Maha Ashour‐Abdalla

Understanding particle acceleration in the magnetotail during substorms requires knowledge of changes in the global magnetospheric configuration and the local regions of intense fields and microinstabilities caused by processes associated with reconnection. We simulated a substorm on 15 February 2008 by coupling the University of California, Los Angeles global magnetohydrodynamic simulation code and a two‐dimensional version of the iPIC3D implicit particle‐in‐cell code. The MHD code provides realistic initial and boundary conditions for the particle‐in‐cell (PIC) code, while the PIC code models the reconnection and evolution of the dipolarization front (DF) self‐consistently with full kinetic physics. In the PIC simulation, after a few seconds, an active X point forms and DF‐like structures form about every 2 s and propagate earthward. In the near‐Earth tail, the earthward moving fronts combine to form thicker structures. The presence of the macroscopic‐scale magnetic field, featuring a significant dipolar component nearer the Earth, affects the reconnection process, chocking the flow that cannot freely propagate earthward, causing the production of multiple repeating DFs. In the region away from the equator and equatorward of the separatrices, the DFs become associated with a series of spatial stripes that are visible in the electron temperature and the magnetic and electric fields and are also caused by the unsteady nature of the process of reconnection. The electrons are preferentially accelerated in the dipolarization region in the extension of the dipolarization field lines to high latitudes and reach energies of 100 keV or more. A streaming instability may be responsible for the parallel acceleration. This acceleration of the plasma associated with the DFs is greater than that occurring near the X point for this substorm.

In this study, we examine the causes of electron precipitation during a substorm on 15 February 2008 that lead to auroral brightening. We use global kinetic simulations along with spacecraft and ground‐based data. We find ~keV electrons in the region modeled in the simulation precipitate into the premidnight sector at latitudes between 71° and 75° due to two distinct physical processes: (1) higher latitude precipitation due to electrons that undergo relatively rapid non‐adiabatic pitch angle scattering into the loss cone just earthward of a reconnection region, and (2) lower latitude precipitation due to electrons that are more gradually accelerated primarily parallel to the geomagnetic field by Fermi acceleration. These latter electrons enter the loss cone much closer to Earth at ~−15 to −10 R_E. The electron precipitation due to the combination of these two mechanisms coincides spatially with observed auroral brightening during the disturbed event.

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.

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 y_GSM = 6 R_E during a very large substorm (AE = 1200) while the Cluster spacecraft were located about 5 R_E 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 (x_GSM = −9 R_E and −8 R_E, 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.

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.

Cluster observations in the near‐Earth plasma sheet boundary layer (PSBL) region have shown the presence of ion shell distributions related to velocity‐dispersed ion structures (VDIS) coincident with electrostatic emissions. We have examined ion shell instabilities in the presence of a cold ion and electron background using linear theory and particle in cell simulations. Linear theory shows that the shell instability is only excited when a cold ion background is present, generating a broad range of ion cyclotron harmonics. Numerical simulations confirm that ion Bernstein modes are preferentially excited transverse to the ambient magnetic field, along with a lower level of wave power at oblique angles. The background ions are heated primarily in the transverse direction because of a linear nonstochastic ion cyclotron heating mechanism, and overall saturation of the instability occurs because of thermalization of the shell combined with heating of the background ions and electrons. Comparison with Cluster observations shows that observed electrostatic waves with a spectrum from a few Hz to several hundred Hz is in good agreement with that expected from the shell instability. The cold background ions are observed to have a temperature anisotropy with T_⊥ > T_∥, while electrons are observed to have T_∥ > T_⊥, also in qualitative agreement with the shell instability. The wave‐particle effects due to the shell instability in the near‐Earth PSBL could have important consequences for auroral potential structure at lower altitudes and may cause the gaps in VDIS structure leading to beamlets.

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 R_E and x = −16 R_E 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.