We construct a simple model for the distant (R > 40 R_E) magnetotail using data from the ISEE 3 magnetic field instrument. The data are divided into plasma sheet and lobe categories using the plasma measurements. We present the average behavior of the field in various projections, and show that a relatively simple mathematical model can be used to describe the distant tail magnetic field. The model results are in good accordance with previously published GEOTAIL observations.

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On March 29, 1993, the Geotail spacecraft was 134R_E downstream from the earth and within 12R_E of the axis of the magnetotail whose orientation was calculated from the solar wind direction measured by IMP 8. The IMF had a magnitude near 12 nT and remained steady and roughly 50° northward for 9 hours. Geotail moved back and forth between two regions, an interior region with characteristics similar to the tail and an exterior region with characteristics similar to the magnetosheath. The flow in the dense rapidly flowing exterior region was tilted 8° toward the tail axis rather than parallel to a cylindrical tail. Calculated boundary normals were primarily in the solar magnetospheric Y direction but with a small X component consistent with a slightly flaring or cylindrical tail. The unusual flow direction in the exterior region along with the presence of slower, less dense, tailward flowing plasma on taillike field lines inside the boundary suggest that the exterior flow has a component across the boundary and into the tail. These data support accumulating evidence from MHD simulations that suggest that the magnetotail assumes a very different configuration when the IMF remains very northward for periods of at least several hours. In this unusual configuration, field lines which would normally extend into the distant tail instead close on the flanks and nearer to the earth, creating a distant tail with less open flux and a reduced Y dimension.

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The distances to the magnetic neutral lines in the magnetotail are derived from the observations of the convection in geomagnetically active times (K_p larger than 3). Observations at seven orbit segments of GEOTAIL that are distributed fromx =-36 to-169R_e, and longer than 24 hours are used. For each data set the ratio between the earthward-moving northward magnetic flux and the tailward-moving southward magnetic flux is derived. The ratio varies systematically with distance, and suggests that the distant neutral line tends to be located at about 140 R_e while the near-Earth neutral line tends to be formed inside 50 R_e. Mainly the open field lines are reconnected at the distant neutral line, while mainly the closed field lines are reconnected at the near-Earth neutral line. The northward magnetic flux transported earthward from the distant neutral line is several times larger than the net southward flux transported tailward from the near-Earth neutral line; the open magnetic flux that is reconnected at the distant neutral line is substantially greater than that reconnected at the near-Earth neutral line. Observations of the field-aligned component of the flow velocity suggests that sometimes these two neutral lines exist together and produce the quasi-stagnant plasmoid.

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The nature of the dawn-dusk component of the motion of magnetic field lines in the plasma sheet is studied at the distances of 40 to 160 R_e in active times. In the presence of the B_y component of the magnetic field, even the flow which is purely parallel to the tail axis (which is taken to be the x direction) produces an apparent velocity of field lines in they direction. However, the more substantial cause of the dawn-dusk motion is the rotation of magnetic field lines. In the region of the earthward flow, the magnetic field lines rotate in such a way as to become closer to the plane which contains the earth's dipole moment; that is, the tilting of field lines caused by the presence of B_y becomes smaller as field lines move closer to the Earth. In the region of the tailward flow, the magnetic field lines rotate from northward to southward in a clockwise or counterclockwise sense depending on the sign of B_y as they move tailward. The net convection in the dawn or dusk direction, if it existed, is masked by this rotational motion in the plasma sheet.

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Prior to the observation of fast tailward plasma flows, earthward plasma flows are often observed in the distant magnetotail. By studying the evolution of the ion distribution functions for the transition flow from earthward to tailward flow, we find a multi-layer structure of beam-like plasma flows in the plasma sheet boundary layer, PSBL: 1) earthward, anisotropic beam plasma can be observed in the outer region of PSBL, i.e., the plasma lobe side of the PSBL; and 2) the counterstreaming ions, which consist of earthward and tailward beam-like plasma flows, are observed in the inner region of the PSBL; i.e., the plasma sheet side. Inside the plasma sheet, an isotropic, hot ion plasma can be observed. These characteristics are similar to those measured in the PSBL closer to the Earth, and the tailward anisotropic ion beams observed in the near-Earth magnetotail are thought to be the mirrored counterpart of the earthward ion beam. In the distant magnetotail, however, it is not easy to explain the tailward beam as the mirrored counterpart, because the reflected ion beams at the Earth will be absorbed into the plasma sheet before the reflected ions can travel to the distant magnetotail. We propose another model of two active magnetic reconnection sites. The tailward ion beams come from a near-Earth reconnection site which is embedded in the distant magnetotail reconnection site ejecting the earthward flow.

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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 (<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 R_E on October 15, 1993. On this day K_p was generally quiet, but Geotail observed two prominent bipolar B_z 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 Auroral 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 not; 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.

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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 Ba/B, 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.

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Although the magnetic field intensity in the lobe of the magnetotail is usually in the range of 7-10 nT at distances beyond 150 R_E as observed by ISEE 3 and GEOTAIL, it becomes at times more than 30 nT. The magnetic field of as large as 53 nT was observed in the distant tail lobe by GEOTAIL at -182 R_E on March 9, 1993, when a moderate magnetic storm was in progress. From the scan of daily plots of magnetic field data during the two year period of the GEOTAIL observations, intervals were chosen when the lobe field strength exceeding 20 nT was registered. Large field events are found to mostly occur during a growing stage of the ring current. Events observed beyond 100 R_E downstream from the earth are associated with intermittent and temporal excursions of the spacecraft into the magnetotail. The characteristic time scale of magnetospheric period is approximately 20 minutes. These entries into the magnetotail are usually preceded by a strong southward component of sheath magnetic field. Since magnetic storms are associated with enhanced solar wind flux and IMF conditions, reconfiguration of the magnetotail due to increases of the total pressure in the sheath and directional changes of the sheath plasma flow should control the probability of spacecraft residence in the magnetotail. It is confirmed that the pressure balance approximately holds at the tail magnetopause under enhanced solar wind conditions by using high resolution plasma data. Observations of large lobe field events suggest that additional magnetic fluxes caused by the merging process near the earth during magnetic storms constrain the magnetotail radius from decreasing the nominal value significantly, although the magnetotail is in a compressed state due to the increase in exterior pressure.

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This paper reports plasma and field changes associated with magnetosheath encounters by GEOTAIL near the center of the tail axis at a down tail distance of X = -94 R_E during the main phase of a weak geomagnetic storm on January 25, 1993. The magnetosheath entries of the satellite occurred in conjunction with an enhancement in the total pressure, having a temporal scale between 2 and 18 minutes. It is suggested that temporal and/or local structures in the solar wind such as pressure pulses caused these brief encounters. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail boundary, we estimate the tail magnetic flux and the dimension of the tail for the encounter events. It is shown that solar wind variations, which generate geomagnetic storms, alter significantly the orientation as well as the size of the distant tail, such that the magnetosheath can be observed even near the center of the “average” tail.

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A set of nineteen specific magnetic field structures in distant (≈220 R_e) ISEE 3 magnetotail plasma sheet observations are analyzed. These were initially chosen on the basis of strong central (core) fields compared to nearby lobe field strengths by Slavin et al. (1995); the selection criteria by design did not include field directional changes. This present study starts with the same set of events as Slavin et al. Further discrimination is done on the basis of how well the signatures are fit by a force-free flux rope field model. This model was earlier applied successfully to magnetotail flux ropes where GEOTAIL field data were used. In general the ISEE 3 study yields average properties for the flux ropes that are similar to those of the GEOTAIL study, which had only about half the number of events, and hence, it strongly supports that earlier study. One difference in the results of these studies, however, is the greater longitudinal spread of rope-axes in the present study, and this feature, related flux rope current systems, and other properties are discussed. The results of this study should provide rigid constraints on global models of tail current systems associated with tailward moving flux ropes related to substorms.

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A new method to estimate the spatial structure of the moving magnetohydrodynamic (MHD) system is presented. We integrate the Faraday's induction equation, ∂B/∂t = -rot E, assuming that the system is one-dimensional. The spatial gradient direction, along which the integration is made, is determined by minimizing the residue of the integrated Faraday's equation against the observed magnetic field. This new method can be regarded as an extension of the conventional magnetic minimum variance method. Sample data analyses using this new method for magnetotail events are also presented.

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We report on the downtail distance, x, variation of energetic ions observed on Geotail in the spatial region dominated by the magnetotail plasma sheet antisunward of Earth, 0 < x < 208R_E. The energy-integrated ∼9.4-210 keV/e ion fluxes presented herein were measured by the Supra-Thermal Ion Composition Spectrometer (STICS) of the Energetic Particles and Ion Composition experiment. H⁺ is the dominant energetic ion species observed. When averaged over geomagnetic activity, the overall x variations of the major ion species are qualitatively similar in that the energy-integrated flux levels decrease with increasing x Earthward of ∼70-100R_E. In this x range tailward, sunward, duskward, and dawnward fluxes are of roughly comparable intensity, although tailward fluxes are slightly more intense. Beyond x ∼ 100R_E, tailward fluxes dominate and their levels do not decrease appreciably, if at all, out to ∼208R_E. Fluxes in the other directions generally continue to decrease with increasing x, albeit with a smaller gradient than closer to Earth. An exception to this exists for high-mass, singly-charged atomic and molecular ions predominantly from the ionosphere, which we group together and call I(O⁺¹). Tailward and duskward I(O⁺¹) fluxes are higher relative to sunward and dawnward I(O⁺¹) fluxes than such intercomparisons for other species. The x variations of H⁺ and He⁺² fluxes are very similar throughout the magnetosphere. We find that the solar wind is the dominant source for H⁺ by intercomparing the x gradient of H⁺ flux to those of: 1) the I(O⁺¹) flux; 2) ion flux consisting of high-mass, high charge state ions from the solar wind, which we call SW(O⁺⁶) flux; and 3) He⁺² flux. He⁺² and O⁺⁶ are the second and third most abundant solar wind species next to H⁺, respectively. At these energies and during the decline to solar activity minimum, the solar epoch of these measurements, the I(O⁺¹) flux is less than He⁺² flux but greater than SW(O⁺⁶) flux. The difference between x gradients of the solar wind and ionospheric origin ion flux increases as geomagnetic activity increases, corresponding to the well-known geomagnetic activity related production of O⁺¹ in the ionosphere. We suggest that the overall difference between H⁺ and I(O⁺¹) x gradients may be explained in part by spatial/temporal differences of the two different respective sources: one extended and steady, the solar wind, the other localized and transient, Earth's ionosphere.

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We present a series of tailward-to-Earthward particle anisotropy reversals observed by the GEOTAIL spacecraft in the distant magnetotail plasma sheet at a distance of | x |≈ -90R_E during the 2.5-day period of 11/01/94 1200 UT to 13/01/94 2400 UT. The reversals were detected in the first order anisotropy of energetic protons and the first moment of the low energy plasma distribution function. The X-component of the cross-field flow exhibits the clearest indicator of a particle anisotropy reversal. A comprehensive examination of ground magnetometer and geosynchronous, satellite data reveals that each distant tall anisotropy reversal occurred during the late expansion or recovery phase of a magnetospheric substorm. Out of 14 substorms during which adequate monitoring of the geomagnetic activity and the plasma sheet was possible all but possibly one exhibited a tailward-to-Earthward reversal in the cross field flow ∼1-2 hours after substorm onset. The median time delay between substorm onset and the beginning of the subsequent Earthward convective flow was 94 min. Some uncertainty is associated with this time delay due to the commonly observed exits of the spacecraft to the lobe/mantle. The lower limit on the time delay based on the time of exit of the spacecraft to the lobe/mantle is 61 min. The implications of the particle anisotropy reversals for substorm dynamics are considered.

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Using data from the energetic particle detector on board the GEOTAIL satellite we report on a case study of electron and ion bursts observed at X_GSM =-128R_E. Based on the arrival times of energetic electrons, protons, Helium and Oxygen, as well as the time of flight channels designed specifically for remote sensing purposes, we estimate that the particle acceleration took place at X_GSM =≅ -103R_E. The ensuing tailward particle anisotropy gives a bulk flow estimate that is consistent with the arrival time of the plasma from the expected location of the acceleration region.

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Brief 0.2-2 MeV electron bursts were detected in the plasma sheet, tail lobes and magnetosheath by the Goddard Space Flight Center Medium Energy Cosmic Ray Experiment during the ISEE 3 geotail mission in 1982-3. We summarize the main features of these data and their implications for the structure and dynamics of the geomagnetic tail. Sharp falls in the intensity and occurrence rate of plasma sheet events at ∼90 Re from Earth suggest a change from predominantly closed field lines Earthward of this location to predominantly open field lines at greater distances downtail. The electron anisotropies and associated plasma flows support this proposal which is also consistent with other ISEE 3 observations suggesting that the tail neutral line typically lies at around this distance. Electron bursts in the tail are closely association with substorm activity onsets as indicated by AL intensifications and particle injections at geosynchronous orbit. Signatures of plasmoid ejection down the tail may be observed following electron bursts. In the deep tail, energetic electron enhancements tend to be observed close to the time of substorm onset and are not generally present in the plasma sheet and plasma sheet boundary layer at other times. This suggests that the electrons are accelerated at the substorm neutral line rather than along the current sheet, and are lost rapidly from the tail along open field lines.

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The large scale dynamics of the tail during substorms are examine during multi-satellite measurements made between 18 and 22:00 UT on August 22, 1983. We use data from ISEE1/2, 1977-007, IMP-8, and ISEE-3 located in the solar wind or tail at distances along the Sun-Earth line ranging from +11 to -205R_E. During the period studied, which corresponded to a southward directed IMF, successive plasma injections were detected at 6.6R_E first during a general increase of the magnetic field in the lobe. The magnetic field increase is attributed to an enhanced reconnection rate, as evidenced by the close relationship between the tail lobe magnetic energy density and the available Poynting energy in the solar wind, for near constant solar wind pressure. A Traveling Compression Region (flux rope) was detected in close association with the first energetic particle injection at geostationary orbit. After the first injections, the series of injections continued and the last ones are shown to be linked with a tailward propagating cross-tail current disruption detected onboard IMP-8 at ∼35R_E. This event is seen in the distant tail at 205R_E onboard ISEE-3 as an increase of the tail radius; the far magnetosphere engulfed the ISEE-3 satellite which was located in the solar wind prior to the event. The characteristic time delay between cross-tail current disruption onset occurring in the near-Earth plasma sheet and the enhancement of the tail diameter at 205R_E is on the order of 40 minutes. Several hours before this event, a similar case was detected in which partial cross-tail current disruption in the mid-tail was also related to a far tail expansion. In both cases, the observed signatures are in agreement with the concept of the ejection of plasmoids from the magnetospheric tail with velocities comparable to that of the solar wind. These results strongly suggest that traveling cross-tail current disruption and plasmoid ejection from the far tail are the signature of the same propagating plasma process.

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Spacecraft observations made at low-altitudes in the nightside auroral region and also farther downtail in the plasma sheet and plasma sheet boundary layer typically exhibit distinctive features in the electron and ion fluxes. At low altitudes, spacecraft often detect velocity-dispersed ion beams, auroral arcs, and a region showing a localized absence in the precipitating ion flux (called the gap). Farther downtail, for example at X_GSM ≈ -15 R_E, spacecraft detect features such as an offset between the electron and ion layers of the plasma sheet boundary layer, field-aligned ion beams, and the evolution of the ion beams to isotropic distributions. Using a 3-D model of electron and ion distributions throughout the magnetotail, we have estimated the spatial relationship between these various features at low and high altitudes and the properties of the downtail plasma sheet that correspond to these small-scale structures. Consistent with previous results, we find that the velocity-dispersed ion structure observed at low altitudes corresponds to the field-aligned ion beams observed farther downtail in the plasma sheet boundary layer. Contrary to previous suggestions, we find that the gap, and therefore the co-located auroral arcs, does not map to the near-Earth plasma sheet. Our results indicate that the gap maps to the location in the plasma sheet boundary layer where the transition is occurring between the field-aligned ion beams and the more isotropic distributions characteristic of the central plasma sheet. In the distant tail neutral sheet, this region maps to where the earthward convection velocity is in transition from high values, just inside the distant reconnection site, to slower values somewhat nearer the Earth.

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The GEOTAIL spacecraft has previously observed cold ion beams of apparent ionospheric origin in the Earth's magnetotail lobes at large geocentric distances (r > 50 R_E). It is proposed here that ion beams originating in certain sections of the polar ionosphere may be on magnetic flux tubes that are not closely linked to the magnetosheath and solar wind beyond. Rather, these flux tubes extend far down the tail and have very weak convective electric fields on them. In other parts of the tail cross-section, it is suggested that there are recently reconnected field lines that are open and connected directly out to the solar wind. These are the flux tubes that represent rotational discontinuities at the magnetopause. ISEE-3, in earlier data, clearly observed parts of the lobe that were relatively devoid of plasma and parts that were densely loaded with plasma, probably due to local magnetosheath plasma entry. We suggest that the cold ionospheric ion streams seen by GEOTAIL could drift very slowly toward the plasma sheet in the regions of weak E-fields. Thus, the beams at large geocentric distances should tend to be seen in relatively “pristine” lobe conditions and would be on flux tubes that are connected to ionospheric regions of low convection (E × B) electric fields.

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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 past, 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.

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During the last decade, satellite observations in the nightside magnetosphere have confirmed the presence of energetic (>20 keV) ionospheric-origin ions in the near-Earth magnetotail. Observations also imply that the feeding of the equatorial magnetosphere with ions from the high-latitude ionosphere can be very fast and intense at times. In the present paper we use observations from the Combined Release and Radiation Effects Satellite (CRRES) to investigate the characteristics of ion energy density variations. Multispecies studies of the energy density give important clues on the coupling of the different plasma sources and their participation in the substorm energization. The measurements, coming from the Magnetospheric Ion Composition Spectrometer (MICS), confirm previous results obtained from the AMPTE/CCE-CHEM spectrometer, with regard to the timing of the high-latitude ionosphere response to the equatorial magnetosphere disturbances. The energy density profile of the ionospheric-origin O⁺ follows the intensity enhancements of the westward electrojet very closely in time, exhibiting a continuing increase, contrary to the behaviour of the other major ion species H⁺ and He⁺⁺. This feature, which is consistent with statistical studies of AMPTE/CCE observations, points towards a fast activation of an extraction/acceleration mechanism which feeds the inner plasma sheet with ions of ionospheric origin during the substorm expansion.

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The magnetic field experiment on board ETS-VI (the Engineering Test Satellite-VI) and initial results from the experiment are presented. ETS-VI was launched on August 28, 1994 and has been placed in anear equatorial orbit with a perigee of 2.3Re, an apogee of 7.1Re, an inclination of 13.4°, and an orbital period of 14.4 hours. The spacecraft is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetometer has a sampling interval of 3 s, and except near perigee, it is operated in the mode having a ±256 nT dynamic range and a 0.125 nT resolution. We have examined the magnetometer data obtained at radial distances from 5.0 to 7.1Re, magnetic latitudes from-10° to 25°, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associated field variations are easily seen at larger radial distances and at local times later than 19 MLT. Using 92 substorm events that occurred in the 21-01 MLT sector, we have constructed the average field configuration during the growth phase. The magnetic field becomes highly taillike and its intensity increases at high latitudes (> 10°), while the field intensity decreases at lower latitudes (<10°). The field configuration suggests that the inner edge of the growth phase current system is located near the synchronous altitude at the end of the growth phase.

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Hybrid simulations are used to investigate the formation of a thin current sheet inside the plasma sheet of a magnetotail-like configuration. The initial equilibrium is subjected to a driving electric field qualitatively similar to what would be expected from solar wind driving. As a result, we find the formation of a new current sheet, with a thickness of approximately the ion inertial length. The current density inside the current sheet region is supplied largely by the electrons. Ion acceleration in the cross-tail direction is absent due since the driving electric field fails to penetrate into the equatorial region. The ions in the thin current sheet region exhibit anisotropies which appear to be driven by the bounce motion in the z direction between the inward moving plasma sheet-lobe boundary.

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We have used a three dimensional global magnetohydrodynamic simulation to model the response of the magnetosphere to an interplanetary magnetic field (IMF) with B_z < 0 and B_y ≠ 0 in order to study the origin and evolution of magnetic flux ropes in the magnetotail. The southward IMF leads to dayside magnetic reconnection followed by reconnection on closed plasma sheet field lines. When there initially was no IMF B_y in the plasma sheet, reconnection led to the formation of a plasmoid composed of a quasi-two dimensional closed magnetic loop structure. When IMF B_y reached the equatorial region the quasi-two dimensional plasmoid became a magnetic flux rope. For IMF B_y ≠ 0 initially in the plasma sheet the reconnection immediately led to the formation of a flux rope structure. The pitch of the flux rope field lines is determined by the amount of B_y. For the case with no initial B_y in the plasma sheet the pitch was relatively large while when B_y ≠ 0 initially the flux rope resembled a flux tube lying across the tail. Flux ropes in the magnetotail consist of closed, open and IMF field lines. The open and IMF field lines in the flux rope become attached to the IMF when closed field lines in the flux rope reconnect with IMF field lines at the flank magnetopause. Flux ropes can move tailward before all of the closed field lines have reconnected. This is primarily caused by the tension on IMF field lines which drape over the flux rope when lobe field lines reconnect. However the tailward velocity is less for flux ropes than for plasmoids because of the connection to the Earth. In the simulation closed flux rope field lines are found over 100R_E down the tail.

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In the end of the 1970ies fast ion flows were discovered, jetting along the boundary of the plasma sheet (PSBL) in the Earth's magnetotail. Now, with the help of new technology devices on board of GALILEO and the GEOTAIL spacecraft, more and more exciting details of the three-dimensional beam velocity distributions are being discovered like, e.g., filaments and rings. In order to understand the origin of these structures and in order to use them for remote diagnostics of acceleration processes, we have derived a quasi-adiabatic theory of the non-linear dynamics of strong acceleration in two-dimensional current sheet fields. Together with the typical magnetotail velocity dispersion our approach reveals, in general, cup-like ion beam velocity distributions. In dependence on the accelerating current sheet electro-magnetic fields the cup-like ion beam velocity distributions may degenerate either to the well known lima-bean shape or to ring distributions. We illustrate our findings by test particle simulations of the beam velocity distribution functions and compare them with ring distributions, observed by GEOTAIL.

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Individual flow bursts in a bursty bulk flow event observed by the AMPTE/IRM satellite at R = (-12, -3, 1)R_e, have been modeled by following large numbers of single particle orbits in a model of the geomagnetic tail current sheet containing both B_z and B_y components and a near Earth neutral line. A flow burst modeled in the central plasma sheet, just after a substorm onset, implied there was a near-Earth neutral line 1 to 1/2 R_e tailward of the satellite. An earlier flow burst at the plasma sheet edge, 1-2 minutes before the substorm onset, implied that at this earlier time the neutral line was already formed and closer to the satellite. To match the centroid of the observations, it was necessary that the source population was strongly earthward and duskward flowing, probably originating from the distant current sheet, and that there must have been a relatively large |B_y| component. With such a B_y component, it is interesting that a secondary feature of the observed distribution can also be explained qualitatively. During the time that we see the need for a strong earthward flow from a distant source, ground measurements indicate a significant increase in magnetospheric convection. A model with weak but non-reversing B_z reproduces some of the observed distribution function features, but not all of them, as well as the neutral line model. However, a secondary ridge seen in the observations implies a mixing of neutral line and weak field topologies, with such mixing occurring on a 1 minute time scale.

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Using noninteracting particle simulation in prescribed fields, we present a range of potential particle signatures for several models of magnetic structures in the geomagnetic tail. We consider X-type neutral lines, weak fieldd regions, O-type neutral lines, flux ropes, and some combinations of structures, both symmetric and asymmetric. We concentrate on predictions of the variety of qualitative features one might be able to observe in the presence of these structures. Results include previously known features such as dips in the distribution function at resonant energies, ridges and sharp dropoffs, and field-aligned beams. In addition to these, new features including “cliffs”, “box canyons”, and quasi-trapped regions are seen. This catalog of potential signatures may be useful in interpreting data from the AMPTE spacecraft and from the recently launched Galileo, GEOTAIL and Interball spacecraft.

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Charged particle motion in magnetic field reversals is well understood in two limiting regimes: the adiabatic regime where the guiding center theory and adiabatic invariance are valid, and the current sheet regime where one can separate timescales of the characteristic z-oscillation and gyromotion about the field normal to the reversal plane. Both approximations fail when the magnetic field curvature radius is on the order of the particle gyroradius, i.e. when the ratio of minimum curvature radius to maximum gyroradius, κ², is near 1. Near κ = 1 the motion has been termed “strongly” or “globally” chaotic, and several proposed applications exploit this chaotic behavior. However, it has been shown recently that there is a characteristic “three-branched” behavior in this regime only part of which is related to nonadiabaticity and the phase dependence which leads to chaos. Moreover, a simple physical model attributing the nonadiabatic jumps in magnetic moment to an impulsive centrifugal force, is able to reproduce this behavior. We give further justification to this centrifugal effect and scrutinize the limits of applicability of this centrifugal impulse model. A phenomenological extension to higher κ values is developed, which extends the model's validity in pitch and phase angle as well.

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The drift kink mode in current sheets of the Harris type is driven by the relative cross-field streaming of the electrons and ions. The properties of this mode are investigated by means of electromagnetic particle simulations and a two-fluid stability analysis, first in a 2-D (y, z) geometry and then in a full 3D. For thin current sheets (ρ_i0/L ≤ 1, where ρ_i0 is the ion gyroradius in the lobe field and L is the current sheet half thickness) the fluid theory indicates that the most unstable mode has k_yL ∼ (M_i/m_e)^1/2/2 and that the associated real frequency extends up to several times the ion gyrofrequency Ω_i0. The simulations indicate, however, that finite gyroradius effects limit the dominant growing modes to longer wavelengths k_yL ∼ 1 and to frequencies ∼Ω_i0. In 2D the current sheet kinks due to a bulk displacement of the plasma in z, and this displacement appears to be limited only by the simulation boundary. This transport of plasma across flux tubes reduces the electron compressibility associated with the finite B_z field, and, in 3D, allows the collisionless tearing instability to grow at a rate comparable to that for the simple 1-D neutral sheet. Implications of these results for the triggering of substorms are discussed.

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We present results of two-dimensional M1113-simulations considering onset and evolution of non-linear resistive instabilities in the near-Earth plasma sheet. We use start configurations with and without local flaring which describe the near-earth plasma sheet including the Earth's dipole field explicitly. These equilibria are linearly stable against large scale tearing instability. This allows to study non-linear instabilities which are not found in usual tail simulations including both plasma sheet and lobe. These instabilities andthe corresponding magneticreconnection occur after a phase of slow diffusive evolution. The corresponding plasma flow differs significantly from the usual flow pattern of a large scale tearing instability. In general, we find earthward flow even tailwards of the reconnection X-line. The results show that a near-Earth flaring of field lines can yield a significant non-linear destabilizing effect.

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The progress in the mapping of the auroral regions in the Earth's polar ionosphere to outer magnetosphere reflects our growing understanding of the gross magnetospheric structure. Several “natural tracers” were identified and used by us for the mapping scheme advocated for more than a decade. A “natural tracer” is a plasma boundary identifiable at different altitudes which, from physical reasons, is aligned along the magnetic flux tube (accounting for cross-field convection). The boundaries' locations describe the current state of the magnetosphere. The following tracers to the tail were used in our studies: the low-latitude Soft Electron precipitation Boundary; the large-scale Convection Boundary, or an Alfven Layer; the Plasmapause; the Stable Trapping Boundary for high energy electrons; the precipitating hot ion Isotropy Boundary; the two types of Velocity-Dispersed Ion Structures: VDIS-1 (adjacent to an electron inverted-V structure within the oval), and VDIS-2 (just poleward from the oval). A new “Wall Region” concept related to non-adiabatic (non-MHD) ion dynamics allows to add its effects in the list of “natural tracers”. Another newly discovered structure in the tail is the Low Energy Layer of counter-streaming low-energy (<100 eV) ions and electrons at the outer edge of the Boundary Plasma Sheet. Its physical origin in the far tail, and respective source location, are debatable. Physical limits to the MHD-mapping approach are placed by plasma and field fluctuations and turbulence, by finite Larmor radius effects including non-adiabatic particle dynamics, by finite Alfven propagation times in the magnetosphere, and by various medium- and large-scale disturbances-“auroral activation”.

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We compare meridional distributions of auroral (0.3-20 keV) and energetic (30-300 keV) particle fluxes measured by two low-altitude polar-orbiting spacecraft which crossed the auroral oval before and shortly after the sudden substorm onset. 3-4 min after the substorm onset a very strong particle energization was observed with the particle flux increased by a factor 100-1000 at energies up to >300 keV. It was found in the middle of auroral oval, poleward of the isotropic boundary of energetic electrons detected prior to the substorm onset. Interpreting the variations of Energetic Particle (EP) flux and anisotropy (ratio of precipitated to trapped particle flux) in terms of particle scattering in the equatorial current sheet, we were able to characterize the configuration of equatorial magnetotail. The major acceleration/precipitation region in the center of auroral zone was inferred to be confined in the region of dipolarized magnetic field in the magnetotail. Further poleward (tailward in the magnetotail) the isotropic electron distributions indicated the presence of current sheet with very stretched magnetic field lines. Based on magnetic flux conservation in the flux tube, we estimate the width of transition region between dipolarized region and current sheet to be ∼0.5R_E (about the gyroradius of plasma sheet proton in the equatorial magnetic field ∼5 nT). Such sharp transition may be formed by fast Earthward flow in the current sheet colliding with the dipolarized magnetic shells. An evidence supporting this interpretation is that the explosive westward electrojet was found at latitudes occupied by the current sheet but poleward of the dipolarized region. We also show another event in which the dipolarized region expanded up to the poleward boundary of auroral oval in 7 minutes after the substorm onset.

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We combine two theoretical models of plasma sheet particle transport to trace plasma sheet particles from the nightside magnetospheric equatorial plane to low altitudes. We predict that the low-altitude signature of the plasma sheet is manifestly different at different local times. The Guiding Center Transport Model (GCTM) (Onsager et al., 1993) uses a given particle energy, position, and an assumed magnetic and electric field to trace the low-altitude particle's trajectory to the plasma sheet. The Finite Tail Width Convection Model (FTWCM) (Spence and Kivelson, 1993) uses two Maxwellian plasma sources, one downtail and one on the dawnside boundary layer, to generate distribution functions across the tail plasma sheet. Using Liouville's theorem, the distribution functions at the equatorial level are projected to low altitudes. When this signature of the plasma sheet is projected to low altitudes near the Earth, the spatial variation in the equatorial plasma properties produce a latitudinal variation in the isoflux contours. Isoflux contours at high latitudes at dawn extend to lower latitudes at dusk, reflecting a larger plasma sheet plasma pressure at dusk than at dawn. This prediction is based solely on a projection of spatial variations in equatorial plasma properties.

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We analyze two strong electric field events observed on ISEE-1 during quiet and moderately disturbed conditions, at distances of a few Earth radii over the auroral ionosphere. These fields were observed over time scales of minutes, and thus have time and spatial scales large compared to previously studied strong electric field events. We interpret the data as quasi-stationary potential electric fields, map the fields to the ionospheric level and to the equatorial magnetotail assuming that magnetic field lines are equipotentials, using the Tsyganenko (1989) magnetic field model, and interpret the mapped electric fields as local E × B plasma flows. We show that the electric field data from close distances to the Earth can be interpreted as local image of the distant -v × B electric field in the magnetotail, and discuss the applicability of this method.

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Substorms can occur in series such as during a geomagnetic storm or as isolated events. We have chosen an isolated substorm from the Dynamics Explorer-2 (DE-2) database and use the Expanding/ Contracting Polar Cap Model to study the substorm-related ionospheric electric fields and polar cap dynamics. The isolated event of October 21, 1981 took place during several consecutive DE-2 passes which occur before the onset, just after the onset, during the substorm maximum, and during the recovery phase of the substorm. The polar cap expands during the growth and expansion phases and contracts little throughout the recovery phase. Tail reconnection, however, dominates the recovery phase. A total of 4 × 10⁸ webers of open flux were reconnected during the recovery phase and removed from the polar cap.

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The ionospheric plasma convection in the midnight sector for stably northward interplanetary magnetic field (IMF) has been examined using the Dynamics Explorer 2 fields and plasma data. From the 12-months of noon-midnight passes, 67 passes were selected. With the electric field data from these passes, we have focused on the convection occurring in the “boundary plasma region.” The boundary plasma region is defined as a region between the equatorward boundary of the polar cap and the poleward boundary of a central/inner plasma sheet. The convection pattern in the boundary plasma region is controlled by the sign of IMF B_Y, the magnitude of (B_Y² + B_Z²)^1/2 and the IMF clock angle. For a large IMF (B_Y² +B_Z²)^1/2 and relatively large IMF clock angles (i.e., larger than 30°), the convection tends to show a By-dependent flow pattern which consists of two or three flow regions having an eastward or westward component. As the clock angle becomes smaller, the convection tends to be an irregular pattern which has multiple flow regions. Large potential difference can occur in the boundary plasma region. When the potential is more than 15 kV, the convection, in most cases, is the B_Y-controlled pattern. This means that for this convection pattern a significant amount of energy can be deposited in the midnight sector from the tail even if IMF B_Z is stably positive.

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In order to understand the processes of formation and motion of sun-aligned polar cap arcs as well as patches, we have analyzed simultaneous observations from the Akebono (EXOS-D) satellite and ground based instruments at Qaanaaq. Results demonstrated the correspondence between the localized electron precipitation signatures observed from Akebono and multiple sun-aligned arcs observed using an all sky camera (ASC) at Qaanaaq and have confirmed that sun-aligned arcs correspond to divE < 0. We also confirmed the time delay of appearance and disappearance of sun-aligned arcs in response to IMF change. As for patches, most of the patches drifted anti-sunward, but some ceased to drift within field of view of the all sky camera. Results of comparison between patch movement and the convection demonstrated a general consistency between them.

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The frequent occurrence of energetic O⁺ upflowing ionospheric ions (UFI) at high altitude during auroral substorms raises the question of the cold plasma source and acceleration altitude for the O⁺ ions, and their possible effects on the substorm-time plasma sheet. Ion composition observations on Akebono in the nightside auroral ionosphere reveal the significant presence of thermal-energy (a few eV) O⁺ ions in the 6000-10, 000 km altitude region both during and between auroral substorms. Their upward flux normalized to 2000 km altitude is about 2 × 10⁸ cm^-2s^-1. They are believed to be a significant source of cold plasma for the energetic UFI. During auroral substorms, Akebono occasionally observes molecular (N₂⁺ and NO⁺) upflowing ions up to ∼60 eV, in or near regions of auroral electron precipitation up to 10, 000 km altitude. This suggests the occurrence of a fast ion acceleration process in the F-region or topside ionosphere, where freshly created molecular N₂⁺ and NO⁺ ions are accelerated to several eV or greater within their dissociative recombination lifetime (∼ a few minutes). Ground photometric observations and simultaneous particle measurements on sounding rockets confirm the presence of transversely accelerated ions (TAI) up to ∼200 eV in the topside ionosphere (near 600 km altitude) within tens of seconds of substorm expansion onset. Such hundred-eV TAI are frequently observed on Akebono in latitudinally confined regions of the nightside auroral ionosphere down to ∼2000 km altitude. They can reach the so-called “parallel accceleration region” at ∼-2 R_E altitude within a few minutes, where they are often accelerated further to keV energy and can typically reach the plasma sheet during a substorm. Their flux is an order of magnitude smaller than the thermal ion flux. They are believed to be a minor source of plasma for the energetic UFI. In contrast, the lower-energy (≤10-eV) O⁺ TAI are typically too slow to reach the parallel acceleration region during the substorm, as they are decelerated by gravitation or trapped by it and traverse repeatedly along the magnetic field line. Hence they constitute a possible source of quiet-time thermal ions in the parallel acceleration region. Another possible source of thermal ions in the region is the polar wind O⁺⁺ ions convected anti-sunward along the auroral oval from the dawn and dusk sectors. The thermal O⁺ ions in the parallel acceleration region at ∼1-2 R_E altitude of the nightside auroral ionosphere are believed to be the dominant source of cold plasma for energetic UFI at high altitude; they can reach the plasma sheet during a substorm, thereby modifying its composition significantly.

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