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We have carried out a global magnetohydrodynamic (MHD) simulation of a geomagnetic storm initiated by a corotating interaction region followed by a high-speed solar wind (HSS) stream that occurred on 8–9 March 2008. The event began with the arrival of a corotating interaction region (CIR) at ~0720 UT on 8 March. The stream interface arrived at Earth at ~1830 UT on 8 March, and the arrival of a second density enhancement (a second CIR) at ~0140 UT on 9 March resulted in the main phase of the storm, with a peak Dst of −97 nT at 0600 UT on 9 March. Our MHD simulation of the event, spanning the interval of 0400 UT on 8 March to 0800 UT on 9 March, shows that the arrival of the first CIR changes the configuration of the magnetotail, and that after a strong substorm at ~1230 UT on 8 March, the tail evolves into a churning state in which the magnetic topology and flow structure of the magnetotail are never steady. In addition, we find that increases in ring current energy density show a nearly one-to-one correspondence to periods of VxBz > 0 (southward interplanetary magnetic field (IMF)). More importantly, we find that the ring current energy density in the MHD simulation shows a nearly linear response to increases in solar wind dynamic pressure, but only for the northward IMF intervals during the initial phase of the event, from 0400 UT to 1800 UT on 8 March.

On October 8, 2004, the Cluster and Double Star spacecraft crossed the near-Earth (12–19 RE) magnetotail neutral sheet during the recovery phase of a small, isolated substorm. Although they were separated in distance by ∼7 RE and in time by ∼30 min, both Cluster and Double Star observed steady, but highly structured Earthward moving >1000 km/s high speed H+ beams in the PSBL. This paper utilizes a global magnetohydrodynamic (MHD) simulation driven by Wind spacecraft solar wind input to model the large-scale structure of the PSBL and large-scale kinetic (LSK) particle tracing calculations to investigate the similarities and differences in the properties of the observed beams. This study finds that the large-scale shape of the PSBL is determined by the MHD configuration. On smaller scales, the LSK calculations, in good qualitative agreement with both Cluster and Double Star observations, demonstrated that the PSBL is highly structured in both time and space, on time intervals of less than 2 min, and spatial distances of the order of 0.2–0.5 RE. This picture of the PSBL is different from the ordered and structured region previously reported in observations.

We have used a global time-dependent magnetohydrodynamic (MHD) simulation of the magnetosphere and particle tracing calculations to determine the access of solar wind ions to the magnetosphere and the access of ionospheric O + ions to the storm-time near-Earth plasma sheet and ring current during the September 24–25, 1998 magnetic storm. We found that both sources have access to the plasma sheet and ring current throughout the initial phase of the storm. Notably, the dawnside magnetosphere is magnetically open to the solar wind, allowing solar wind H + ions direct access to the near-Earth plasma sheet and ring current. The supply of O + ions from the dayside cusp to the plasma sheet varies because of changes in the solar wind dynamic pressure and in the interplanetary magnetic field (IMF). Most significantly, ionospheric O + from the dayside cusp loses access to the plasma sheet and ring current soon after the southward turning of the IMF, but recovers after the reconfiguration of the magnetosphere following the passage of the magnetic cloud. On average, during the first 3 h after the sudden storm commencement (SSC), the number density of solar wind H + ions is a factor of 2–5 larger than the number density of ionospheric O + ions in the plasma sheet and ring current. However, by 04:00 UT, ∼4 h after the SSC, O + becomes the dominant species in the ring current and carries more energy density than H + ions in both the plasma sheet and ring current.

We have investigated the population of the magnetosphere by ionospheric O + ions from the dayside during the first 8 h of the September 24–25, 1998 magnetic storm by tracing ion trajectories from the ionosphere in time-dependent electric and magnetic fields obtained from a three-dimensional global magnetohydrodynamic (MHD) simulation of the magnetosphere during this storm event. The MHD simulation used WIND data upstream of Earth as input for this storm that began at 2345 UT on September 24, 1998, when a magnetic cloud impacted Earth’s magnetosphere. Ions were launched from both hemispheres on the dayside, in a region extending from 11 to 13 MLT and from 70° to 85° invariant latitude at five minute intervals, beginning 2 h before storm onset and extending to 8 h after the storm commenced. Ions were launched with energies that reflected the effects of ion energization along field lines during this event (e.g. [Cladis, J.B., Collin, H.L., Lennartsson, O.W., Moore, T.E., Peterson, W.K., Russell, C.T., 2000. Observations of centrifugal acceleration during compression of magnetosphere. Geophys. Res. Lett. 27, 915.]), as these effects were not a priori included in the MHD simulation of the event. The ion launch rate was dynamically normalized to observations by using the [Pollock Jr., C.J., Chappell, C.R., Gurnett, D.A., 1990. A survey of upwelling ion event characteristics. J. Geophys. Res. 95, 18–969.] and [Moore, T.E., Peterson, W.K., Russell, C.T., Chandler, M.O., Collier, M.R., Collin, H.L., Craven, P.D., Fitzenreiner, R., Giles, B.L., Pollock, C.J., 1999. Ionospheric mass ejection in response to a CME. Geophys. Res. Lett. 26, 2339.] relationship between the standard deviation of solar wind dynamic pressure and dayside O + outflow. We found that ionospheric O + ions had access to the plasma sheet beyond a radial distance of 10 R E before the storm, but gained access to the near-Earth region and partial ring current soon after the sudden commencement. In addition, significant changes to the magnetospheric configuration caused by the variations in solar wind dynamic pressure, most notably the two pressure peaks at 2345 and ∼0145 UT resulted in a relative absence of O + ions from the magnetotail extending from ∼0140 to ∼0300 UT. After 0300 UT, and for the next hour, the O + density in the plasma sheet increased to >1 cm −3 , and O + was more abundant in the magnetotail compared even to the period immediately following the storm commencement.

We extend our large-scale kinetic (LSK) simulation of the magnetotail by including the global electrostatic effects generated by the field-aligned motion of electrons. Differences in electron and ion dynamics result in significant electrostatic fields near the current sheet (especially near X-lines) and in the auroral zone. In addition, E ƒ and E ⊥ alter the ion precipitation profile and affect particle loss from the system through the flanks and downtail. This work provides a basis for including transverse electron currents in our calculations.

The large-scale kinetic technique has been used in the last decade to address many of the intriguing features of the magnetotail revealed by spacecraft observations of the region. In this paper, we present a brief overview of the results achieved by using this technique and present our most recent effort, a time-dependent, self-consistent model of the magnetotail in which the ion current is used to update the ambient magnetic field. This model indicates that the magnetotail exhibits intrinsic variability in the absence of external stimuli and reproduces many of the observed features of the magnetotail, including periodic ion precipitation profiles. Enhancements of this model promise to reveal more of the intricacies of the magnetotail when applied to studying the branching and percolation of the cross-tail current and to the influence of electron and ion behavior on macroscopic processes before and during substorms.

This study of the magnetotail employs a large-scale kinetic ion model specifically developed to consider the region's approach to equilibrium and its dynamics under the influence of an imposed electric field. Results from the self-consistent, two-dimensional model indicate that the magnetotail achieves equilibrium by maintaining a delicate balance between the influx of plasma from a mantle source and the accelerated loss of these particles resulting from nonadiabatic particle dynamics. The quasi-steady oscillations that result from instantaneous imbalances between sources and losses occur independent of solar wind variations. This variability occurs even for steady solar wind conditions and is caused by the rapid nonadiabatic energization of ions incident on the current sheet and their loss through the flanks of the magnetotail faster than they can be replenished. To determine the influence of external parameters on the dynamical state of the magnetotail, we performed a parameter search in which we varied the magnitude of the convection electric field and the influx of plasma mantle ions. This parameter search showed that the magnetotail configuration can adjust itself to compensate for the various combinations of solar wind parameters. As a rule, this self-adjustment occurs in the form of quasi-steady magnetotail states, where only the average characteristics of the configuration are steady but pronounced cyclic variations occur in the magnetotail topology. For cases in which the magnetotail is underpopulated, the current sheet is depleted via the flanks. In overpopulation cases a flaring of magnetotail field lines caused by the additional current in the neural sheet limits access of additional mantle particles to the region. The magnetotail system is flexible enough to self-consistently regulate its own population. However, the resulting configuration becomes dynamic and only a quasi-steady state is achieved.

This study uses observations from the Wind, Geotail and Interball spacecraft together with global MHD simulation to investigate the onset of a substorm on Dec. 22, 1996. At 1240 UT, during the growth phase, a small localized neutral line formed in the dawn sector at x ∼ −10 RE and initially extended less than 3 RE in azimuth. The formation of this neutral line was associated with the total Poynting flux being focused in the region close to the neutral line. During the growth phase the neutral line expanded in azimuth and moved tailward. At the onset of the expansion phase lobe field lines began to reconnect, and a second small, localized neutral line formed in the dusk magnetotail at ∼1256 UT. Lobe reconnection at this neutral line corresponded to a second intensification of the substorm at ∼1316 UT.

This study employs Geotail plasma observations and numerical modeling to determine sources of the ions observed in the near-Earth magnetotail near midnight during a substorm. The growth phase has the low-latitude boundary layer as its most important source of ions at Geotail, but during the expansion phase the plasma mantle is dominant. The mantle distribution shows evidence of two distinct entry mechanisms: entry through a high-latitude reconnection region resulting in an accelerated component, and entry through open field lines traditionally identified with the mantle source. The two entry mechanisms are separated in time, with the high-latitude reconnection region disappearing prior to substorm onset.

We have developed a technique for including large-scale magnetospheric current systems in magnetic field models. In this paper, we incorporate the Region I and Region II Birkeland current systems into the Source Surface Model (SSM) of the terrestrial magnetosphere. The original SSM was a prescribed-magnetopause model that used a spherical harmonic expansion and a variational principle to obtain the magnetic field in the near-Earth region. Field lines in the tail were constructed geometrically. This model is ideal for studying the effects of Birkeland currents since it has a well-defined boundary between open and closed field lines, magnetopause shielding currents separate the geomagnetic and interplanetary magnetic fields in the absence of a specified connection between the two, and Birkeland currents are not included in the model either implicitly or explicitly. The Region I and II Birkeland currents are added to the model using a series of field-aligned, infinitely thin wire segments. The magnetic field produced by these currents is calculated from the Biot-Savart law and its normal component on the surface of the magnetopause is shielded (minimized) by image currents carried on wires placed outside the magnetosphere. The result of these added currents is a sunward shift of the separatrix between open and closed field lines. Since Region I currents are regarded here as flowing on the separatrix, this shift necessitates an iteration of our procedure. Three iterations were required to reach a satisfactory solution in our case. We find that inclusion of Birkeland currents in the SSM results in a northward Bz in the near-midnight tail region, and a southward Bz at the flanks. The closure of previously open flux near midnight accounts for the sunward shift of the separatrix. Moreover, the sunward shift is found to increase with increasing Birkeland current strength by an amount that agrees with observations of the auroral oval.

This study investigates the sources of the ions up the complex and nonisotropic H(+) velocity distribution functions observed by the Geotail spacecraft on May 23, 1995, in the near-Earth magnetotail region and recently reported by Frank et al. [1996]. A distribution function observed by Geotail at -10 R(sub E) downtail is used as input for the large scale kinetic (LSK) technique to follow the trajectories of approximately 90,000 H(+) ions backward in time. Time-dependent magnetic and electric fields are taken from a global magnetohydrodynamic (MHD) simulation of the magnetosphere and its interactions with appropriate solar wind and IMF conditions. The ion population described by the Geotail distribution function was found to consist of a mixture of particles originating from three distinct sources: the ionosphere, the low latitude boundary layer (LLBL), and the high latitude plasma mantle. Ionospheric particles had direct access along field lines to Geotail, and LLBL ions convected adiabatically to the Geotail location. Plasma mantle ions, on the other hand, exhibited two distinct types of behavior. Most near-Earth mantle ions reached Geotail on adiabatic orbits, while distant mantle ions interacted with the current sheet tailward of Geotail and had mostly nonadiabatic orbits. Ions from the ionosphere, the LLBL, and the near-Earth mantle were directly responsible for the well-separated, low energy structures easily discernible in the observed and modeled distribution functions. Distant mantle ions formed the higher energy portion of the Geotail distribution. Thus, we have been successful in extracting useful information about particle sources, their relative contribution to the measured distribution and the acceleration processes that affected particle transport during this time.

On May 23, 1995, the Comprehensive Plasma Instrumentation (CPI) onboard the Geotail spacecraft observed a complex and structured ion distribution function near the magnetotail midplane at x approximately -10 R(sub E). On the same day, the Wind spacecraft observed a very high density (approximately 40/cubic cm) solar wind and an interplanetary magnetic field (IMF) that was predominantly northward but had several southward turnings. We have inferred the sources of the ions in this distribution function by following approximately 90,000 ion trajectories backward in time using time-dependent electric and magnetic fields obtained from a global MHD (magnetohydrodynamic) simulation. Wind data were used as input for the MHD model. We found that three sources contributed to this distribution: the ionosphere, the plasma mantle which had near-Earth and distant tail components, and the low latitude boundary layer (LLBL). Moreover, distinct structures in the low energy part of the distribution function were found to be associated with individual sources. Structures near 0 deg pitch angle were made up of either ionospheric or plasma mantle ions, while structures near 90 deg pitch angle were dominated by ions from the LLBL source. Particles that underwent nonadiabatic acceleration were numerous in the higher energy part of the ion distribution function, whereas ionospheric and LLBL ions were mostly adiabatic. A large proportion of the near-Earth mantle ions underwent adiabatic acceleration, while most of the distant mantle ions experienced nonadiabatic acceleration.

Experimental results from Geotail and Galileo, as well as results from large-scale kinetic modeling, demonstrate the characteristic structuring of ion distribution functions, especially in the distant and middle parts of the magnetotail. This structuring of the distributions is associated with nonadiabatic acceleration processes in the magnetotail. This paper suggests that the complexity of ion distributions can be quantified by using the analog of the fractal dimension D of the isodensity contours of the ion velocity distributions. It is found that D is usually larger than 1 for various locations within the tail and that as a result of the smearing of the structuring, the value gradually approaches the topological dimension D = 1 as the Earth is approached. The use of the parameter D may be helpful in comparing the degree of granularity of the distribution functions obtained in different regions of the magnetotail, as well as in comparing experimental and theoretical results.

This paper reports the development and performance of a large scale kinetic simulation using a three-dimensional model of the terrestrial electric and magnetic fields in an effort to reach a better understanding of the ionospheric contribution to the near-Earth (x < 10 R E ) region during quiet and slightly disturbed times. The simulation employed the Tsyganenko [1989] magnetic field model and an electric field derived from the Heppner and Maynard [1987] ionospheric potentials. For the conditions considered in this study (southward interplanetary magnetic field (IMF), φ XT = 20 - 40 kV), it was found that the cleft ion fountain plays a relatively minor role in supplying particles to the near-Earth region. The ionospheric contribution to the near-Earth proton population is significant during quiet times with the bulk of the O + ions in the near-Earth region coming from the auroral zone upwelling region. However, the plasma mantle becomes the dominant hot proton source during more active times. Using the nightside auroral zone as a source, we launched distributions of H + , He + , and O + ions and calculated densities, pressures, and other bulk parameters in the near-Earth plasma sheet and partial ring current. Because of the static nature of the model, ionospheric ions had very limited access to the trapped ring current, but the ions formed a reservoir of energetic particles just outside this region that in theory could act as a source for the ring current during more active times. The residence time of ions in the model is too short for charge exchange losses to become significant, and the principal loss mechanism is through the dusk flank of the magnetopause, with precipitation into the ionosphere playing a minor role.

The structure of a plasma sheet ion distribution is modeled by using the large-scale kinetic approach in which the trajectories of a distribution of plasma mantle particles are followed through the magnetotail. In the plasma sheet boundary layer, beamlets that originate from localized regions in the current sheet maintain their identities even after multiple interactions with the current sheet. Because of their origin, beamlet distribution functions appear to be phase bunched and azimuthally asymmetric, even far from the current sheet. Structures in the distribution of ions manifest themselves in both configuration space and velocity space. At the outer edge of the plasma sheet the particles experiencing the greatest acceleration in the distant magnetotail form a structure with multiple peaks in velocity space. When earthward and tailward beamlets are found in the same region, there is an increase in density and a decrease in bulk velocity. At the outer edge of the plasma sheet and thus near the boundary layer, counterstreaming distributions are observed. In the plasma sheet, two populations are found: beamlets with large drift velocity and a chaotic counterpart which has been thermalized by multiple crossings of the current sheet. In the vicinity of the earthward edge of the plasma sheet, the two populations merge to become an isotropic distribution function. A detailed comparison of these results with observations from the plasma instrument (PLS) on the Galileo spacecraft during the Earth 1 flyby is presented. The principal features of the observed and computed distributions are in substantial qualitative agreement.

[1] The contribution of solar wind and ionospheric ions to the ion population of the magnetotail during the 17 April 2002 geomagnetic storm was investigated by using large-scale kinetic (LSK) particle tracing calculations. We began our investigation by carrying out a global magnetohydrodynamic simulation of the storm event by using upstream solar wind and interplanetary magnetic field data from the ACE spacecraft. We launched solar wind H+ ions and ionospheric O+ ions beginning at 0900 UT, ∼2 h prior to the sudden storm commencement (SSC), until 2000 UT. We found that during this Dst ∼ −98 nT storm, solar wind ions carried the bulk of the density and energy density in the nightside ring current and plasma sheet, with the notable exception of the 90 min immediately after the SSC when O+ densities in the ring current exceeded those of H+ ions. The LSK simulation did a very good job of reproducing ion densities observed by the Los Alamos National Laboratory spacecraft at geosynchronous orbit and reproduced the changes in the inner magnetosphere and the injection of ions observed by the IMAGE spacecraft during a substorm that occurred at 1900 UT. These comparisons with observations serve to validate our results throughout the magnetotail and allow us to obtain time-dependent maps of H+ and O+ density and energy density where IMAGE cannot make measurements. In essence, this work extends the viewing window of the IMAGE spacecraft far downtail.

We investigated the injection of ions into the inner magnetosphere during the sudden storm commencement (SSC) of the 28 October 2001 geomagnetic storm by carrying out a large‐scale kinetic particle tracing study of the event. We launched H+ ions from the solar wind and O+ ions from the ionosphere in global time‐dependent electric and magnetic fields obtained from a global magnetohydrodynamic (MHD) simulation of the storm, and compared our results to those from IMAGE/HENA and the LANL geosynchronous spacecraft 1995‐095. We show that the SSC resulted in a dramatic increase in the density and energy density of both ion species. In addition, our results show an injection of solar wind ions from the dawn flank that is consistent with observations and with previously published results.

We have compared the AUREOL 3 (A3) observations of auroral ion precipitation, particularly ion beams, with the results from the global kinetic model of magnetotail plasma of Ashour-Abdalla et al. (1993). We have identified 101 energetic keV H(+) velocity dispersed precipitating ion structures (VDIS) with fluxes above 10(exp -3) ergs./sq cm./s in the A3 record between the end of 1981 and mid-1984. These beams display a systematic increase in energy with increasing latitude and were observed in a narrow region within less than 1 deg in latitude of the polar cap boundary. The VDIS are the most distinctive feature in the auroral zone of the plasma sheet boundary layer. We report first on a statistical analysis of the possible ralationships between magnetic activity or substorm phase and the VDIS properties. Our particle simulations of the precipitating ions have been extended by using a series of modified versions of the Tsyganenko (1989) magnetic field model and by varying the cross-magnetosphere electric field. In the simulations, plasma from a mantle source is subject to strong nonlinear acceleration, forming beams which flow along the PSBL. Only 3 to 4% of these beams precipitate into the ionosphere to form the VDIS while the majority return to the equatorial plane after mirroring and form the thermalized central plasma sheet. The final energy and the dispersion of the beams in the model depend on the amplitude of the cross-tail electric field. Two unsual observations of low-energy (less than 5 keV) O(+) VDIS, shifted by 4 deg 5 deg in invariant latitude equatorward of H(+) VDIS are analyzed in detail. The sparsity of such O(+) events and the absence of the changes in the flux and frequency of occurrence indicate a solar wind origin for the plasma. Finally, large-scale kinetic modeling, even with its simplifications and assumptions (e.g., static magnetic field, solar wind source), reproduces low-altitude auroral ion features fairly well; it may therefore be presented as an appropriate framework into which data on energization and transport of the hot plasma, obtained in the equatorial plane, could be inserted in the near future.

Results from modeling ion distribution functions in a two-dimensional reduction of the Tsyganenko [1989] magnetic field model have enabled us to calculate the full ion pressure tensor inside the model magnetotail. A thin current sheet is formed in the distant tail and the pressure tensor within this sheet has significant off-diagonal terms. These terms resulting from quasiadiabatic ion trajectories create azimuthally asymmetric distribution functions which are capable of maintaining stress-balance. Outside the current sheet the off-diagonal terms disappear and moderate anisotropy builds up with P⟂/P∥ ∼ 0.8. Closer to the Earth rapid isotropization of the distribution occurs.

[1] We investigated a highly structured plasma sheet populated by counterstreaming beams observed by the Cluster and Double Star spacecraft on October 10, 2005, during an extended interval of northward interplanetary magnetic field (IMF). We used Wind spacecraft observations as input into our global magnetohydrodynamic (MHD) simulation of the event, and launched ions from the low-latitude boundary layer, determined to be the source of the observed ions, in our large-scale kinetic (LSK) particle tracing calculation that used the time-dependent global fields from the MHD simulation. We found that a highly structured plasma sheet was formed in which the dawn and dusk halves were temporarily decoupled because of the presence of flux ropes in the center of the tail, possibly the result of localized near-Earth reconnection. Our LSK simulation results showed good agreement with observed spectrograms and velocity distribution functions. On the dawn side, the ions launched in our simulation first formed a plasma sheet boundary layer (PSBL). Subsequent interactions of these ions with the current sheet scattered them and formed the central plasma sheet (CPS). On the dusk side, however, the topology of the magnetotail, including the formation of highly stretched field lines in the dusk flank and of a flux rope on open field lines in the center of the tail resulted in a structured plasma sheet, with counterstreaming beams present throughout the plasma sheet, including its center portion. This is very different from the classical picture of the PSBL in which layered counterstreaming beams exist only at the outer edges of the plasma sheet and do not penetrate the CPS.

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly nonadiabatic ion acceleration (known as the wall region) exists in the near earth tail and demarcates two very different regimes of ion motion: adiabatic and quasi-adiabatic. A strong enhancement of the cross-tail current occurs on the tailward side of the wall. A comparison of numerical and adiabatic pressure profiles indicates that nonadiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasi-steady magnetospheric convection.

Mass-resolved ion, electron, and plasma wave data obtained from several low-latitude boundary layer (LLBL) crossings by the AMPTE CCE satellite are analyzed. The data clearly separate the LLBL from the adjacent magnetosheath and magnetosphere. Attention was focused on wave-particle interactions involving electrons. Electron beams were found to be present in the LLBL during the southward interplanetary magnetic field, along with a simultaneous enhancement of electrostatic waves with parallel polarization. Linear theory analysis shows that for plasma conditions in the LLBL, electron beams are unstable to electrostatic waves that propagate parallel to the local magnetic field, in agreement with observations. A numerical simulation study of the beam-plasma interaction in the LLBL shows that the instability saturates by thermalization of the beam but that a beamlike structure can still remain in the electron distribution for certain initial parameters. It is suggested that peaks in the electron velocity distribution function may be found in the LLBL away from the beam source region.

[1] Energetic ions with hundreds of keV energy are frequently observed in the near-Earth tail during magnetospheric substorms. We examined the sources and acceleration of ions during a magnetospheric substorm on 1 March 2008 by using Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Cluster observations and numerical simulations. Four of the THEMIS spacecraft were aligned at yGSM = 6 RE during a very large substorm (AE = 1200) while the Cluster spacecraft were located about 5 RE above the auroral ionosphere. For 2 h before the substorm, Cluster observed ionospheric oxygen flowing out into the magnetosphere. After substorm onset the THEMIS P3 and P4 spacecraft located in the near-Earth tail (xGSM = −9 RE and −8 RE, respectively) observed large fluxes of energetic ions up to 500 keV. We used calculations of millions of ions of solar wind and ionospheric origin in the time-dependent electric and magnetic fields from a global magnetohydrodynamic simulation of this event to study the source of these ions and their acceleration. The simulation did a good job of reproducing the particle observations. Both solar wind protons and ionospheric oxygen were accelerated by nonadiabatic motion across large (>∼5 mV/m) total electric fields (both potential and induced). The acceleration occurred in the “wall” region of the near-Earth tail where nonadiabatic motion dominates over convection and the particles move rapidly across the tail. The acceleration occurred mostly in regions with large electric fields and nonadiabatic motion. There was relatively little acceleration in regions with large electric fields and adiabatic motion or small electric fields and nonadiabatic motion. Prior to substorm onset, ionospheric ions were a significant contributor to the cross-tail current, but after onset, solar wind ions become more dominant.

[1] The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near-Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near-Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.

[1] The Cluster mission offers a unique opportunity to investigate the origin of the energy-dispersed ion structures frequently observed at 4.5–5 RE altitude in the auroral region. We present a detailed study of the 14 February 2001 northern pass, characterized by the successive observation by three spacecraft of a series of energy-dispersed structures at ∼72–75° ILAT in a region of poleward convection. Equatorward, the satellites also observed a localized, steady, and intense source of outflowing energetic (3–10 keV) H+ and O+ ions. These substructures were modeled by launching millions of H+ ions from this ionospheric source and following them through time-dependent electric and magnetic fields obtained from a global MHD simulation of this event. Despite the complexity of ion orbits, the simulations showed that a large number of ions returned to the Cluster location, poleward of their source, in a number of adjacent or overlapping energy-latitude substructures with the correct dispersion. The first dispersed echo was unexpectedly generated by “half-bouncing” ions that interacted with the current sheet to return to the same hemisphere. The time-shifted observations made by two Cluster (SC1 and SC3) spacecrafts were correctly reproduced. Almost all the ions returning to the spacecraft underwent a ∼2–5 keV nonadiabatic acceleration at each interaction with the current sheet in a very confined resonant region. This acceleration explains the overall energy increase from one structure to the next. This event confirms the importance of the ionospheric source in populating bouncing ion clusters within the magnetosphere, even at high latitudes.

[1] We have examined the validity of assuming that the solar wind observed at the L1 Lagrange point is a good predictor of the environment at the Earth by using solar wind observations during a magnetic storm from four spacecraft (ACE, Wind, IMP-8, and Geotail). The storm on 23 and 24 May 2000 occurred when a magnetic cloud reached the Earth. We carried out a running cross-correlation analysis between the solar wind parameters observed on pairs of the solar wind spacecraft and found good agreement early in the storm. The peak cross-correlations between magnetic field components exceeded 90% throughout this interval, while the peak cross-correlations between the dynamic pressure observations was about 70% or better. The main differences were at higher frequencies. However, later after the main cloud had passed the Earth the peak cross-correlations decreased, especially between ACE at L1 and spacecraft nearer the Earth. Late in the storm the monitors were not observing the same solar wind. We examined the effect of these differences by using our global magnetohydrodynamic simulation to model the magnetosphere using observations from three monitors. Early in the event we found very good agreement between simulated magnetospheres based on input from different solar wind monitors but later the agreement became very bad. If we assume that the monitors near the Earth provide the best measurement of the solar wind that interacts with the magnetosphere, then later in the magnetic storm the solar wind observations at ACE were not a good indication of the solar wind reaching the Earth.

[1] We investigated the storm-time entry of solar wind ions into the magnetosphere by carrying out a large-scale kinetic particle tracing calculation for the 24–25 September 1998 and the 17 April 2002 geomagnetic storms. For each storm, we simulated the magnetosphere by using a global magnetohydrodynamic (MHD) code that was driven by solar wind and interplanetary magnetic field (IMF) data from spacecraft upstream of Earth. We then launched ions in the solar wind in the global time-dependent fields obtained from the MHD simulation, beginning prior to storm onset and extending into the main phase. We collected those ions that successfully reached the nightside plasma sheet and ring current, and then determined the mechanisms of entry and transport responsible for the injection of these ions into the magnetotail. We found that, in agreement with quiet time entry results, the IMF By component strongly influenced whether ions entered through the dawn or dusk flanks, and changes in IMF By were immediately reflected on the ion entry pattern. Also, in addition to the usual dayside merging source, ion entry into the magnetosphere was facilitated by dynamic pressure enhancements, during which entry occurred over a wide swath of latitudes and extended from the dayside to locations far downtail. More significantly, because of the warmer storm-time ion population in the magnetosheath, ions were more readily affected by inhomogeneities and rapid changes in the IMF, which caused solar wind ions to gradient drift onto open field lines and reach the inner magnetosphere.

[1] We investigate the nonlinear influence of the cross-tail currents carried by beamlets (substructures of PSBL ion beams) on the topology of the magnetic field, and, correspondingly, on the dispersion properties of these substructures self-consistently generated in this field. We found that some of the peculiarities of beamlet shapes found recently in CLUSTER data could be explained by taking into account the nonlinearity of the system. This model explains the steepening of local beamlets dispersion in comparison with the global dispersion of the enveloping VDIS structure. At the same time we found that velocity filter effects operating during beamlets propagation toward the Earth prevent the sign's reversal of this local dispersion.

[1] This paper presents a detailed study of the Feb. 14, 2001 Cluster northern auroral pass at mid-altitudes (4–5 RE), characterized by observations of a series of energy-dispersed ion structures in a region of poleward convection. In contradiction with one current view, that ions populating these energy-dispersed signatures originate sporadically in the magnetotail, Cluster directly observed energetic (0.2–15 keV), field-aligned H+ ions of ionospheric origin. The ions were ejected at the top of a steady auroral acceleration region near 72.5° ILAT, then bounced on closed field-lines, and were finally dispersed poleward in latitude by the E × B drift effect. Simple but realistic latitudinal drift computations demonstrate that the anticipated location of successive bouncing echoes coincides rather well with the Cluster observations. Best agreement is reached when the particles are further accelerated (presumably nonadiabatically) by 1–2 keV, as they periodically cross the tail neutral sheet.

[1] On 14 February 2001 the Cluster Ion Spectrometry (CIS) experiment onboard three of the Cluster spacecraft observed velocity-dispersed ion structures (VDIS) as the spacecraft passed from the tail lobes into the plasma sheet boundary layer. These are the first multiple spacecraft observations of the VDIS phenomenon. The Cluster 1 spacecraft (SC1) observed a dispersed ion signature with beamlets and a second structure like that expected to be produced by an echo, while Cluster 3 (SC3) observed much less pronounced structuring a few minutes later. During this same event and over an extended interval the ACE spacecraft observed an interplanetary magnetic field that was directed southward. We have inferred the sources and acceleration mechanisms of the ions in these VDIS observations by following millions of ion trajectories backward and forward in time through time-dependent electric and magnetic fields obtained from a global MHD simulation. ACE data were used as input for the MHD model. We found that almost all of the particles comprising the first (A1) and second (A2) beamlets observed by SC1 had been nonadiabatic earlier in their history, while particles in the A3 beamlet exhibited a combination of adiabatic and nonadiabatic behavior. Beamlet A4 particles were always adiabatic. Moreover, for all of the beamlets the current sheet crossing that took place prior to their detection occurred between x = −13 RE and x = −16 RE in the tail, well earthward of the permanent stochastic “sea” from which all of the beamlets originated. Our model does not favor the multiple source scenario suggested by A. Keiling et al. Instead, it indicates that the source regions of the structures are spatially correlated. We have carried out a similar analysis of the SC3 observations. In general, SC3 beamlets have higher κ values, partly because of the depolarization of the field lines during these observations. In time forward calculations only a small fraction of ions from SC1 A structures returned to the spacecraft location. “Echoes” were more pronounced on SC3. In addition, in our calculations, some particles from SC1 A structures interacted with the current sheet and returned to the SC3 location, at the time when SC3 observed the A structures. When Cluster observations were organized by latitude instead of time, we found that all three Cluster spacecraft seemed to observe the same primary structure that persisted throughout the interval of observation.

[1] This study investigated the entry of solar wind ions into the magnetosphere by following particle orbits from the solar wind through the magnetopause boundary in time-dependent electric and magnetic fields obtained from a 3D global magnetohydrodynamic (MHD) simulation of the magnetosphere; five different orientations of the interplanetary magnetic field (IMF) were considered. We found the region of the upstream solar wind that mapped to the magnetopause entry regions to be parallel to the y-z orientation of the IMF. However, significant asymmetries were caused by the history of the IMF, the orientation of the interplanetary electric field (IEF), the nonadiabatic acceleration of ions at the magnetopause reconnection region, and acceleration by the magnetopause electric fields. Ions generally entered the magnetosphere through extended dayside regions corresponding to the reconnection region for each IMF and were accelerated by that region's electric fields. Entry also occurred along high-latitude open field lines convecting into the magnetotail.

We have investigated the entry of solar wind ions into the magnetosphere by tracing particle orbits in time-dependent electric and magnetic fields obtained from a three-dimensional global MHD simulation of the magnetosphere. The MHD simulation was developed and carried out for the Geospace Environment Modeling (GEM) program General Geospace Circulation Model (GGCM) campaign and included several different orientations of the interplanetary magnetic field (IMF). Ions were launched in the solar wind at four different time intervals and collected on the outer surface of the magnetopause. We found that all IMF orientations examined shared subsolar entry. Additionally, all of the cases showed an entry region that was parallel to they - z orientation of the IMF. Ions entering in the subsolar point had converted their streaming energy to thermal energy. Ions entering through the high-latitude reconnection regions showed evidence of acceleration in this region.

We have used global magnetohydrodynamic, simulations of the interaction between the solar wind and magnetosphere together with single particle trajectory calculations to investigate the sources of plasma entering the magnetosphere. In all of our calculations solar wind plasma primarily enters the magnetosphere when the field line on which it is convecting reconnects. When the interplanetary magnetic field has a northward component the reconnection is in the polar cusp region. In the simulations plasma in the low latitude boundary layer (LLBL) can be on either open or closed field lines. Open field lines occur when the high latitude reconnection occurs in only one cusp. In the MHD calculations the ionosphere does not contribute significantly to the LLBL for northward IMF. The particle trajectory calculations show that ions preferentially enter in the cusp region where they can be accelerated by non-adiabatic motion across the high latitude electric field. For southward IMF in the MHD simulations the plasma in the middle and inner magnetosphere comes from the inner (ionospheric) boundary of the simulation. Solar wind plasma on open field lines is confined to high latitudes and exits the tailward boundary of the simulation without reaching the plasma sheet. The LLBL is populated by both ionospheric and solar wind plasma. When the particle trajectories are included solar wind ions can enter the middle magnetosphere. We have used both the MHD simulations and the particle calculations to estimate source rates for the magnetosphere which are consistent with those inferred from observations.

This study investigates the evolution of the magnetotail's magnetic field with the aid of a self-consistent two-dimensional model in which the ion current periodically updates the magnetic field. The plasma mantle supplies particles continuously to the magnetotail, and the perturbation magnetic field is calculated from the ion current using the Biot-Savart law. The simulated magnetotail evolves into a quasi-steady state, characterized by the periodic motion of the near-Earth X-line in the model. This variability is caused by the nonadiabatic acceleration of ions in the current sheet and their rapid loss from the tail. Particularly noteworthy is the value found for the characteristic time scale of variability in the magnetotail. on the order of 4 - 5 minutes.

We investigate the origins and the transport of ions observed in the near-Earth plasma sheet during the growth and expansion phases of a magnetospheric substorm that occurred on November 24, 1996. Ions observed at Geotail were traced backward in time in time-dependent magnetic and electric fields to determine their origins and the acceleration mechanisms responsible for their energization. Results from this investigation indicate that, during the growth phase of the substorm, most of the ions reaching Geotail had origins in the low latitude boundary layer (LLBL) and had alread@, entered the magnetosphere when the growth phase began. Late in the growth phase and in the expansion phase a higher proportion of the ions reaching Geotail had their origin in the plasma mantle. Indeed, during the expansion phase more than 90% of the ions seen by Geotail were from the mantle. The ions were accelerated enroute to the spacecraft; however, most of the ions' energy gain was achieved by non-adiabatic acceleration while crossing the equatorial current sheet just prior to their detection by Geotail. In general, the plasma mantle from both southern and northern hemispheres supplied non-adiabatic ions to Geotail, whereas the LLBL supplied mostly adiabatic ions to the distributions measured by the spacecraft.

This study investigates the evolution of the magnetotail’s magnetic field with the aid of a self-consistent two-dimensional model. In this model the plasma mantle continuously supplies particles to the magnetotail, the ion current periodically updates the magnetic field using the Biot-Savart law. The simulated magnetotail evolves into a quasi-steady state, characterized by the periodic motion of the model’s near-Earth X-line. This variability results from the nonadiabatic acceleration of ions in the current sheet and their rapid loss from the tail. The characteristic time scale of variability in the magnetotail is on the order of 4–5 minutes. We also investigate how the magnetotail’s topology responds to increased convection electric fields, and show examples of observations of variability in the magnetotail.

We calculate polar cap boundaries and polar cap equipotentials obtained by using two modifications of the source surface model (SSM) for three intervals specified by the Geospace Environment Modeling (GEM) grand challenge and compare with results from the assimilative mapping of ionospheric electrodynamics (AMIE) code. In the first modification Birkeland currents are self-consistently added to the SSM. In the second modification the magnetotail radius is increased, and the lobe magnetic field is modified to obtain a more realistic magnetotail. Both modifications of the SSM, although simple, are found to reproduce many of the features seen in the AMIE results. These include a duskward (dawnward) shift of the separatrix in the northern (southern) hemisphere because of negative IMF By effects, and a good agreement for southern hemisphere equipotentials and for the locations or the nightside separatrix between open and closed field lines. However, both versions of our model give northern hemisphere separatrices that are more nearly circular than those observed and show IMF By effects not present in the AMIE results; they also displace the dayside separatrix poleward of the positions indicated by observations.

[1] A global simulation of a December 22, 1996 substorm revealed a previously unknown state of magnetospheric convection. Earthward flow from a tail neutral line reversed direction in the inner magnetosphere and formed a large-scale vortical nightside convection pattern. The dawnside vortex formed a flux rope just prior to the substorm onset, but this was not the cause of the substorm breakup. The vortex structure of the dusk-side flow created strong field-aligned currents directed away from the Earth that connected to the region of maximum auroral luminosity in the ionosphere. These currents are probably related to the westward traveling surge. The flow reversal and formation of the tail vortices may be a result of the simulation's earthward convection being limited by the high near-Earth plasma pressure and/or by line-tying caused by the high ionospheric conductance.

The trajectories of a large ensemble of particles are calculated in a modified Tsyganenko magnetic field model with a uniform cross-tail electric field. The model magnetotail can be divided into several distinct dynamical regimes of ion motion. Near Earth, where the field lines are dipolar the adiabatic formalism is adequate. In the mid-tail and distant tail, guiding-center theory breaks down and must be replaced by a quasi-adiabatic formalism. There is an important transition region between the adiabatic and quasi-adiabatic regions where ion trajectories become more complicated and no simple analytical description holds. This wall region is characterized by rapid ion acceleration and a major loss of particles to the dusk flank. The moments of the ion distribution function are constructued from the ion trajectories, including density, temperature, and pressure in the x-z and x-y planes. In the noon-midnight meridian plane, parameters are relatively constant except near the Earth, while the x-y plots show strong gradients across the magnetotail. Magnetotail plasma convects earthward, drifts toward dusk, and is squeezed out of the tail in the near-Earth region. A thin current sheet forms in the quasi-adiabatic region, and the pressure tensor has significant off-diagonal terms at its edges. These terms are the result of quasi-adiabatic ion trajectories which lead to azimuthally asymmetric distribution functions capable of maintaining approximate stress balance across the current sheet. Simplified analytical descriptions provide further physical insight into ion dynamics that are observed.

International audience; Knowledge of the ion composition in the near-Earth’s magnetosphere and plasma sheet is essential for the understanding of magnetospheric processes and instabilities. The presence of heavy ions of ionospheric origin in the magnetosphere, in particular oxygen (O ), influences the plasma sheet bulk properties, current sheet (CS) thickness and its structure. It affects reconnection rates and the formation of Kelvin-Helmholtz instabilities. This has profound consequences for the global magnetospheric dynamics, including geomagnetic storms and substorm-like events. The formation and demise of the ring current and the radiation belts are also dependent on the presence of heavy ions. In this review we cover recent advances in observations and models of the circulation of heavy ions in the magnetosphere, considering sources, transport, acceleration, bulk properties, and the influence on the magnetospheric dynamics. We identify important open questions and promising avenues for future research.