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[1] The Jupiter orbiting spacecraft Galileo has provided evidence that the Jovian magnetotail is subject to a periodic process with typical timescales of several days by which the Jovian system is presumably releasing its excess iogenic mass. The mass release process resembles a terrestrial substorm in the sense of a global reconfiguration of the magnetotail. During the initial “loading” phase the plasma convection is at a moderate speed in the corotation direction and the Jovian plasma sheet appears to be in a stable configuration. In the release phase, reconnection through a thinned current sheet leads to radially inward and outward plasma flows and the ejection of plasmoids. Storage of magnetic energy in the lobe region seems not to be the prime driver of the reconfiguration process. Therefore the role of the solar wind as energy source is of less importance than for terrestrial substorms. Instead, it can be envisaged that plasma loading of fast rotating magnetic flux tubes and the associated centrifugal forces drive the reconfiguration process.

[1] Finite ion cyclotron effects play a significant role in determining the dynamics of the neutral sheet. The demagnetization of the ions facilitates reconnection and produces an electric field perpendicular to the direction of the tail currents. This in-plane electric field drives field-aligned currents and an out-of-plane (or core) magnetic field in conjunction with the generation of flux ropes. In addition to these electromagnetic effects, it is shown that ion cyclotron effects lead to the preferential convection of plasma from the dawnside to the duskside. This convection is consistent with results from single-particle tracking but differs from ideal MHD treatment where the flow occurs symmetrically around the Earth. A physical manifestation of these asymmetric particle trajectories is the wrapping of the field-aligned current between the region 1 currents and the region 2 and/or region 0 currents. In addition, localized density enhancements and depletions are seen in the tail where the local heavy ion density can be substantially elevated over ionospheric conditions. Because of the local density variations, reconnection across the tail is inhomogeneous. Reconnection is initiated postmidnight and then sweeps across to the dawn and dusk flanks within a few minutes. Because of this spatial variation, the ejection plasmoid is actually U-shaped and the subsequent flux rope formation is highly skewed.

Accepted: 2004-08-24, 資料番号: SA1002559000

[1] Magnetic reconnection in a plane current sheet is investigated in both two and three dimensions, using three different types of simulation codes, Hall MHD, hybrid (electron fluid, kinetic ions), and a new code called Hall-less hybrid. The latter code, which is similar to the hybrid code but has the Hall term removed, enables us to clarify the differences between kinetic ion and Hall MHD approaches. The major findings of this research are (1) Sweet-Parker regime of reconnection cannot be maintained and does not reach a steady state in a kinetic plasma for physically interesting parameter regimes. (2) Fast asymptotic reconnection rate of 0.15VA0B0 is obtained both in the hybrid and Hall-less hybrid simulations with outflow boundaries. VA0 and B0 are the Alfven velocity and magnetic field strength in the upstream region. This finding has two immediate implications. First, ion kinetics are sufficient to lead to fast reconnection even in the absence of the Hall term, and second, explanation of fast reconnection in terms of quadratic disperion of whistlers needs to be reconsidered, as whistlers are dispersionless in Hall-less hybrid limit. (3) Unlike in MHD, diffusion region is different in size that the region of localized resistivity. (4) While both Hall and hybrid codes show that reconnection is inherently asymmetric in three dimensions, there are differences in the nature of the asymmetry. In Hall MHD we show that the X-line grows in the direction of the electron drift, propagating as a (reconnection) wave because the current is carried by electrons, although the wave direction can change in the presence of a substantial ion flow. However, in the hybrid simulations here, as is the case for typical conditions at the magnetopause and magnetotail, ions carry the bulk of the current, and the observed asymmetry is found to be due to ion flow and not a wave motion unless the extent of finite resistivity in the third dimension is very thin, comparable to the current sheet thickness. Thus aside from scenarios where electrons are the dominant current carriers, such as very thin current sheets that are on electron scales, we do not expect the reconnection wave to form. This result is relevant to the magnetotail where dawn-dusk asymmetries are observed in the motions of auroral brightenings and surges, as well as in the statistical location of pressure decreases, flows, and magnetic signatures associated with the near-Earth neutral line and early plasmoids. We attribute the observed dawn-dusk asymmetries to ion flows. One interesting question left for future work is the possibility that reconnection waves may form in thin electron-scale current layers that are sometimes observed embedded within a thicker sheet in the magnetotail.

plasma density N � 0.3 cm � 3 and the plasma ion b � 1.5. The Walen analysis applied to the tailward flow interval from 0400:36 UT to 0403:14 UT is consistent with the ions being accelerated to � 73% of the Alfven speed across a slow-mode shock connected to a near-Earth neutral line located on the earthward side of the spacecraft. The occurrence of a small plasmoid-type magnetic flux rope during the leading edge of the tailward flows provides further support in favor of an active region of magnetic reconnection earthward of Cluster. The more field-aligned earthward flows between 0406 UT and 0408 UT, however, failed to satisfy the Walen test. Rankine-Hugoniot analyses of upstream and downstream plasma and magnetic field parameters confirm the presence of a slow-mode shock in connection with the passage of the tailward flow region but not with the 0406 UT to 0408 UT earthward flow interval. The confirmed shock satisfies the critical slow-mode requirements: MI * � 1.0 and MSM * > 1.0 on the upstream side and MSM * � 19 RE of the near-Earth magnetotail. INDEX TERMS: 7835 Space Plasma Physics: Magnetic reconnection; 7851 Space Plasma Physics: Shock waves; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: magnetic reconnection, shock waves, magnetotail boundary layers

Pressure balance structures (PBSs) are a common high plasma beta feature in high latitude, high speed solar wind. They have been proposed as remnants of coronal plumes. If true, they should reflect the observation that plumes are rooted in unipolar magnetic flux concentrations in the photosphere and are heated as oppositely directed flux is advected into and reconnects with the flux concentration. A minimum variance analysis (MVA) of magnetic discontinuities in PBSs showed there is a larger proportion of tangential discontinuities than in the surrounding high speed wind, supporting the hypothesis that plasmoids or extended current sheets are formed during reconnection at the base of plumes. To further evaluate the character of magnetic field discontinuities in PBSs, differential streaming between alpha particles and protons is analyzed here for the same sample of PBSs used in the MVA. Alpha particles in high speed wind generally have a higher radial flow speed than protons. However, if the magnetic field is folded back on itself, as in a large amplitude Alfven wave, alpha particles will locally have a radial flow speed less than protons. This characteristic is used here to distinguish between folded back magnetic fields (which would contain rotational discontinuities) and tangential discontinuities using Ulysses high latitude, high speed solar wind data. The analysis indicates that almost all reversals in the radial magnetic field in PBSs are folded back field lines. This is found to also be true outside PBSs, supporting existing results for typical high speed, high latitude wind. There remains a small number of cases that appear not to be folds in the magnetic field and which may be flux tubes with both ends rooted in the Sun. The distinct difference in MVA results inside and outside PBSs remains unexplained.

[1] The present study examines the tailward propagation of substorm-associated variations of the tail current intensity. In the substorm event of 24 November 1996, the Interball and IMP 8 satellites were located in the midnight sector at X = −26 and −36 RE, respectively, and observed an increase and a decrease of the lobe magnetic field strength corresponding to the storage and release of the lobe magnetic energy. Both spacecraft observed BZ to decrease initially and then increase in the course of the decrease in ∣BX∣, a feature that was reported previously as a manifestation of the tailward expansion of the current disruption region. The delay of the signatures between the two satellites confirms that the associated current system moved tailward. Motivated by this fortuitous coordination of the satellite observation, the present study revisits a global MHD simulation previously conducted specifically for this substorm event [Raeder et al., 2001]. The most noticeable feature of the modeled tail dynamics is the repeated occurrence of tail current surges, that is, temporal intensifications of the tail current that propagate tailward. The first tail current surge is accompanied by the stretching of the tail magnetic field, which starts in the inner magnetosphere and extends tailward. The associated tailward flow redistributes the plasma pressure in such a way that the tail current is reduced in its intensity in the near-Earth region, while the pressure gradient increases at the propagation front, which intensifies the local current. The last major tail current surge is caused by the near-Earth reconnection. Inside a plasmoid, the pressure gradient current is intensified on the tailward side of the O-line, and it propagates tailward as the plasmoid grows and is released. For each tail current surge, irrespective of its cause, the intensification of the tail current is followed by the reduction, and its tailward propagation creates the aforementioned phase relationship between BX and BZ. It is probably difficult to determine based on lobe magnetic field observations whether it is caused by the tail stretching or a neutral line motion. The present study not only sheds new light about tail substorm dynamics but also provides a good exercise for evaluating the potential of the modeling effort for substorm study in general.

The entropy parameter Pn � 5/3 was found to be relatively uniform throughout the region studied. The energy parameter TV 2/3 decreased by 40% for ions and 10% for electrons near midnight between � 29.5 and � 11.5 RE. These energy parameter changes suggest that the most energetic ions and electrons are either being deenergized or preferentially lost, processes that may be associated with gradient and curvature drifts through the sides of the convecting flux tubes or by wave instabilities and a parallel heat flux. INDEX TERMS: 2764 Magnetospheric Physics: Plasma sheet; 2744 Magnetospheric Physics: Magnetotail; 2760 Magnetospheric Physics: Plasma convection; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; KEYWORDS: plasma sheet, magnetotail, pressure balance inconsistency, 3-D models

[1] We present the results of magnetohydrodynamic (MHD) simulations of the nonlinear development of instabilities of magnetically sheared arcades and show how they relate to plasmoid ejections and coronal mass ejections (CMEs). To model the arcade disruptions, we capitalize on a family of analytical solutions for initial states; these describe magnetic arcades in uniform gravity that are characterized by magnetic shear. Two-and-a-half-dimensional (2.5-D) time-dependent simulations were performed with the ZEUS-2D code to show the response of the arcades to small velocity perturbations. The model arcades respond by rising and expanding, and most significantly, we find shearing motions naturally arise in conjunction with the instability. This field line shearing is in response to the Lorentz force, which drives large-amplitude Alfven waves, which in turn transport magnetic shear from the lower to the upper extremities of the arcades. The self-induced shear Alfven waves, coupled with magnetic buoyancy, provide a feedback mechanism that drives the arcades to a loss of equilibrium and disruption following a long period of slow rise and expansion. These simulations of arcade disruptions may be relevant with regard to CME initiation for three major reasons. First, the arcade disruptions are the result of undriven magnetic buoyancy instabilities. Second, magnetic field line shearing is an intrinsic aspect of the instability that occurs spontaneously and is driven only by the magnetic tension force. (No imposed shear motions are specified.) Third, the arcade disruption process has been found to repeat without prompting.

[1] Quasi-periodic, sawtooth-like variations of energetic plasma particle fluxes are often measured at geosynchronous orbit during magnetic storms. The outstanding problems are whether the sawtooth flux variations represent particle injections from the tail to the inner magnetosphere, whether the sawtooth variations correspond to substorm onsets, and what mechanism is responsible for the generation of periodic particle injections (or periodic substorms). In this paper, we present the measurements of multiple space-based and ground-based instruments in the magnetosphere and ionosphere during two magnetic storms. The Geotail satellite was located in the near tail between XGSM = −20 and −30 RE and detected periodic southward turnings of the magnetospheric magnetic field. The southward turnings of Bz are interpreted as the signature of periodic near-tail reconnection and plasmoid formations. Geosynchronous satellites measured periodic gradual dropouts and rapid increases of energetic charged fluxes and magnetic field dipolarization. The IMAGE satellite measured enhanced emissions of energetic neutral atoms in the ring current and auroral brightenings. The Canadian Auroral Network for the OPEN Program Unified Study's photometers measured intensifications and latitudinal motion of auroral emissions. The auroral electrojet index showed periodic increases. These magnetospheric and ionospheric phenomena have the same periodicity and indicate the occurrence of periodic substorms. There is an excellent correspondence among the substorm signatures from all the measurements. The results show that periodic magnetospheric substorms can indeed occur during continuous southward interplanetary magnetic field, and the mean period in these two cases is ∼2.7 hours. The sawtooth variations of energetic plasma fluxes at geosynchronous orbit represent true particle injections from the tail to the inner magnetosphere. The sawtooth injections are related to the near-tail reconnection and correspond to the onsets of periodic substorms.

[1] Examination of Geotail measurements in the near-tail (X > � 30 RE) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of � 1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hVxi = 431 km/s, and 38 moving tailward, hVxi = � 451 km/s. We refer to these two populations as ‘‘BBF-type’’ and ‘‘plasmoid-type’’ flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal Bz perturbations, i.e., � for the ‘‘BBF’’ events and ± for the ‘‘plasmoid’’ events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within � 1–2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 RE and 20 nT and 4.4 RE and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X–Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events. INDEX TERMS: 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2764 Magnetospheric Physics: Plasma sheet; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms

Accepted: 2002-09-27, 資料番号: SA1002552000

[1] Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A one-dimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst-plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X ∼ −15 to −18 RE, respectively, about 2–5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X ∼ −9 RE. The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high-latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high-latitude inner magnetosphere, ∼103 km s−1. In summary, the results of our investigation strongly support the modern near-Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed ∼2–5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within ∼1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, ∼10–20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.

The highly structured magnetic field and plasma properties observed in the heliospheric extension of the coronal streamer belt have been interpreted as evidence for multiple current sheets originating at coronal helmet streamers. We explore the linear stability of a simple case: a triple current sheet, as would exist above two neighboring helmets of the same polarity. The behavior of the triple current sheet when perturbed by small disturbances can be described (in the incompressible limit) by MHD Orr-Sommerfeld and Squire equations, which we solve with a Chebyshev-τ method. We show the velocity and magnetic fields which characterize the three unstable modes and describe the modal dependence on fieldwise wavenumber and current sheet separation. At long wavelengths an unexpected phenomenon occurs: two modes degenerate into unstable traveling modes. We also explore the three-dimensional behavior and the modal variation with both large and small values of the resistivity and viscosity. We conclude that the magnetic topology in closely packed streamers is susceptible to instabilities with growth times of the order of hours. Our predictions indicate that the resultant plasmoid structures should be observable with the large angle and spectrometric coronagraph (LASCO) and ultraviolet coronagraph spectrometer (UVCS) instruments on the upcoming Solar and Heliospheric Observatory (SOHO) mission.

Helios 1 observed a magnetic cloud on the spacecraft's closest approach to the Sun (0.3 AU) on March 26, 1976. Fluxes of energetic particles detected by the spacecraft generally decrease during the passage of a cloud, but in this case the smooth intensity depression was interrupted by a sharp peak-like increase coinciding with a deep drop of the magnetic field magnitude. This behavior is observed for protons and electrons in several energy bands. A possible explanation of this event is given by the spheroidal model of magnetic clouds, applicable for the plasmoid geometry (having field lines fully disconnected with the Sun).

Accepted: 1995-05-17, 資料番号: SA1002514000

In contrast to the extensively studied growth and expansion phases of magnetospheric substorms, the substorm recovery phase has not received much attention in the published literature. It has generally been considered as a period in which all disturbances caused by the previous two phases decay, the magnetosphere “recovers” to reach a quiet state again. Using mainly ground-based data, we show that the “recovery phase” contains a number of features which are qualitatively different from the expansion phase, that is, not just a decay of previously excited forms of energy release. Typical recovery phase phenomena include intense electrojet activity in the morning sector, high-energy particle precipitation, the development of large-scale auroral vortex streets (so-called Ω bands and associated magnetic Ps 6 pulsations), and very often new eastward expanding active auroral phenomena, not unlike evening-sector expansion phase features but concentrated to the morning sector of the auroral oval. Such features must be associated with the substorm mechanism itself. We suggest that our observations during the substorm recovery phase are explained by the magnetospheric reorganization after the ejection of a plasmoid. We show that a more detailed investigation of the late substorm features can increase our understanding of the physical processes leading to the complete cycle of magnetospheric energy release.

A survey of Interplanetary Monitoring Platform (IMP 8) magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south-north bipolar lobe signatures. The properties of these relatively near-Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in International Sun-Earth Explorer (ISEE) 3 deep-tail observations. The occurrence frequency of these near-Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid- and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south-north bipolar turning in the Geocentric Solar Magnetospheric (GSM) B(sub z) component, are localized near the noon-midnight meridional plane, and are strongly correlated with interplanetary magnetic field B(sub z) north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are 'proto-plasmoids,' i.e., plasmoids for which near-Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto-plasmoids are then 'trapped' inside closed magnetic field lines and propagate earthward owing to the effect of the distant X-line's earthward plasma flow. We suggest that the two different 'types' of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.

We present results of a three-dimensional magnetohydrodynamic (MHD) simulation of magnetotail evolution initiated by a sudden occurrence or increase of spatially localized resistivity as the major expected concequence of some localized microinstability. Because of the absence of a quantitative model, possible variations of resistivity levels with current density, or the reduction thereof, are not incorporated in the present investigation. The emphasis of the study is on an investigation of the changes to the overall evolution brought about by this localization, in particular, on the disruption and diversion of the cross-tail current and the nonlinear evolution of the magnetotail instability. The immediate consequences of the occurrence of the localized resistance and the resulting electric field are a reduction and diversion of the electric current around the region of high resistivity, associated with an increase of B(sub z) ('dipolarization') at the earthward edge and a decrease of B(sub z) at the tailward edge of this region. These effects, however, are localized and do not involve a reduction of the total cross-tail current and hence do not lead to the global development of a 'substorm current wedge,' which includes not only the reduction of the cross-tail current but also the buildup of a global field-aligned current system of 'regional 1' type (toward the Earth on the dawnside and away on the duskside of the tail). Such signatures develop at a later time, as consequences of a three-dimensional tearing instability, which is triggered by the occurrence of the resistivity. These features are found in combination with plasmoid formation and ejection, quite similar to results of earlier simulations with uniform resistivity. Differences are found in the timescale of the evolution, which tends to be shorter for localized resistivity, and in the propagation of the dipolarization effects in the equatorial plane. Whereas for uniform resistivity the temporal increase in B(sub z) tends to propagate tailward, apparently due to a pileup effect in the near-Earth region, an earthward propagation is found for the localized resistivity. This propagation results from an earthward convection of the gradient in B(sub z), which is set up by the reconnection process further tailward.

Two-dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the impulsive penetration process when a magnetosheath plasma irregularity interacts with the magnetopause. The irregularity is modeled as a field-aligned filament with an excess density impinging on the magnetopause. Simulations with strictly parallel or antiparallel magnetic fields in the magnetosheath and in the magnetosphere are performed in order to investigate ion kinetic effects during the penetration of the plasma irregularity. The results show that a MHD approach is not appropriate when describing the impulsive penetration of small-scale irregularities. If the fields in the magnetosheath and magnetosphere are antiparallel, the magnetic field gradient near the magnetopause boundary leads to an asymmetric propagation of the magnetosheath plasma filament. In contrast, for moderate field changes across the magnetopause the parallel case exhibits an almost straight-line propagation of the filament, while its main part is focused into a small region. In both cases, twin vortices are set up in the magnetosphere due to the filament motion. In the antiparallel case the shape of the filament is totally distorted by these vortices; these fluidlike structures stretch the magnetopause boundary and thereby increase the line to surface ratio of the boundary layer. The filament breaks in the antiparallel as well as in the parallel case into smaller parts which leads to isolated islands of magnetosheath like plasma and field in the magnetosphere of the order of the ion gyroradius.

Recent simulations of magnetotail reconnection have pointed to a link between plasma flows, dipolarization, and the substorm current wedge. In particular, Hesse and Birn (1991) have proposed that earthward jetting of plasma from the reconnection region transports flux into the near-Earth region. At the inner edge of the plasma sheet this flux piles up, producing a dipolarization of the magnetic field. The vorticity produced by the east-west deflection of the flow at the inner edge of the plasma sheet gives rise to field-aligned currents that have region 1 polarity. Thus in this scenario the earthward flow from the reconnection region produces the dipolarization ad the current wedge in a self-consistent fashion. In this study we examine observations made on April 8, 1985 by the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Ion Release Module (IRM), the geosynchronous satellites 1979-053, 1983-019, and 1984-037, and Syowa station, as well as AE. This event is unique because IRM was located near the neutral sheet in the midnight sector for am extended period of time. Ground data show that there was ongoing activity in the IRM local time sector for several hours, beginning at 1800 UT and reaching a crescendo at 2300 UT. This activity was also accompanied by energetic particle variations, including injections, at geosynchronous orbit in the nighttime sector. Significantly, there were no fast flows at the neutral sheet until the great intensification of activity at 2300 UT. At that time, IRM recorded fast eartheard flow simultaneous with a dipolatization of the magetic field. We conclude that while the aforementioned scenario for the creation of the current wedge encounters serious problems explaining the earlier activity, the observations at 2300 UT are consistent with the scenario of Hesse and Birn (1191). On that basis it is argued that the physics of substorms is not exclusively rooted in the development of a global tearing mode. Processes at the inner edge of the cross-tail current that cause a disruption of the current and a consequent dipolarization and current wedge may be unrelated to the formation of a macroscale reconnection region. Thus the global evolution of a substorm is probably a complicated superposition of such processes operating on a very localized scale and a global macroscale process that allows for such things as releasing te energy stored in lobe flux and creation of plasmoids.

A comprehensive study is conducted of traveling compression regions (TCRs) in the distant magnetotail; a total of 116 TCRs were studied from ISEE 3 observations. Strong support is obtained for the interpretation of TCRs as large-scale compressions of the lobes that are caused by the rapid downtail motion of plasmoids. TCRs furnish information on the 3D shape and volume of the plasmoid bulge. The close association noted between the substorm expansion phase onset and the TCRs provides strong support for the plasmoid model of magnetotail dynamics.

We have used a new high-resolution global magnetohydrodynamic simulation model to investigate the onset of reconnection in the magnetotail during intervals with southward interplanetary magnetic field (IMF). After the southward IMF reaches the dayside magnetopause reconnection begins and magnetic flux is convected into the tail lobes. After about 35 min, reconnection begins within the plasma sheet near midnight at x = −14RE. Later the x line moves toward dawn and dusk. The reconnection occurs just tailward of the region where the tail attaches onto the dipole-dominated inner magnetosphere. The simulation shows that prior to the onset of reconnection, the Poynting flux is concentrated in this region. The time required for the start of reconnection depends on the component of the magnetic field normal to the equator (BZ). Reconnection occurs only after the BZ component has been reduced sufficiently for the tearing mode to grow. Later, when all the plasma sheet field lines have reconnected, a plasmoid moves down the tail.

An overview of the Galileo magnetometer observations from the crossing of the tail magnetopause at an antisolar distance of close to 100 R(E) through exit into the solar wind on the dayside is presented. These measurements are linked with correlative data from ground stations and from IMP 8 which was ideally located to serve as a monitor of the solar wind upstream of the bow shock. A time line of the important geomagnetic events of the day that provides a framework for the full multiinstrument analysis of the flyby data is presented. The observations are used to investigate apsects of the relationship between magnetotail dynamics and the separate intensifications of a multiple onset substorm inferred from ground-based data. It is proposed that the signatures associated with individual substorm intensifications are localized in the dawn-to-dusk extent even at remote locations in the magnetotail, just as they are in the ionosphere, and that the tail disturbances associated with successive substorm intensifications step across the tail towards the dusk flank.

The paper discusses the characteristics of quasi-steady plasma and field structures with field-aligned flow. Explicit solutions are developed for modeling the compressible flow around a plasmoid in the distant magnetotail. The expected and observed plasmoid signatures are found. Field signatures outside the plasmoid are typically those of encounters of traveling compression region: a north-south signature of Bz accompanied by an enhancement of Bx.

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high-speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well-defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.

A substorm which occurred on April 24, 1979 was the subject of the CDAW 7 workshop and several in-depth studies of ISEE and ground-based data. In this paper we continue the analysis by looking in detail at geosynchronous energetic particle data from the Los Alamos charged particle analyzer instruments. Observations show three distinct injection events during this substorm. We have used a drift shell tracing technique to determine the time and location on the geosynchronous drift shell of each injection region. We then compare the timing of events on the ground, in the tail and at geosynchronous orbit. The first injection is very closely associated with observations from ISEE and ground-based measurements which have been interpreted as the signatures of plasmoid formation in the near-Earth tail. The later two injections occurred while ISEE 1 and 2 were in the lobes following the exit of the plasmoid downtail. All three injections are calculated to have originated in the same local time sector in which the ISEE spacecraft were located and also to have overlapped one another in local time extent. These observations show that the spatial and temporal evolution of the substorm is more dynamic than previous studies have implied. We have considered possible interpretations of the more complete set of observations the context of the both the traditional near-Earth neutral line model and the inner magnetospheric current disruption model. Either model can provide an interpretation of the data but neither by itself, provides a completely satisfactory explanation. We suggest a new synthesis of these two models which provides a simple interpretation for all the observations. We propose that reconnection in the tail initiates the substorm but diverts much of the cross-tail current around the reconnection region enhancing the near-Earth cross-tail current. This creates an unstable situation and current disruptions dipolarize the field in three steps creating the three injections.

Approximate equilibrium solutions for the two-dimensional magnetohydrodynamic equilibrium equations with flow along magnetic field lines applied to modeling the Earth`s magnetic tail are constructed. An alternative version of the condition for whether a given magnetospheric configuration is open or closed is presented. Also developed are examples of open and closed magnetospheric configurations with flow occurring in a layer modeling the plasma sheet boundary layer. Examples of finite-size compressible plasmoids traveling in the magentotail are presented. The relation between the profile of the total pressure (thermal plus magnetic) and the shape of the plasmoid is also discussed. 30 refs., 5 figs.

A global simulation of the magnetosphere with a long tail ({approximately}1,000 R{sub E}) is performed. A magnetosphere with a neutral sheet is constructed from a dipole field by solar wind dynamic pressure (no interplanetary magnetic field (IMF)). Concentration of the plasma sheet current occurs preferentially at 14-18 R{sub E} on the tail side of the Earth, which is an indication that, magnetically, this is the most fragile region of the tail structure. It is the demarcation region between the dipolar and streaming field line structures. Therefore, in the presence of resistivity, magnetic reconnection can be preferentially driven here by a compression disturbance of some sort. In the present initial-value problem, reconnection occurs at 15-20 R{sub E} in the tail and develops into a large lump of plasma surrounded by reconnected field lines, a plasmoid, which is ejected tailward. The time scale of the plasmoid formation and ejection process is very slow, of the order of several hours, when no IMF exists. After ejecting one plasmoid, reconnection occurs again in the plasma sheet, and a second plasmoid is formed and ejected. This result shows that a magnetosphere that has a sufficiently long tail and a neutral sheet is fragile and subjectmore » to plasmoid formation. The authors also show that the hot plasma, when mapped down to the ionosphere along the field lines, encompasses the auroral oval in agreement with the DE satellite observations.« less

A spherical plasmoid is injected into a representative solar wind at 18 solar radii, which is chosen as the lower computational boundary of a 3-dimensional MHD model. The field line topology of the injected plasmoid resembles the streamline topology of a spherical vortex. Evolution of the plasmoid and its surrounding interplanetary medium is described out to approximately 1 AU for three cases with different velocities imparted to the plasmoid. In the first case a plasmoid enters the lower boundary with a velocity of 250 km s{sup {minus}1} equal to the steady state background solar wind velocity at the lower boundary. In the second and third cases the plasmoid enters with peak velocities of twice and 3 times the background velocity. A number of interesting features are found. For instance, the evolving plasmoid retains its basic magnetic topology although the shape becomes distorted. As might be expected, the shape distortion increases with the injection velocity. Development of a bow shock occurs when the plasmoid is injected with a velocity greater than the sum of the local fast magnetosonic speed and the ambient solar wind velocity. The MHD simulation demonstrates magnetic draping around the plasmoid.

The formation and acceleration of plasmoids is investigated by two-dimensional magnetohydrodynamic simulations. The initial equilibrium contains a plasma sheet with a northward magnetic field (Bz) component and a tailward pressure gradient. Reconnection is initiated by three different methods: Case A, a constant resistivity is applied everywhere and a tearing mode evolves, case B, a spatially localized resistivity is fixed in the near-Earth region, and case C, the resistivity is allowed to depend on the electrical current density. In case A we obtain the same results as have been presented by Otto et al. (1990): the tearing instability releases the tension of the closed field lines so that the inherent pressure gradient of the two-dimensional system is not balanced anymore. The pressure gradient then sets the plasmoid into motion. Any sling-shot effect of open magnetic field lines is of minor importance. A completely different behavior has been found in cases B and C. In these cases the high-speed flow in the wedge-shaped region tailward of the near-Earth neutral line pushes against the detached plasmoid and drives it tailward. The ideal terms contributing to the acceleration are still only the pressure and the magnetic field term. However, in these cases the pressure is due to the dynamic pressure of the fast outflow from the reconnection region. The outflow in the wedge-shaped region on both sides of the neutral line is due to acceleration of plasma by tangential magnetic stresses at the slow mode shocks extending from the X line.

The formation and evolution of a three-dimensional magnetotail plasmoid is investigated by means of a three-dimensional resistive MHD simulation. In contrast to earlier simulations, the present one takes place in a tail configuration that includes the transition from a closed field line region to an open far-tail region with a distant X line or separator. This is necessary to identify the plasmoid and to study its properties during the separation from Earth. Particular emphasis is placed on the plasma acceleration and the relative importance of pressure forces and Lorentz forces. Two distinct phases of the plasmoid evolution are analyzed, the growth, characterized by accumulation of plasma that has already been accelerated by Lorentz forces outside the plasmoid, and the severance of the plasmoid from Earth, which takes a finite amount of time, propagating from midnight toward the flanks. This phase is characterized by a decrease in plasmoid speed due to the fact that accumulation of plasma of slower speed continues near the flanks and at the leading edge, while the plasmoid becomes severed near midnight. This phase is also characterized by a complicated topological structure of the plasmoid consisting of intermingled flux tubes with different connections. The total force on the plasmoid itself is initially dominated by the pressure gradients and later by the Lorentz forces. These forces, however, do not contribute significantly to the plasmoid motion in comparison to the accumulation of momentum.

The influence of magnetosphere-ionosphere coupling on the dynamic evolution of the magnetotail is investigated by a three-dimensional resistive MHD code that includes the effects of the closure of field-aligned currents in a simple resistive model ionosphere. Particular emphasis is on the role of this coupling during substorm evolution and the modification of the latter by the convection driven by the ionospheric electric fields. For comparison, results are presented from a simulation which uses an infinitely conducting ionosphere but is otherwise identical. Comparison of the two simulations shows that the major impact of magnetosphere-ionospheric communication is an acceleration of magnetotail evolution. Otherwise, phenomena in the two models are qualitatively similar. It is concluded that ionospheric effects do not significantly affect substorm associated magnetotail dynamics.

Solar energetic particles (SEPs) have been used as probes of magnetic cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.

The dynamic evolution of a nonsymmetric magnetotail configuration initiated by the sudden occurrence of (anomalous) resistivity are examined using a three-dimensional resistive MHD simulation developed by Birn (1990) that includes a net cross-tail field and thus breaks the mirror symmetry around the neutral sheet. Results show that the field evolution is similar to that of a symmetric configuration studied by Birn and Hones (1981), pointing to the formation and ejection of a plasmoid. On the other hand, the topological structure of the magnetic field, defined by the field line connections, was remarkably different from the symmetric case. The plasmoid in this case became 'open', connected initially with the earth but gradually becoming connected with the interplanetary field. The openness of the plasmoid and its magnetic connection with interplanetary field lines suggest the possibility of a heat flux out of the plasmoid on interconnected flux tubes.

MHD simulations have studied the evolution of an electric current system associated with fast reconnection development in three dimensions. The initial equilibrium is a one-dimensional antiparallel magnetic field with a current sheet (or plasma sheet) in the middle. Fast reconnection is initiated by a localized resistivity and spontaneously develops in a finite region of the current sheet, leading to an Alfvenic plasma jet. Ahead of the plasma jet the plasma sheet is notably compressed, forming a plasmoid which swells and propagates. In the fast reconnection region, notable plasma rarefaction occurs so that the plasma flow converges, whereas in the plasmoid, plasma is compressed so that the plasma flow diverges. The resulting velocity shear gives rise to a magnetic shear along the plasmoid boundary, which is directly related to a (positive) field-aligned current along the zero-order magnetic field. The field-aligned current is thus formed like a tube extending along the plasmoid boundary, and it becomes larger as the plasma compression in the plasmoid becomes larger. The fast reconnection development in three dimensions causes a drastic change in the overall current system to generate a redistribution of current.

The formation and dynamical evolution of plasmoids are investigated by two-dimensional resistive MHD computations. It is shown that a magnetotail equilibrium has to include a distant neutral line for a quantitative evaluation of the relevant dynamical properties of plasmoids. Special attention is given to the acceleration mechanism, which exhibits qualitatively a universal pattern but depends quantitatively on the details of the initial field structure and on the type of the microscopic dissipation. In addition to pressure and magnetic forces, mass and momentum transfer from the surrounding plasma to the plasmoid contributes to the acceleration process. In the beginning of the plasmoid formation process, a unique feature of the acceleration is the dominant contribution of momentum transfer and pressure forces. For the cases considered, the pressure forces strongly depend on the chosen initial equilibrium state. Although magnetic forces increase before the plasmoid detaches from the near-Earth X line, the contribution of mass transfer to the plasmoid becomes dominant, which leads to rapid growth and small deceleration of the plasmoid. When the detachment has occurred, mass and momentum transfer is amll, and the relative importance of magnetic forces and pressure forces depends on the chosen initial state and the chosen resistivity. Largermore » Lundquist numbers and/or a current-dependent resistivity similar to the one used in the computations seem to enhance magnetic forces. The qualitative picture of a slingshot effect, i.e., plasmoid acceleration by the tension of open interplanetary magnetic flux, is shown to be misleading for the cases considered in this paper. For magnetotail configurations that are more realistic than those considered so far, the results confirm that spontaneous magnetic reconnection resulting in plasmoid formation and acceleration represents a sufficiently fast process in order to explain magnetotail dynamics during substorms.« less

We investigate the time development of a magnetotail like plasma sheet equilibrium, using the two-dimensional implicit electromagnetic particle code VENUS. As a starting point, we treat the simplified case of equal ion and electron masses. It is shown that a tearing instability develops if the magnetic field component perpendicular to the sheet is small enough and if the ion gyroradius is comparable to the plasma sheet thickness. An earlier estimate of the stability criterion has been confirmed approximately. In contrast to the case of a one-dimensional sheet equilibrium, the instability does not saturate. Instead, the unstable dynamics continues to evolve by formation, acceleration, and ejection of a plasmoid structure. The analysis of the distribution function in the nonlinear stage shows anisotropic heating and the generation of a bulk flow in the tail direction, caused by the development of the plasmoid structure. There is a local enhancement of the drift velocity in the cross-tail direction in the vicinity of the neutral line. Detailed investigation of the particles at the neutral line presents evidence that the current is significantly carried by resonant particles, which is typical for a kinetic tearing instability. We briefly address the case of an ion to electron mass ratio of 10, where the ion tearing instability still develops. Compared to the case of equal masses, electron stabilization leads to smaller growth rate and to an enhanced wavelength, according to theory.

Explicit solutions are presented for quiet, quasi-static magnetotail configurations in the presence of a net cross-tail magnetic field component in addition to field components associated with tail flaring. The solutions are derived using the general three-dimensional equilibrium theory of Birn (1987). It is found that the net By(N) field varies only weakly along field lines and across the tail. The presence of the net By(N) in the neutral sheet implies that a part of the cross-tail current becomes field-aligned, flowing from the southern to the northern hemisphere for By(N) larger than zero and from north to south for By(n) less than zero. The deformations of magnetic flux surfaces suggest that there may be a class of field lines extending from the earth into the magnetosheath region through the low-latitude flanks of the tail. A topological connection is found of the low-latitude region near the magnetopause with the plasma sheet boundary layer, separating closed plasma sheet and open lobe field lines.

A fairly well known and documented fireball phenomenon that is occasionally and accidentally formed aboard certain types of submarines is discussed. The fireball forms when a highly charged battery bank is accidentally connected across a dead generator. The reverse current relay disconnects the circuit, but when the amount of charge left in the batteries creates a sufficiently intense current through the arc, a fireball will float off. (W.D.M.)

Two- and three-dimensional computer models of the dynamics of the magnetosphere and in particular the magnetotail have shown, that the basic features of the idealized linear or steady state reconnection theory are still found in time dependent and spatially more complicated configurations such as the magnetotail, which basically resembles a plane sheet pinch but in addition has small magnetic field components perpendicular to the sheet, field line flaring and variations along both directions parallel to the current sheet. These basic features are the formation of a magnetic neutral x-line or separator, where two surfaces separating magnetic fluxes of different topology intersect, with the generation of an electric field along the separator and the production of strong plasma flows parallel to the current sheet away from the separator in opposite directions. In addition, the computer models of magnetotail dynamics have produced many large scale features that are directly observed or deduced from observation in relation with magnetospheric substorms. Among those features are: the thinning of the plasma sheet, the formation of a plasmoid, a region of closed magnetic loops detached from Earth, which moves tailward at a speed of several hundreds of km/sec, and the generation of field-aligned currents. In viewmore » of the recent discovery of plasmoid signatures in the distant magnetotail at about 200 R/sub E/ from ISEE-3 satellite measurements, we discuss the properties of the plasmoid in the computer simulations, in particular its topology, spatial extent and speed, the current system associated with it and its local appearance at a fixed location in space. Furthermore, we discuss the conversion of the energy flux around the separator, current deviations and the occurrence of field-aligned currents and their generation by shear flows. 10 references, 21 figures, 2 tables.« less

The changes of the magnetic field topology and the field line connections are examined in detail using a simple explicit magnetic field model of a plasmoid in different stages from its formation, penetration through a separatrix connected with a distant neutral line, to its complete disconnection. It is shown that complications arise from the fact that separatrix surfaces become very complicated, folded, and filamented in the presence of a small but finite magnetic field in the reconnection region. The filamentary mixing of topologically different field lines may lead to a mixing of different plasma populations as well, and thereby possibly to a more efficient dissipation.

Magnetotail reconnection is studied under the influence of a dawn to dusk electric field applied at the high-latitude tail boundary using a two-dimensional resistive MHD code. It is found that the basic unstable sequence of tearing and plasmoid formation and ejection was not changed if the external electric field was fairly uniform. The externally forced compression, however, could shorten the onset time and the growth time of the evolution. The faster growth can be understood by the fact that the forced compression gradually alters the lobe field strength and the plasma sheet width and reduces thereby the relevant Alfven time. In fact, if the evolution of the cases with imposed electric field is rescaled on the basis of these characteristic parameters, a good quantitative agreement with the case without external electric field is found.

This paper presents a representative example of an enhancement in energetic ion flux associated with the International Sun-Earth Explorer 3 (ISEE 3) spacecraft's encounter with a traveling compression region (TCR). Data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3 are studied, along with magnetic field data from the vector helium magnetometer. It is concluded that the ion enhancements seen are spatial in nature, thus supporting the idea that TCRs are the lobe signatures of plasmoids moving along the magnetotail, away from earth.

We simulate, on the basis of a two-dimensional incompressible MHD code, the plasma dynamics of a long magnetotail (∼200 RE) under a constant and time-varying driving force delivered from the solar wind. It is found that under a constant driving force the magnetic fields in the magnetotail tend to reconnect impulsively and the formation of X line and plasmoids occurs intermittently and repeatedly every 2–4 hours. Under a time-varying driving force the post-driven phase of a plasmoid formation and the spontaneous reconnection process are also studied. Our results show that magnetic reconnection in the magnetotail during magnetospheric substorms and storms is basically a driven process. A complete sequence of the event, going from the gradual pile-up of magnetic flux to the formation and the antiearthward convection of the new X line and to the ejection of plasmoids from the antiearthward end of the magnetotail, requires the presence of the driving force. The simulation results may also be useful in unifying the different interpretations (1) of the plasmoid formation and plasma sheet thinning and (2) of the relative importance of magnetotail processes with respect to the direct solar wind effects during substorms.

Magnetotail observations from the ISEE 3 distant (1983) tail mission taken during the Coordinated Data Analysis Workshop 8 (CDAW 8) A and G events are investigated. The ISEE 3 magnetic field, plasma, and energetic particle measurements taken in these two plasmoids have been analyzed and compared with various equilibrium structures and propagating waves/tail oscillation modes. Results indicate general agreement with either the closed-loop (Hones, 1977) or very small pitch angle flux rope (Hughes and Sibeck, 1987; Birn et al., 1989) models of plasmoid structure and poorer agreement with other hypotheses. Calculations based upon typical plasmoid and tail parameters are presented, indicating that the J and B force associated with the disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. Overall, the energy expended in accelerating the plasmoids down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. The study produces strong evidence in favor of the plasmoid model of substorm tail dynamics.

A two-dimensional, resistive-MHD computer code is used to investigate the spontaneous reconnection of magnetotaillike configurations. The initial conditions adopted in the simulations are of two types: (1) in which the equatorial normal magnetic field component B(ze) declines monotonically down the tail, and (2) in which B(ze) exhibits a deep minimum in the near-earth plasma sheet. To represent the case where the earthward convection stops before the X line forms, zero-flow boundary conditions are imposed at the edges of the computational box. The initial configurations are in equilibrium and table within ideal MHD. The dynamic evolution of the system starts after the resistivity is turned on. The main results of these simulations basically support the neutral-line model of substorms and confirm Birn's (1980) computer studies. Spontaneous formation of an X-type neutral point and a single O-type plasmoid with strong tailward flow on the tailward side of the X point is found. The time interval from the turning on of the resistivity to the formation of a plasmoid is much shorter in the case where there is an initial deep minimum. A simple analytic calculation is also carried out to demonstrate why the configuration with a deep minimum is more susceptible to the development of the neutral point.

The outflow from a reconnection region in a realistic magnetotail geometry is studied using a two-dimensional time-dependent, compressible, resistive MHD code. Two cases are emphasized: (1) the evolution of near-earth reconnection, which grows in the form of an internal unstable mode after gradual externally forced changes have initiated an anomalous dissipation process, and (2) the evolution of more distant reconnection under influence of a nonuniform inflow that forces reconnection to occur at a given location in the distant tail. In both cases, it is demonstrated that plasma flow is primarily parallel to the magnetic field in regions away from the localized area of reconnection and outside a narrow central layer.

A distinctive magnetic structure in which azimuthal and latitudinal angles of field vectors are closely related to each other has been found in the interplanetary magnetic field data obtained by Sakigake at 0.8-1.0 AU. In this structure, termed a planar magnetic structure (PMS), the magnetic field vectors are nearly parallel to a fixed plane. This plane includes the spiral direction but is inclined to the ecliptic plane from 30{degree} to 85{degree}. The field vectors take almost all directions parallel to this plane. The PMS consists of several segments in which field directions are almost constant, and the segments are separated by tangential discontinuities where directional changes of the field vector occur abruptly without showing any preferred polarization. The ion number density, the ion temperature, and the plasma {beta} tend to be higher in the PMS than in the surrounding plasma. The PMS events are clearly distinct from magnetic clouds both in the field configuration and in the plasma conditions. During the 25-month period from July 1985 to July 1987, eight PMS events with durations of several hours have been identified. The PMS events may be associated with newly emerging magnetic structure in the photosphere from which magnetic tongues are extendedmore » into interplanetary space.« less

From the analysis of ISEE 3 data it is found that a plasmoid is sometimes formed in the middle tail outside the intervals of the substorm expansion phase. This plasmoid is produced by reconnection at the X-type neutral line, which is located earthward of the distant neutral line but beyond the substorm-associated near-tail neutral line, and it is almost stagnant in that the associated flow speed is less than 300 km/s. The blocking effect of the distant neutral line is the most probable reason for the slow movement. The quasi-stagnant plasmoid is observed at x = -60 to - 100 earth radii for a duration of a few tens of minutes, and in about one half of the cases it is followed by the fast tailward streaming. The onset of this streaming tends to coincide with the onset of the substorm expansion phase, and this probably occurs when the reconnection at the middle-tail neutral line comes close to processing the last closed field line. Intensification of the dawn-to-dusk electric field that causes the mantle plasma to reach the plasma sheet boundary closer to the earth is suggested as the reason for the formation of the middle-tail neutral line earthward of the distant neutral line. The effects on the energetic particle flux and relation to the near-tail reconnection are also discussed.