長谷川 洋

We present observations on 19-20 November 2006 by the Cluster spacecraft that were skimming the high-latitude dusk-flank magnetopause, which are consistent with more than one reconnection X-line present on the tailward side of the cusp under northward IMF. Evidence of quasi-continuous reconnection over 16 hours exists in the form of Alfvenic acceleration of magnetosheath ions found almost always when either of the satellites traversed the boundary. The data indicate that a dominant X-line was sunward of Cluster for most of the time, but ion velocity distributions consisting of two magnetosheath populations demonstrate that for part of the time, more than one X-line existed. Further, the motion of reconnected field lines shows that some X-line(s) retreated tailward. It is inferred that following the X-line retreat, another X-line reformed sunward of Cluster, leading tomultiple X-lines.

Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B(z) ( magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At similar to 2313 UT on 13 August 2002, Cluster encountered a bipolar B(z) at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov ( GS) reconstruction, the technique for recovery of two-dimensional ( 2D) magnetohydrostatic structures, suggests that a flux rope with diameter of similar to 2 R(E) was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional ( 3D) MHD simulation, in which a bipolar B(z) can be produced through localized ( 3D) reconnection in the presence of guide field B(y) ( Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B(z) thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some ( e. g., force-free, or magnetohydrostatic) model-based results.

[1] We present first results of a novel technique for producing a two-dimensional (2-D) map of the velocity field from single-spacecraft observations of the bulk plasma parameters and magnetic field. For flow transverse to a unidirectional magnetic field, the MHD equations of motion can be reduced to a Grad-Shafranov- type (GS-type) equation for the stream function (Sonnerup et al., 2006), provided that the plasma structure is 2-D and time-independent when seen in its proper frame. We show how this equation can be used to recover the flow field in regions surrounding a spacecraft path, as was first done in the GS reconstruction of the magnetic field by Sonnerup and Guo ( 1996). The new method is benchmarked by use of an exact solution of the GS-type equation and further by use of synthetic data from 2-D MHD simulations of the Kelvin-Helmholtz instability (KHI). Streamline maps of reasonable accuracy can be generated even when temporal evolution during the KHI development expected at the flank magnetopause is present. Application of the technique to a Geotail encounter with a train of KH waves in the low-latitude flank boundary layer indicates that a chain of vortices ( each of size similar to 3 R(E) by 1 R(E)) existed and moved tailward along the magnetopause.

[ 1] Recent numerical simulations suggest that as soon as the Kelvin-Helmholtz instability (KHI) has grown nonlinearly to form a highly rolled-up vortex, plasma mixing is inevitably achieved within the vortex. Identification of rolled-up vortices by in situ measurements is therefore an important task as a step to establish the mechanism by which solar wind plasmas enter the magnetosphere and to understand conditions under which the vortices form. In the present study we show that the rolled-up vortices are detectable even from single-spacecraft measurements. Numerical simulations of the KHI indicate that in the rolled-up vortex the tailward speed of a fraction of low-density, magnetospheric plasmas exceeds that of the magnetosheath flow. This feature appears only after a vortex is rolled up and thus can be used as a marker of roll-up. This signature was indeed found in the Cluster multispacecraft measurements of the rolled-up vortices at the flank magnetopause. By use of this marker, we have searched for events consistent with the roll-up from Geotail observations showing quasi-periodic plasma and field fluctuations in the flank low-latitude boundary layer (LLBL) under northward interplanetary magnetic field ( IMF), presumably associated with KH waves. The survey shows that such rolled-up events do occur on both dawn and dusk flanks and are not rare for northward IMF conditions. In addition, in all the rolled-up cases, magnetosheath-like ions are detected on the magnetospheric side of the boundary. These findings indicate that the KHI plays a nonnegligible role in the formation of the flank LLBL under northward IMF.

The structure and formation mechanism of a total of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster spacecraft, with separation of similar to 5000 km, are studied by applying the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assumption that the structure is two-dimensional (2-D) and time-independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggesting that multiple X-line reconnection was involved in generating the observed FTEs. The dimension of the flux ropes in the direction normal to the magnetopause ranges from about 2000km to more than I R-E. The orientation of the flux rope axis can be determined through optimization of the GS map, the result being consistent with those from various single-spacecraft methods. Thanks to this, the unambiguous presence of a strong core field is confirmed, providing evidence for component merging. The amount of magnetic flux contained within each flux rope is calculated from the map and, by dividing it by the time interval between the preceding FIE and the one reconstructed, a lower limit of the reconnection electric field during the creation of the flux rope can be estimated; the estimated value ranges from similar to 0.11 to similar to 0.26 mV m(-1), with an average of 0.19 mV m(-1). This can be translated to the reconnection rate of 0.038 to 0.074, with an average of 0.056. Based on the success of the 2-D model in recovering the observed FTEs, the length of the X-lines is estimated to be at least a few R-E.

Using Geotail observations, we investigate characteristics of field-aligned, suprathermal electron fluxes in the transition region between the magnetosheath and the distant (X < -100R(E)) tail lobe in order to clarify their relationship with field line topologies. The behavior of 100-300 eV electrons in the boundary layer is closely correlated with the solar wind strahl electrons, suggesting that the electrons result from the direct entry of the solar wind electrons along reconnected field lines. A remarkable finding is the frequent occurrence of unidirectionally streaming electrons, which implies that field lines threading the boundary layer are of IMF-type, i.e., they are connected at one end to the sun and at the other end to the interplanetary medium beyond the Earth. The unidirectional electron flux usually coexists with double-component ion populations, one part of which streams faster than the magnetosheath flow. This fact again supports the view that the field lines are disconnected from the Earth through magnetic reconnection occurring on lobe open field lines, since the high-speed ion population would be generated tailward of the high-latitude reconnection site. We suggest that the tailward unidirectional electron fluxes are found in the boundary region when the northern (southern) lobe field lines are reconnected with the IMF in toward (away) sectors, while the sunward unidirectional electron fluxes are found when the northern (southern) field lines are reconnected with the IMF in away (toward) sectors. Our statistics show that the tailward and sunward unidirectional electron fluxes are encountered with similar occurrence probabilities. It is inferred that, in each hemisphere, high-latitude reconnection takes place for both IMF sectors, that is, regardless of the polarity of IMF B, (c) 2005 Published by Elsevier Ltd on behalf of COSPAR.

The Grad-Shafranov (GS) reconstruction technique, a single-spacecraft based data analysis method for recovering approximately two-dimensional (2-D) magnetohydrostatic plasma/field structures in space, is improved to become a multi-spacecraft technique that produces a single field map by ingesting data from all four Cluster spacecraft into the calculation. The plasma pressure, required for the technique, is measured in high time resolution by only two of the spacecraft, C1 and C3, but, with the help of spacecraft potential measurements available from all four spacecraft, the pressure can be estimated at the other spacecraft as well via a relationship, established from C1 and C3 data, between the pressure and the electron density deduced from the potentials. Consequently, four independent field maps, one for each spacecraft, can be reconstructed and then merged into a single map. The resulting map appears more accurate than the individual single-spacecraft based ones, in the sense that agreement between magnetic field variations predicted from the map to occur at each of the four spacecraft and those actually measured is significantly better. Such a composite map does not satisfy the GS equation any more, but is optimal under the constraints that the structures are 2-D and time-independent. Based on the reconstruction results, we show that, even on a scale of a few thousand km, the magnetopause surface is usually not planar, but has significant curvature, often with intriguing meso-scale structures embedded in the current layer, and that the thickness of both the current layer and the boundary layer attached to its earthward side can occasionally be larger than 3000 km.

Establishing the mechanisms by which the solar wind enters Earth's magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae(1). It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause(2). But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions(3,4), contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions - in the absence of active reconnection at low latitudes - there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin - Helmholtz instability(5). This can supply plasma sources for various space weather phenomena.

The ion behavior in the low-latitude boundary region is studied based on Geotail data accumulated over several years, toward understanding the formation mechanism of the cold-dense plasma sheet under prolonged northward interplanetary magnetic field (IMF). A statistical survey shows that, during extended northward IMF, (1) dense magnetosheath-like ions appear far more often, especially on the flanks, (2) the dense ions are mostly stagnant, in contrast to those in the classical low-latitude boundary layer (LLBL), (3) a substantial fraction of the dense and stagnant ions is spatially mixed with hot magnetospheric ions, and (4) the mixed ion population has magnetic local time dependence in ion mixing state in energy space. Based on these findings, we argue that, under extended northward IMF, a significant transfer of the solar wind/LLBL ions onto the magnetospheric field lines occurs on the flanks, and the transport/heating process of the entrant ions is different for different local times.

A recently developed technique for reconstructing approximately two-dimensional (partial derivative/partial derivative z approximate to 0), time-stationary magnectic field structures in space is applied to two magnetopause traversals on the dawnside flank by the four Cluster spacecraft, when the spacecraft separation was about 2000 km. The method consists of solving the Grad-Shafranov equation for magnetohydrostatic structures, using plasma and magnetic field data measured along a single spacecraft trajectory as spatial initial values. We assess the usefulness of this single-spacecraft-based technique by comparing the magnetic field maps produced from one spacecraft with the field vectors that other spacecraft actually observed. For an optimally selected invariant (z)-axis, the correlation between the field components predicted from the reconstructed map and the corresponding measured components reaches more than 0.97. This result indicates that the reconstruction technique predicts conditions at the other spacecraft locations quite well. The optimal invariant axis is relatively close to the intermediate variance direction. computed from minimum variance analysis of the measured magnetic field, and is generally well determined with respect to rotations about the maximum variance direction but less well with respect to rotations about the minimum variance direction. In one of the events, field maps recovered individually for two of the spacecraft, which crossed the magnetopause with an interval of a few tens of seconds. show substantial differences in configuration. By comparing these field maps, time evolution of the magnetopause structures. such as the formation of magnetic islands, motion of the structures, and thickening of the magnetopause current layer, is discussed.

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

[1] A total of 1263 lobe magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft at large downtail distances (-100 R-E > X > -210 R-E). On the basis of these crossing events, we compare the observed variations in the plasma and magnetic field parameters with the ones predicted from the MHD framework and study the structure of the tail boundary layer as well as the nature of the solar wind plasma entry. The above crossing events have been classified into three (open, closed, and ambiguous) types based on the jump condition for the MHD discontinuities [Hasegawa et al., 2002]. For the open-type boundaries, the changes in the plasma flow velocity agree well with the Walen relation, i.e., the tangential stress balance relation expected for a rotational discontinuity (RD), at the external interface of the magnetopause transition region. In the lobe/mantle region inside the interface, however, the velocity changes deviate from the Wale'n relation and the plasma and field parameters exhibit a smooth variation from the values characteristic to the magnetosheath to those characteristic to the core lobe. This behavior is in good agreement with the prediction that a slow-mode expansion fan is formed in the tail lobe inside the RD magnetopause. Our comparison suggests that the magnetosheath plasma is decelerated and rarefied mainly across the expansion fan rather than across the RD, as it is transferred to the inner part of the tail lobe.

[1] As many as 1800 magnetopause crossings were identified utilizing the plasma and magnetic field data obtained by the GEOTAIL spacecraft in the distant (-100 R-E > X > -210 R-E) tail region. We classified the magnetopause crossings into three (open, closed, and ambiguous) types based on the jump conditions for the MHD discontinuities and statistically investigated the properties of these boundaries. The most conspicuous finding is that for a considerable percentage (similar to25%) of the lobe-magnetosheath crossings, the magnetotail boundary is locally open for northward as well as southward IMF polarities, indicating that magnetic reconnection frequently occurs regardless of the polarity of the IMF Bz. Our statistics confirm that the magnetic field has a finite normal component for the locally open boundaries but a negligibly small one for the closed boundaries. The field lines threading the open boundary are directed inward (outward) in the Northern (Southern) Hemisphere, supporting the view that such field lines are connected to the Earth. The jump in the plasma flow velocity at the open boundaries agrees well in magnitude and direction with the ones expected for the rotational discontinuity. The flow direction is directed into the magnetotail, which is again consistent with the model of the rotational discontinuity. The spatial distribution of the open boundaries suggests that the locally open portions of the magnetotail boundary migrate toward high latitudes for southward IMF cases and toward low latitudes (toward the plasma sheet) for northward IMF cases. These results indicate that the solar wind plasma is effectively transferred to the lobe/mantle region through the open portion of the boundary whose location is controlled by the direction of the IMF.

Utilizing the GEOTAIL data obtained in the distant (X < -110 Re) magnetotail and the simultaneous solar wind and interplanetary magnetic field data from IMP 8, we investigate how the solar wind plasma and magnetic field affect the location of the distant tail magnetopause. Our analyses are performed in a coordinate system whose x axis is parallel to the incident direction of the upstream solar wind. The distant tail magnetopause is found to respond to the changes in the solar wind static (magnetic plus thermal) pressure rather than the dynamic pressure, in contrast with the past near-tail observations. This result confirms that in the distant region, the flaring of the magnetotail almost ceases and the solar wind dynamic pressure contribution to the pressure balance at the magnetopause is small. For northward interplanetary magnetic field, the power law index of the tail radius dependence on the solar wind static pressure is consistent with the -1/4 power law that is theoretically expected when the total magnetic flux in the tail does not depend on the solar wind pressure. On the other hand, during southward interplanetary magnetic field, the magnetic flux is often much larger than that during northward case. A likely cause of this feature is the pile up of the dayside-eroded magnetic flux. (C) 2000 COSPAR. Published by Elsevier Science Ltd.