Mostafa El-Alaoui

Current disruption during November 24, 1996, substorm

This study uses global magnetohydrodynamic (MHD) simulations driven by solar wind data along with Geotail, Interball, and IMP 8 observations of the magnetotail to investigate the dynamics of the near-Earth plasma sheet during a substorm that occurred on November 24, 1996. The MHD simulation shows that prior to the onset of the substorm, the magnitude of the current density decreases in a small region in the near-Earth plasma sheet. During and after the substorm onset this region of weak current becomes larger and more pronounced, expanding dawnward, duskward, upward, downward, and tailward. Part of the cross-tail current is redirected to the ionosphere via an earthward field-aligned current on the dawnside and a tailward return current on the duskside. The simulation showed that the field-aligned current was associated with velocity shear and flow vortices.

Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble

Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X = -18 R-E. The bubble is bounded by both sharp leading (partial derivative b(z)/partial derivative x 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvenic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E x B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B-z, or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed. Citation: Zhou, M., X. Deng, M. Ashour-Abdalla, R. Walker, Y. Pang, C. Tang, S. Huang, M. El-Alaoui, Z. Yuan and H. Li (2013), Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble, J. Geophys. Res. Space Physics, 118, 674-684, doi:10.1029/2012JA018323.

Ion sources and acceleration mechanisms inferred from local distribution functions

This study investigates the sources of the ions making 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 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.

Localized reconnection and substorm onset on Dec. 22, 1996

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.

Generation of electromagnetic fpe and 2fpe waves in the Earth's electron foreshock via linear mode conversion

This study examines the mode conversion from electrostatic to electromagnetic waves near the plasma frequency in the Earths electron foreshock. The conversion and reflection coefficients are obtained by solving coupled differential equations in a weakly mag-netized warm plasma with a longitudinal linear density gradient. Results indicate that the foreshock first harmonic electromagnetic emissions and the backward-propagating Langmuir waves required for the generation of the second harmonic electromagnetic waves may be efficiently produced by the linear conversion process in an inhomogeneous plasma.

Multiscale study of electron energization during unsteady reconnection events

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.

Electron energization and transport in the magnetotail during substorms

It has been suggested recently that electrons are accelerated both near the reconnection site and during subsequent earthward transport. We provide global and quantitative evidence for this two-step process by examining electron energization during a substorm event that occurred on 11 March 2008, when the Earths magnetosphere was immersed in southward interplanetary magnetic field and high-speed solar wind. The approach is to integrate coordinated measurements by Time History of Events and Macroscale Interactions during Substorms (THEMIS), global magnetohydrodynamic (MHD) modeling of the magnetosphere driven by upstream solar wind conditions, and large-scale kinetics (LSK) simulation. In this event, THEMIS P2 at XGSM ~ − 15 RE detected a dipolarization pulse-like magnetic structure coupled with energetic electron flux increase. About 1 min later, P3 and P4 at XGSM ~ − 10 RE observed dipolarized magnetic field and even larger flux increase. These multipoint data are applied to benchmark the MHD/LSK simulation. We quantify electron energization near the reconnection site and during earthward transport. We show that electrons were initially energized near the reconnection site, and subsequently further adiabatically accelerated far away from the reconnection site as the dipolarization front formed and grew. The adiabatic enhancement was rather effective given steep source electron distribution as a function of energy. In the outer magnetosphere with strong flows, E × B drift statistically dominated electron guiding center motion; thus, electrons were transported along the flow channel with small spatial diffusion. Close to the Earth, gradient and curvature drifts became appreciable; therefore, electrons circled around the region of large magnetic field.

Plasma waves in the Earth's electron foreshock: 2. Simulations using time‐of‐flight electron distributions in a generalized Lorentzian plasma

This study considers the level of occurrence of nonlinear wave-wave interactions in foreshock nonthermal plasma and whether such interactions are significant in producing observed electromagnetic emissions. To this end, we performed two-dimensional electromagnetic particle-in-cell (PIC) simulations using the time-of-flight electron distribution model described in the paper proceeding this [Yin et al., this issue], and input parameters based on two sets of Wind/3DP distribution measurements made when narrowband and broadband waves were detected by the Wind/WAVES instruments. In the simulations we limited the wave energy density to W ≤ 10 -3 , a value inferred from the largest wave amplitude detected by Wind. We consider the dynamics of narrowband and broadband wave instabilities, focusing on wave-coupling processes in the presence of foreshock hot ions as well as the most intense narrowband waves, which are the most likely candidates for producing the detected electromagnetic emissions. Results are compared with recent Wind measurements. We showed that when foreshock conditions exist, quasilinear diffusion dominates the instabilities, that weak wave coupling does not prevent beam flattening, and that no discernible wave backscattering occurs. The study indicates that the weak nonlinearity of foreshock electrostatic waves in a homogeneous plasma and in the presence of foreshock nonthermal electrons and moderate beam parameters is not responsible for the generation of electromagnetic emissions at either the fundamental or the second harmonic of the plasma frequency.

Source Distributions of Substorm Ions Observed in the Near-Earth Magnetotail

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.

Ion energization and transport associated with magnetic dipolarizations

Ion energization in the magnetotail during substorms is examined by simulating a modest substorm event that occurred on 7 February 2009. The simulation scheme combines global magnetohydrodynamic (MHD) modeling of the magnetosphere driven by realistic upstream solar wind conditions, with a large-scale kinetics (LSK) simulation. Multiple earthward propagating dipolarizations driven by reconnection outflow jets are modeled by the MHD simulation. Ion trajectories in the LSK simulation show that ions that originated near the reconnection site first gained energy nonadiabatically and then gained energy adiabatically as they “caught up with and then rode on” the earthward propagating dipolarizations. Consequently, the high-energy (>25 keV) ion fluxes were enhanced where and when the dipolarizations intensified. High-speed flows in narrow channels controlled the ion earthward transport in the outer magnetosphere due to the dominant E × B drift. The mechanisms of nonlocal energization by dipolarizations and transport controlled by high-speed flows operate similarly for electrons as described by Ashour-Abdalla et al. (2011) and Pan et al. (2014).