (100-500 eV) are found to show bi-directional anisotropy. This suggests that part of the magnetosheath plasma is captured on closed field lines at the inner part of the low-latitude boundary layer (LLBL) in the tail flanks. Another study inside the plasma sheet at (XGSM, YGSM) = (-17, -12) RE (a position well inside the magnetopause) shows a 2.5-hours period of the plasma sheet filled with cold (500 eV) and dense (2/cc) plasma. Bi-directional thermal electrons are seen to accompany this population. This may imply that relatively unheated plasma from the LLBL of the near-Earth magnetotail is filling a substantial part of the plasma sheet.

This letter reports on leakage ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) as observed by Geotail at the dayside magnetopause. A reconnection of the interplanetary magnetic field (IMF) with magnetospheric field lines is shown to be responsible for the leakage. Based on analysis of a variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions, we propose a "velocity filter effect" model with a finite source region to explain this variation. The location of the reconnection region is subsequently calculated to be 2.2RE north of the geomagnetic equatorial plane under the proposed velocity filter effect model.

Nishida et al. (1986) proposed the category of quasi-stagnant plasmoids on the basis of ISEE-3 observations. Their quasi-stagnant plasmoids occurred during intervals of low geomagnetic activity, and the electron plasma moment data suggested that the plasmoids moved tailward very slowly (&lt 300 km/s). This paper reports for the first time the low-energy (32 eV/e-43 keV/e) plasma ion signatures of a quasi-stagnant plasmoid, which was observed with Geotail at X=-170 RE on October 15, 1993. On this day Kp was generally quiet, but Geotail observed two prominent bipolar Bz perturbations identifiable as plasmoids. The first plasmoid moved tailward very slowly (~250 km/s), and it was associated with a Pi2 onset possibly localized at high latitudes and with a gradual enhancement in the power of Aurora] Kilometric Radiation (AKR) emissions. On the other hand, the second plasmoid moved tailward fairly rapidly (~500 km/s), and it was associated with a Pi2 onset observable at wide range of latitudes and with, a sharp enhancement in the AKR power. Thus the first plasmoid falls into the category of quasi-stagnant plasmoids but the second plasmoid would notj this is consistent with the previous observations by Nishida et al. that quasi-stagnant plasmoids are sometimes followed by substorm onset-related plasmoids. A unique feature of the plasma ions in the first plasmoid is that the earthward field-aligned beam was observed first and then the tailward field-aligned beam was observed, as the spacecraft entered into the plasmoid. This time sequence is rarely observed for plasmoids during geomagnetically active times. It is explained in the framework of the quasi-stagnant plasmoid model. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term 'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic B-y fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the B-y/B-z field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic B-x field is very small, but significant B-y and/or B-z fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term 'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic By fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the By/Bz field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic Bx field is very small, but significant By and/or Bz fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

GEOTAIL data from the low-latitude flanks of the magnetotail at -30R(E) < X(GSM) < -15R(E) are analyzed. An example presented in this paper, which is representative of a group of the cases at the dawnside, shows that the region is characterized by highly varying magnetic field and the appearance of cold (<1 keV) and dense (>0.5/cc) plasma. While some of these cold-dense plasma are detected to flow tailward at 300-500 km/s, others are found to flow only slowly tailward or even sunward sometimes. Electrons of >100 eV are seen to be depleted in the tailward flowing part, suggesting that these data are taken when the satellite was located on open field lines. When the sunward flows are detected, the denisty tends to show an intermediate value, and thermal electrons (100-500 eV) are found to show bi-directional anisotropy. This suggests that part of the magnetosheath plasma is captured on closed field lines at the inner part of the low-latitude boundary layer (LLBL) in the tail flanks. Another study inside the plasma sheet at (X(GSM), Y-GSM) = (-17, -12)R(E) (a position well inside the magnetopause) shows a 2.5-hours period of the plasma sheet filled with cold (500 eV) and dense (2/cc) plasma. Bi-directional thermal electrons are seen to accompany this population. This may imply that relatively unheated plasma from the LLBL of the near-Earth magnetotail is filling a substantial part of the plasma sheet.

For the steady-state three-dimensional reconnection problem in a compressible plasma, we have found a new class of exact solutions, which consist of ideal-MHD regions, MHD singularities (slow shocks and rotational discontinuities), and a neutral line. This solution predicts the existence of plasma jetting parallel to the neutral line, in addition to the usual plasma jetting in the direction perpendicular to the neutral line. However, contrary to the suggestion by LAu and FINN (1990), and PRIEST and FORBES (1992) that three-dimensional reconnection regions should have velocity singularities, our solution has well-behaved velocity fields without any singularity.

The Kelvin-Helmholtz instability in a thin velocity shear layer is analyzed in a two-fluid plasma. In the system treated here, electrons are treated as a massless, charge neutralizing fluid, and the ion inertia effects appearing through the Hall term and the electron pressure term in the generalized Ohm's law are added to the usual MHD system of equations. Linearized equations are derived and solved numerically for the parallel (V0 parallel-to B0) and the perpendicular (V0 perpendicular-to B0) geometries. Although the growth rates are only slightly affected by the ion inertia effect for both configurations, the structures of the eigenmodes become highly different when the modes have spatial dependences along the background magnetic field. The implication of the present results for the magnetopause boundary layer is discussed.