Electron-ion Drift Research Articles

Pulsed townsend electron and ion swarm parameter measurements of the trigger RPC standard mixture

The electron swarm parameters and ion drift velocities of the standard mixture R134a/iC4H10/SF6 95.2/4.5/0.3 used in trigger-purpose resistive plate chambers are examined in the pulsed Townsend experiment. Reduced electric fields up to 330Td at pressures from 400Pa to 12kPa are investigated, whereby no significant pressure dependence of the reduced swarm parameters are observed. The acquired effective ionization coefficient and the bulk drift velocity agree with predictions made using the updated R134a cross-sections from the swarm parameter simulation software Magboltz version 11.19.

Impacts of Subauroral Polarization Streams on Storm‐Enhanced Density Plume and Consequently on Polar Tongue of Ionization

AbstractThe influences of subauroral polarization streams (SAPS) on storm‐enhanced density (SED) plume and consequently on polar tongue of ionization (TOI), an important topic in the field of magnetosphere‐ionosphere‐thermosphere coupling, however, remain undetermined. The Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) with/without an empirical SAPS model has been used to investigate the impacts of SAPS on local ionosphere‐thermosphere system, which in turn affect the SED plume and consequently the TOI. The modeled total electron content and ion drift velocities agree reasonably well with the observations of Global Navigation Satellite System and Defense Meteorological Satellite Program satellites on 17 March 2013. The TIEGCM simulations show that SAPS can significantly affect the electron density of SED plume and consequently of TOI. The ionospheric‐thermospheric effect of SAPS can reduce the electron density of the SED plume in the afternoon sector and enhance the SED plume in the noon sector, which moves the SED plume dawnward. SAPS‐induced electron depletions in the throat region undergo anti‐sunward convection into the polar cap to weaken the TOI. A term‐by‐term analysis of the O+ ion continuity equation in the F‐region shows that the electron density depletions on the duskward edge of the SED plume are mainly due to the increased local plasma loss rates because of SAPS elevated plasma‐neutral temperatures and O/N2 reduction because of thermosphere upwelling. The electron density enhancements on the edge of the SED plume near noon are mainly due to the SAPS‐induced westward plasma E × B transports and the O/N2 increment because of thermospheric downwelling.

The High‐Latitude Trough in the Southern Hemisphere Observed by Swarm‐A Satellite

AbstractBased on the electron density, field‐aligned currents (FACs) and ion drift velocity data from the Swarm‐A satellite during 2013–2018, we investigate the features of the high‐latitude trough in the Southern Hemisphere. The results show that the high‐latitude trough in the Southern Hemisphere is a persistent postmidnight feature in winter. It is observed mainly in the eastern longitudes under low solar activity conditions and in a smaller longitudinal range under high solar activity conditions. The high‐latitude trough moves to higher latitudes at later local times, which is more obvious under low solar activity conditions. We also find that the features of FACs and ion drift velocity distribution in the high‐latitude trough region depend on longitude. In the longitude sector of 0°–70°E, the high‐latitude trough positions are always located at the poleward boundary of the downward FAC region, and collocated with the convection reversal boundary in the postmidnight region. In the longitude sector of 80°–130°E, the high‐latitude trough positions are located at the poleward boundary of the downward FAC region in the midnight region and at the strong westward or antisunward ion flow region in the postmidnight region. It is suggested that the high‐latitude trough formation and evolution is possibly associated with downward FACs, flow stagnation in the convection reversal boundary, and westward or antisunward ion flow.

Theoretical Prediction of Asymmetric Instability of Drift Kinetic Alfven Waves in Anisotropic Plasmas

AbstractThe asymmetric instability of drift kinetic Alfvén waves (DKAWs) is analyzed to study the interplay between the density inhomogeneity and the temperature anisotropy, including the wave‐particle interaction and finite ion Larmor radius effects. The asymmetric behavior arises due to the opposite diamagnetic drifts of electrons and ions and quantifies the contribution of electron drift waves and ion drift waves in altering the dispersion characteristic of DKAWs. It is shown that the coupling of drift waves with kinetic Alfvén waves leads to the generation of three different frequency (i.e., higher, intermediate, and lower) modes of DKAWs. The comparison of their propagation characteristics and stability mechanisms applicable to a wide range of plasma parameters is provided. The analytical dispersion relation can be used as an improved theoretical tool for identifying the existence of DKAWs and their source region in the multisatellite observations, especially near the reconnection X‐line.

The effect of ring current electron scattering rates on magnetosphere‐ionosphere coupling

AbstractThis simulation study investigated the electrodynamic impact of varying descriptions of the diffuse aurora on the magnetosphere‐ionosphere (M‐I) system. Pitch angle diffusion caused by waves in the inner magnetosphere is the primary source term for the diffuse aurora, especially during storm time. The magnetic local time (MLT) and storm‐dependent electrodynamic impacts of the diffuse aurora were analyzed using a comparison between a new self‐consistent version of the Hot Electron Ion Drift Integrator with varying electron scattering rates and real geomagnetic storm events. The results were compared with Dst and hemispheric power indices, as well as auroral electron flux and cross‐track plasma velocity observations. It was found that changing the maximum lifetime of electrons in the ring current by 2–6 h can alter electric fields in the nightside ionosphere by up to 26%. The lifetime also strongly influenced the location of the aurora, but the model generally produced aurora equatorward of observations.

MAGNETOHYDRODYNAMIC-PARTICLE-IN-CELL METHOD FOR COUPLING COSMIC RAYS WITH A THERMAL PLASMA: APPLICATION TO NON-RELATIVISTIC SHOCKS

We formulate a magnetohydrodynamic-particle-in-cell (MHD-PIC) method for describing the interaction between collisionless cosmic ray (CR) particles and a thermal plasma. The thermal plasma is treated as a fluid, obeying equations of ideal MHD, while CRs are treated as relativistic Lagrangian particles subject to the Lorentz force. Backreaction from CRs to the gas is included in the form of momentum and energy feedback. In addition, we include the electromagnetic feedback due to CR-induced Hall effect that becomes important when the electron-ion drift velocity of the background plasma induced by CRs approaches the Alfv\'en velocity. Our method is applicable on scales much larger than the ion inertial length, bypassing the microscopic scales that must be resolved in conventional PIC methods, while retaining the full kinetic nature of the CRs. We have implemented and tested this method in the Athena MHD code, where the overall scheme is second-order accurate and fully conservative. As a first application, we describe a numerical experiment to study particle acceleration in non-relativistic shocks. Using a simplified prescription for ion injection, we reproduce the shock structure and the CR energy spectra obtained with more self-consistent hybrid-PIC simulations, but at substantially reduced computational cost. We also show that the CR-induced Hall effect reduces the growth rate of the Bell instability and affects the gas dynamics in the vicinity of the shock front. As a step forward, we are able to capture the transition of particle acceleration from non relativistic to relativistic regimes, with momentum spectrum $f(p)\sim p^{-4}$ connecting smoothly through the transition, as expected from the theory of Fermi acceleration.

A Particle-in-Cell Simulation of Double Layers and Ion-Acoustic Waves

Double layers and ion-acoustic waves are investigated by using a one-dimensional electrostatic particle-in-cell simulation code. Our results show that double layers can be formed even when the drift velocity between electrons and ions is less than the electron thermal velocity. Electron and ion density depressions were clearly seen. Electrons gradually developed a distribution comprising both background and beam components. In fact, as the initial electron-ion drift velocity was less than the electron thermal velocity, intense ion-acoustic waves could be found only at the places where the electron beam was located, suggesting that they are excited by the self-consistently developed electron beam. Besides the Langmuir waves and ion-acoustic waves, the beam mode excited by electron beams produced in our simulation has been clearly found.

Kinetic model of the inner magnetosphere with arbitrary magnetic field

Theoretical and numerical modifications to an inner magnetosphere model—Hot Electron Ion Drift Integrator (HEIDI)—were implemented, in order to accommodate for a nondipolar arbitrary magnetic field. While the dipolar solution for the geomagnetic field during quiet times represents a reasonable assumption in the near‐Earth closed field region, during storm activity this assumption becomes invalid. HEIDI solves the time‐dependent, gyration‐ and bounce‐averaged kinetic equation for the phase space density of one or more ring current species. New equations are derived for the bounce‐averaged coefficients for the distribution function, and their numerical implementation is discussed. Also, numerically solving all the bounce‐averaged coefficients for the dipole case does not change the results significantly from the analytical approximation of Ejiri (1978). However, distorting the magnetic field changes all bounce‐averaged coefficients that make up the kinetic equation. Initial simulations show that changing the magnetic field changes the whole topology of the ring current. This is because the drifts are altered due to dayside compression and nightside stretching of the field. Therefore, at certain locations, the nondipolar magnetic drifts can dominate the convective drifts, considerably altering the pressure distribution in the equatorial plane.

Drift instabilities in the solar corona within the multi-fluid description

Context. Recent observations have revealed that the solar atmosphere is highly structured in density, temperature, and magnetic field. The presence of these gradients may lead to the appearance of currents in the plasma, which in the weakly collisional corona can constitute sources of free energy for driving micro-instabilities. Such instabilities are very important since they represent a possible source of ion-cyclotron waves that have been conjectured as playing a prominent role in coronal heating, but whose solar origin remains unclear. Aims. Considering a density stratification transverse to the magnetic field, this paper aims at studying the possible occurrence of gradient-induced plasma micro-instabilities under the conditions typical of coronal holes. Methods. Taking the WKB (Wentzel-Kramers-Brillouin) approximation into account, we performed the Fourier plane wave analysis using the collisionless multi-fluid model. By neglecting the electron inertia, this model allowed us to take into account ion-cyclotron wave effects that are absent from the one-fluid model of magnetohydrodynamics (MHD). Realistic models of density and temperature, as well as a 2D analytical magnetic-field model, have been used to define the background plasma in the open-field funnel in a polar coronal hole. The ray-tracing theory has been used to compute the ray path of the unstable waves, as well as the evolution of their growth rates during the propagation. Results. We demonstrate that in typical coronal hole conditions, and when assuming typical transverse density length scales taken from radio observations, the current generated by a relative electron-ion drift provides enough free energy for driving the mode unstable. This instability results from coupling between slow-mode waves propagating in opposite directions. However, the ray-tracing computation shows that the unstable waves propagate upward to only a short distance but then are reflected backward. The corresponding growth rate increases and decreases intermittently in the upward propagating phase, and the instability ceases while the wave is propagating downward. Conclusions. Drift currents caused by fine structures in the density distribution in the magnetically-open coronal funnels can provide enough energy to drive plasma micro-instabilities, which constitute a possible source of the ion-cyclotron waves that have been invoked for coronal heating.

Lower-hybrid drift and Buneman instabilities in current sheets with guide field

Lower-hybrid drift and Buneman instabilities operate in current sheets with or without the guide field. The lower-hybrid drift instability is a universal instability in that it operates for all parameters. In contrast, the excitation of Buneman instability requires sufficiently thin current sheet. That is, the relative electron-ion drift speed must exceed the threshold in order for Buneman instability to operate. Traditionally, the two instabilities were treated separately with different mathematical formalisms. In a recent paper, an improved electrostatic dispersion relation was derived that is valid for both unstable modes [P. H. Yoon and A. T. Y. Lui, Phys. Plasmas 15, 072101 (2008)]. However, the actual numerical analysis was restricted to a one-dimensional situation. The present paper generalizes the previous analysis and investigates the two-dimensional nature of both instabilities. It is found that the lower-hybrid drift instability is a flute mode satisfying k⋅B=0 and k⋅∇n=0, where k represents the wave number for the most unstable mode, B stands for the total local magnetic field, and ∇n is the density gradient. This finding is not totally unexpected. However, a somewhat surprising finding is that the Buneman instability is a field-aligned mode characterized by k×B=0 and k⋅∇n=0, rather than being a beam-aligned instability.

Drift instabilities in the solar corona within the multi-fluid description

Recent observations revealed that the solar atmosphere is highly structured in density, temperature and magnetic field. The presence of these gradients may lead to the appearance of currents in the plasma, which in the weakly collisional corona can constitute sources of free energy for driving micro-instabilities. Such instabilities are very important since they represent a possible source of ion-cyclotron waves which have been conjectured to play a prominent role in coronal heating, but whose solar origin remains unclear. Considering a density stratification transverse to the magnetic field, this paper aims at studying the possible occurrence of gradient-induced plasma micro-instabilities under typical conditions of coronal holes. Taking into account the WKB (Wentzel-Kramers-Brillouin) approximation, we perform a Fourier plane waves analysis using the collisionless multi-fluid model. By neglecting the electron inertia, this model allows us to take into account ion-cyclotron wave effects that are absent from the one-fluid model of magnetohydrodynamics (MHD). Realistic models of density and temperature, as well as a 2-D analytical magnetic-field model, are used to define the background plasma in the open-field funnel in a polar coronal hole. The ray-tracing theory is used to compute the ray path of the unstable waves, as well as the evolution of their growth rates during the propagation. We demonstrate that in typical coronal hole conditions, and when assuming typical transverse density length scales taken from radio observations, the current generated by a relative electron-ion drift provides enough free energy for driving the mode unstable. This instability results from a coupling between oppositely propagating slow-mode waves.

Numerical Study for the Bursty Nature of Spontaneous Fast Reconnection

In this paper, spontaneous fast reconnection in a neutral current sheet, which is initially perturbed by a localized resistivity, is studied by the newly developed Space-Time Conservation Element and Solution Element (CESE) method. After the initial perturbation is switched off, an anomalous resistivity is allowed to occur if a threshold of the local electron-ion drift velocity is exceeded. For a given threshold value, the amount of the reconnected magnetic flux introduced by the initial perturbation is very crucial for the onset of the anomalous resistivity. The numerical results indicate that fast reconnection can develop self-consistently with slow shocks extending between the diffusion region and a large-scale plasmoid-like structure, which is pushed forward by the reconnection outflow. A Petschek-like configuration is then built up, but it can not be sustained as a quasi-steady state. In fact, during the reconnection evolution, the diffusion region undergoes an elongation process so that after the dynamic process is nonlinearly saturated secondary tearing is subject to occur at the center of the system. This leads to enhanced and time-dependent reconnection. The reconnection evolution is further studied in various physical situations, also confirming the bursty nature of the spontaneous fast reconnection mechanism.

First tomographic image of ionospheric outflows

An image of the dayside low‐energy ion outflow event that occurred on 16 December 2003 was constructed with ground‐ and space‐based GPS (Global Positioning System) Total Electron Content (TEC) data and ion drift meter data from the DMSP (Defense Meteorological Satellite Program). A tomographic reconstruction technique has been applied to the GPS TEC data obtained from the GPS receiver on the Low Earth Orbit (LEO) satellite FedSat. The two dimensional tomographic image of the topside ionosphere and plasmasphere reveals a spectacular beam‐like dayside ion outflow emanating from the cusp region. The transverse components of the magnetic field in FedSat's NewMag data show the presence of field aligned current (FAC) sheets, indicating the existence of low‐energy electron precipitation in the cusp region. The DMSP ion drift data show upward ion drift velocities and upward fluxes of low‐energy ions and electrons at the orbiting height of the DMSP spacecraft in the cusp region. This study presents the first tomographic image of the flux tube structure of ionospheric ion outflows from 0.13 Re up to 3.17 Re altitude.

Conditions for the fast reconnection mechanism in three dimensions

Physical conditions for the fast reconnection mechanism to be realized as an eventual solution of magnetohydrodynamic equations are examined by three-dimensional (3D) simulations for different resistivity models. Initiated by a small disturbance in a current sheet, all the phenomena grow by the self-consistent interaction between the 3D reconnection flow and the effective resistivity. For the classical resistivity η due to Coulomb collisions, where η∝T−3∕2, no effective reconnection occurs, since the resistivity becomes reduced with the increase in temperature T as magnetic reconnection proceeds, which indicates the negative feedback between the reconnection flow and the resistivity. For the anomalous resistivity, where η increases with the current density when a threshold is exceeded, the positive feedback eventually leads to the fast reconnection mechanism as a nonlinear instability. In this case, the resistivity is distinctly enhanced at the slow shock layer attached to the diffusion region, so that the shock layer becomes thicker, and resistive tearing is more likely to take place in the diffusion region. For the anomalous resistivity, where η increases with the relative electron-ion drift velocity when a threshold is exceeded, the fast reconnection mechanism evolves most effectively as a nonlinear instability and is sustained steadily. It is concluded in general three-dimensional situations that the fast reconnection mechanism can be realized as an eventual solution for current-driven anomalous resistivities, whereas in usual circumstances with the classical resistivity no substantial magnetic reconnection takes place.

Plasma density enhancements in the high‐altitude polar cap region observed on Akebono

The plasma density in the polar cap ionosphere is generally low (<103 cm−3 above 3000 km), mainly because of plasma escape from the ionosphere along open magnetic‐field lines. The Akebono satellite occasionally encounters regions of unusually high plasma density (≥103 cm−3) above 4000 km altitude, in which the thermal plasma exhibits a distinctively low electron temperature (<3000 K) and low parallel ion drift velocity (≤1 km/s). Such events are almost always observed on the dusk side. The occurrence of low electron temperature and ion drift velocity appears to suggest the antisunward convection of high‐density plasma into the polar cap, and the decrease in electron temperature due to the disruption of field‐aligned heat flux in the high‐altitude polar cap.

Computer studies on the spontaneous fast reconnection evolution in various physical situations

The spontaneous fast reconnection evolution is studied in a long current sheet system in various physical situations, where the threshold of current-driven anomalous resistivity is assumed to increase with the thermal velocity. If the initial threshold VC0 is sufficiently large in a low-β plasma, the fast reconnection mechanism can fully be set up; on the other hand, if VC0 is so small that the anomalous resistivity can easily occur in the usual circumstances, the resulting diffusion region notably lengthens so that the reconnection process becomes much less effective. Also, the fast reconnection evolution is strongly influenced by plasma β in the ambient magnetic field region, and an essential condition for the fast reconnection mechanism to evolve explosively is that the plasma β is sufficiently small. In fact, only in a low-β plasma does the magnetic tension force involved play the dominant role in the overall system dynamics and in the drastic magnetic energy release. It is also demonstrated that the fast reconnection evolution does not depend on the detailed functional form of the (current-driven) anomalous resistivity model. This is because the positive feedback between the anomalous resistivity and the reconnection flow effectively works so long as an anomalous resistivity is assumed to increase with the relative electron-ion drift velocity.

Three-dimensional spontaneous magnetic reconnection in neutral current sheets

Magnetic reconnection in an antiparallel uniform Harris current sheet equilibrium, which is initially perturbed by a region of enhanced resistivity limited in all three dimensions, is investigated through compressible magnetohydrodynamic simulations. Variable resistivity, coupled to the dynamics of the plasma by an electron–ion drift velocity criterion, is used during the evolution. A phase of magnetic reconnection amplifying with time and leading to eruptive energy release is triggered only if the initial perturbation is strongly elongated in the direction of current flow or if the threshold for the onset of anomalous resistivity is significantly lower than in the corresponding two-dimensional case. A Petschek-like configuration is then built up for ∼102 Alfvén times, but remains localized in the third dimension. Subsequently, a change of topology to an O-line at the center of the system (“secondary tearing”) occurs. This leads to enhanced and time-variable reconnection, to a second pair of outflow jets directed along the O-line, and to expansion of the reconnection process into the third dimension. High parallel current density components are created mainly near the region of enhanced resistivity.

Flow shear stabilization of a hybrid dissipative trapped electron-ion temperature gradient mode in tokamaks

A simple physical model for toroidal flow shear stabilization of a hybrid dissipative trapped electron-ion temperature gradient mode (namely, the hybrid electron-ion drift mode) is presented. Coupling between the sheared flows and non-adiabatic electrons is proposed as the stabilization mechanism of the hybrid electron-ion drift mode. This implies that the confinement improvement can be achieved not only through the poloidal sheared rotation flow in the edge region of tokamaks for the L-H transition but also through the toroidal sheared rotation in the core region for further confinement improvement associated with the internal transport barrier and negative shear.

Elucidation of the physics of the gravity wave‐TID relationship with the aid of theoretical simulations

The physics of the relationship between atmospheric gravity waves (AGWs) and traveling ionospheric disturbances (TIDs) is thoroughly investigated with emphasis on large‐scale AGW/TIDs at F region heights at middle and high latitudes (provided field‐perpendicular drifts are small). In support, simulations using a realistic AGW model (“Clark‐based AGW model”) in combination with a realistic ionospheric model (“Graz Ionospheric Flux Tube Simulation (GIFTS) model”) were performed. All fundamental AGW/TID quantities are treated consistently, i.e., perturbations in neutral densities, wind, and temperature as well as disturbances in electron density, ion drift, and ion and electron temperature. The AGW‐induced ionospheric response is inspected for all TlD quantities, based on their governing conservation equations. The results are discussed by means of detailed and approximative formulae as well as instructive figures which provide a firm quantitative understanding significantly beyond the current state of knowledge. Especially the physics of the electron temperature disturbance (Te‐TID), up to now not yet quantitatively inspected, is thoroughly explored. A major finding is that the disturbance in specific terms of the electron energy equation is an order of magnitude more pronounced than the net disturbance determining the strength of the Te‐TID. Furthermore, simulation results illustrating the natural variability of the AGW/TID quantities (1) due to varying AGW properties and (2) due to changing thermosphere/ionosphere background conditions are discussed. Features observed include the following: AGW period and magnetic field line‐induced south‐north asymmetries are major causes of variability. Change from high/moderate to low solar activity enhances amplitudes of most AGW/TID quantities, electron density and temperature being likely exceptions; nighttime conditions tend to lower amplitudes versus daytime. The insight gained is valuable from a basic research point of view and also for suitable AGW/TID descriptions for thermosphere/ionosphere weather modeling.

Particle dynamics and current-free double layers in an expanding, collisionless, two-electron-population plasma

The expansion of a two-electron-population, collisionless plasma into vacuum is investigated experimentally. Detailed in situ measurements of plasma density, plasma potential, electric field, and particle distribution functions are performed. At the source, the electron population consists of a high-density, cold (kTe≂4 eV) Maxwellian, and a sparse, energetic ( (1)/(2) mv2e≂80 eV) tail. During the expansion of plasma, space-charge effects self-consistently produce an ambipolar electric field whose amplitude is controlled by the energy of tail electrons. The ambipolar electric field accelerates a small number (∼1%) of ions to streaming energies which exceed and scale linearly with the energy of tail electrons. As the expansion proceeds, the energetic tail electrons electrostatically trap the colder Maxwellian electrons and prevent them from reaching the expansion front. A potential double layer develops at the position of the cold electron front. Upstream of the double layer both electron populations exist; but downstream, only the tail electrons do. Hence, the expansion front is dominated by retarded tail electrons. Initially, the double layer propagates away from the source with a speed approximately equal to the ion sound speed in the cold electron population. The propagation speed is independent of the tail electron energy. At later times, the propagating double layer slows down and eventually stagnates. The final position and amplitude of the double layer are controlled by the relative densities of the two electron populations in the source. The steady-state double layer persists till the end of the discharge (Δt≂1 msec), much longer than the ion transit time through the device (t≂150 μsec). On the low-potential side, the double layer generates a monoenergetic, neutralized ion beam with no stationary background plasma. The field-aligned double layer contains no trapped or counter-streaming ions, and is current free; i.e., the relative electron–ion drift is zero.