Maha Ashour-Abdalla

The structure of the distant geomagnetic tail during long periods of northward IMF

We have used a newly developed, parallelized, global MHD magnetosphere-ionosphere simulation model with a 400 RE long tail to study the evolution, structure, and dynamics of the distant magnetotail during extended periods of northward IMF. We find that the tail evolves to a nearly time stationary structure about one solar wind transit time after the IMF turns northward. Four regions of different magnetic topology can be distinguished which extend at least to the end of the simulation box at 400 RE. Besides lobe field lines and open solar wind field lines tailward of an X-line, there is a broad boundary layer of closed field lines which we call the tail flank boundary layer (TFBL). Just inside the TFBL there is a region of closed field loops. Besides the X-line we find two O-lines which are enclosed by the closed field loops and are roughly aligned with the tail axis. Together they form a U shaped separator between the northward and the southward plasma sheet fields.

Shaping of the magnetotail from the mantle - Global and local structuring

This paper discusses kinetic modeling of the properties of magnetotail formation from a plasma mantle source and develops a unified view of the structure of the central part of the magnetotail plasma sheet as well as the structure of its boundary layer. Trajectories of mantle protons in the presence of a uniform dawn-dusk electric field have been traced using the Tsyganenko magnetic field model for quiet periods of magnetospheric activity. The most important portion of the particle trajectories is each particles first interaction with the sharp reversal of the magnetic field in the tail midplane, because this interaction results in particle energization and chaotic scattering. The closer this interaction takes place to the X line, the larger a particles energy will become. The intensity of the chaotic scattering in the tail midplane depends also on the position in the tail at which it occurs. The energization and scattering result in a significant restructuring of the tail ion distributions, both in space and in velocity coordinates. Our model shows the evolution of the global structure of the tail with a clearly defined central plasma sheet and plasma sheet boundary layer developing from its beginnings as a plasma “nucleus” in the distant tail current sheet. This large-scale restructuring is accompanied by the creation of small-scale features in the particle distribution functions. For example, the model not only correctly reproduces the spatial distribution and velocity dispersion of the fast ion beams moving both earthward and tailward in the plasma sheet boundary layer, but also indicates that these beams should be highly structured spatially into 5-6 smaller beamlets with distinct velocities. In addition the model shows that complementary ring distribution structures should also exist in the central plasma sheet. Our analysis indicates that the ion distribution functions in the central plasma sheet should take a rather specific form in velocity space with loss regions oriented predominantly orthogonal to the magnetic field. Our results also emphasize the importance of counterstreaming populations, not only in the boundary layer, but also in the central part of the plasma sheet. Analytical calculations indicate that the properties of chaotic scattering in the magnetotail under realistic conditions (x dependence of the normal magnetic field and dawn-dusk electric field) are quite different from those predicted by earlier simple xindependent models. Finally the model results are compared with recent observations of ion distribution functions and their moments for various regions of the magnetotail, and quantitative estimates from the model are shown to be in good agreement with observations. Small-scale structuring and the presence of counterstreaming are also discussed, as well as their possible importance in explaining the observed intermittency in the plasma sheet bulk flows.

Consequences of magnetotail ion dynamics

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 constructed 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.

A global magnetohydrodynamic simulation of the response of the magnetosphere to a northward turning of the interplanetary magnetic field

We have used a global magnetohydrodynamic simulation model to investigate the time series of events which occur in the magnetosphere when the interplanetary magnetic field (IMF) changes from southward to northward. Within 15 min of the northward turning the magnetopause reconnection site moves from the subsolar point to the high-latitude tail. The high-latitude reconnection converts tail lobe field lines into closed dayside field lines and new field lines in the IMF. The removal of lobe flux plus continuing reconnection at the near-Earth neutral line cause the field lines which thread the plasma sheet to relax to a more dipolar state and a very slow tailward retreat of the near earth neutral line. The newly closed dayside field lines are convected around the flanks of the magnetosphere and into the tail. The closed flux convected from the dayside to the tail is the main source of closed flux into the tail. As a result, the plasma sheet recovers first along the flanks of the magnetosphere and then nearer midnight. The near-Earth neutral line shrinks toward midnight and then moves rapidly tailward.

Wave collapse at the lower‐hybrid resonance

The modulational instability and collapse of waves in the vicinity of the lower‐hybrid resonance including both magnetosonic and lower‐hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower‐hybrid waves with the much lower‐frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high‐frequency field producing elongated, cigar‐shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self‐similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two‐ and three‐dimensional cavity geometries. It is shown that in three‐dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower‐hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.

A global magnetohydrodynamic simulation of the magnetosheath and magnetosphere when the interplanetary magnetic field is northward

A new high-resolution global magnetohydrodynamic simulation model is used to investigate the configuration of the magnetosphere when the interplanetary magnetic field (IMF) is northward. For northward IMF the magnetospheric configuration is dominated by magnetic reconnection at the tail lobe magnetopause tailward of the polar cusp. This results in a local thickening of the plasma sheet equatorward of the region of reconnection and the establishment of a convection system with two cells in each lobe. In the magnetosheath the plasma density and pressure decrease near the subsolar magnetopause, forming a depletion region. Along the flanks of the magnetosphere the magnetosheath flow is accelerated to values larger than the solar wind velocity. The magnetopause shape from the simulations is consistent with the empirically determined shape. >

Geotail waveform observations of broadband/narrowband electrostatic noise in the distant tail

Broadband electrostatic noise (BEN) and narrowband electrostatic noise (NEN) are common wave activities in the plasma sheet boundary and the tail lobe regions, respectively. Similar wave emissions can be observed in the magnetosheath region. We demonstrate the nature of these waves based on the waveform observations by the plasma wave instrument on board the Geotail spacecraft. The above observed waveforms are divided into two types of classifications. The BEN type emissions observed in the plasma sheet boundary and magnetosheath consist of a series of isolated bipolar pulses. They are termed after their waveforms as plasma sheet boundary layer electrostatic solitary waves (PSBL ESW) and magnetosheath electrostatic solitary waves (MS ESW). On the other hand, the waveforms of the NEN type emissions are quasi-monochromatic. They are termed as lobe electrostatic quasi-monochromatic waves (lobe EQMW) and magnetosheath electrostatic quasi-monochromatic waves (MS EQMW). The waveform observations with the high time resolution show that one of the common features of these waves is the burstiness. The burstiness means that their amplitudes or frequencies rapidly change of the order of a few milliseconds to a few hundreds of milliseconds. Further, we show that the PSBL ESW, lobe EQMW, MS ESW, and MS EQMW are parallel propagating waves relative to the ambient magnetic field. The similarities of the ESW and EQMW in the magnetosheath and magnetotail suggest the possibility that these waves are generated by the same generation mechanism.

Dispersed ion structures at the poleward edge of the auroral oval: Low‐altitude observations and numerical modeling

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 2-20 keV H+ velocity dispersed precipitating ion structures (VDIS) with fluxes above 10−3 ergs. cm−2. s−1 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 relationships between magnetic activity or substorm phase and the VDIS properties. The VDIS are found on 15-18% of the A3 orbits. In general their frequency of occurrence is not correlated with activity. However in the 2200-0200 MLT sector, the probability of observing more energetic VDIS increases for larger values of the AE index. 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 unusual observations of low-energy (< 5 keV) O+ VDIS, shifted by 4°-5° 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.

The formation of the wall region: Consequences in the near Earth magnetotail

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly non-adiabatic ion acceleration (known as the [open quotes]wall[close quotes] region) exists in the near Earth tail and demarcates two very different regimes of ion motion: Adiabatic and quasiadiabatic. A strong enhancement of the cross-tail current occurs on the tailward side of the wall. The authors comparison of numerical and adiabatic pressure profiles indicates that non-adiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasisteady magnetospheric convection. 23 refs., 4 figs.

The quasi-adiabatic ion distribution in the central plasma sheet and its boundary layer

This paper derives for the first time, the consequences of quasi-adiabatic ion motion in the magnetospheric tail, which is in fact a chaotic scattering of the ions due to their essentially nonadiabatic behavior. This is caused by the nonlinearity of their equations of motion in the strongly curved tail magnetic field and results from separatrix traversals in the phase space. In this paper it is shown how the continuous violation of adiabaticity of the ion motion in the Earths magnetotail leads to a redistribution of ions between the central plasma sheet (CPS) and the plasma sheet boundary layer (PSBL). It is also shown that, due to the mechanism described in this paper, ions are accelerated to the PSBL even without any assumption of the existence of an electric field caused by reconnection with an χ line. For the CPS ion distribution this acceleration also results in the depletion of some domains in velocity space that can lead to substantial non-Maxwellian features.