Stochastic acceleration in turbulent electric fields generated by 3D reconnection

Electron and proton acceleration in three-dimensional electric and magnetic fields is studied through test particle simulations. The fields are obtained by a three-dimensional magnetohydrodynamic simulation of magnetic reconnection in slab geometry. The nonlinear evolution of the system is characterized by the growth of many unstable modes and the initial current sheet is fragmented with formation of small scale structures. We inject at random points inside the evolving current sheet a Maxwellian distribution of particles. In a relatively short time (less than a millisecond) the particles develop a power-law tail. The acceleration is extremely efficient and the electrons absorb a large percentage of the available energy in a small fraction of the characteristic time of the MHD simulation, suggesting that resistive MHD codes are unable to represent the full extent of particle acceleration.     

Formation of inner structure of a reconnection separatrix region

We present multipoint spacecraft observations at the dayside magnetopause of a magnetic reconnection separatrix region. This region separates two plasmas with significantly different temperatures and densities, at a large distance from the X line. We identify which terms in the generalized Ohm's law balance the observed electric field throughout the separatrix region. The electric field inside a thin approximately c/omega pi Hall layer is balanced by the j x B/ne term while other terms dominate elsewhere. On the low density side of the region we observe a density cavity which forms due to the escape of magnetospheric electrons along the newly opened field lines. The perpendicular electric field inside the cavity constitutes a potential jump of several kV. The observed potential jump and field aligned currents can be responsible for strong aurora.     

Sounding rocket measurements of suprathermal ion acceleration

Transverse ion acceleration has been observed at rocket altitudes between 500 and 1000 km due to the injection of 100-200-eV argon plasma, auroral electron precipitation, and the injection of electromagnetic waves. Field-aligned currents necessary to neutralize the plasma injection payloads and those naturally occurring in the aurora could be responsible for the ions observed in the first two observations. Associated with the aurora, both bulk heating and tail heating are observed, sometimes simultaneously. In this case, either different masses are accelerated and/or different mechanisms are responsible. The bulk heating is closely correlated with the aurora structure while tail heating is not so well correlated. High-time-resolution rocket ion data have revealed that the transverse acceleration process is of very short duration (~100 ms) and occurs in a very limited volume (a few hundred kilometers along B and on the order of the ion gyrodiameter across B). Such impulse acceleration events are correlated with waves near the lower hybrid resonance. Wave injections of electromagnetic waves near the lower hybrid frequency result in the transverse acceleration of ambient ions     

Electron temperature gradient scale at collisionless shocks

Shock waves are ubiquitous in space and astrophysics. They transform directed flow energy into thermal energy and accelerate energetic particles. The energy repartition is a multiscale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. While large scale features of ion heating are known, the electron heating and smaller scale fields remain poorly understood. We determine for the first time the scale of the electron temperature gradient via electron distributions measured in situ by the Cluster spacecraft. Half of the electron heating coincides with a narrow layer several electron inertial lengths (c/ω(pe)) thick. Consequently, the nonlinear steepening is limited by wave dispersion. The dc electric field must also vary over these small scales, strongly influencing the efficiency of shocks as cosmic ray accelerators.     

Electric fields in the magnetosphere-the evidence from ISEE, GEOSand Viking

Electric field measurements on the satellites GEOS-1, GEOS-2, ISEE-1, and Viking have extended the empirical knowledge of electric fields in space to include the outer regions of the magnetosphere. While the measurements confirm some of the theoretically expected properties of the electric fields, they also reveal unexpected features and a high degree of complexity and variability. The existence of a magnetospheric dawn-to-dusk electric field, as expected on the basis of extrapolation from low-altitude measurements, is confirmed in an average sense. However, the actual field exhibits large spatial and temporal variations, including strong fields of inductive origin. At the magnetopause, the average (dawn-to-dusk directed) tangential electric field component is typically obscured by irregular fluctuations of larger amplitude. In addition, data from electric-field measurements provide further support for the conclusion that a nonvanishing magnetic-field aligned electric field exists in the auroral acceleration region     

The Armstrong experiment revisited

When a high-voltage direct-current is applied to two beakers filled with water or polar liquid dielectrica, a horizontal bridge forms between the two beakers. This experiment was first carried out by Lord Armstrong in 1893 and then forgotten until recently. Such bridges are stable by the action of electrohydrodynamic (EHD) forces caused by electric field gradients counteracting gravity. Due to these gradients a permanent pumping of liquid from one beaker into the other is observed. At macroscopic scale several of the properties of a horizontal water bridge can be explained by modern electrohydrodynamics, analyzing the motion of fluids in electric fields. Whereas on the molecular scale water can be described by quantum mechanics, there is a conceptual gap at mesoscopic scale which is bridged by a number of theories including quantum mechanical entanglement and coherent structures in water – theories that we discuss here. Much of the phenomenon is already understood, but even more can still be learned from it, since such “floating” liquid bridges resemble a small high voltage laboratory of their own: The physics of liquids in electric fields of some kV/cm can be studied, even long time experiments like neutron or light scattering are feasible since the bridge is in a steady-state equilibrium and can be kept stable for hours. It is also an electro-chemical reactor where compounds are transported through by the EHD flow, enabling the study of electrochemical reactions under potentials which are otherwise not easily accessible. Last but not least the bridge provides the experimental biologist with the opportunity to expose living organisms such as bacteria to electric fields without killing them, but with a significant influence on their behavior, and possibly, even on their genome.     

Intense laser-driven energetic proton beams from solid density targets

The effects of target density on proton acceleration driven by an intense sub-ps laser pulse are investigated using two-dimensional hybrid particle-in-cell simulations. Results show that at higher density the target-normal-sheath acceleration (TNSA) is more effective than shock acceleration for protons from a plastic target. Furthermore a lower-density target is favorable to higher energy of the TNSA protons. Moreover, the longitudinal electric fields at the target surfaces may reveal typical inhomogeneous structures for a long acceleration time. The conversion efficiency of laser energy into particle (electron, proton, and C(+) ion) energy is found to increase with decreasing target density.     

Electron energy gain in the fundamental mode of an elliptical waveguide in the presence of static helical magnet by microwave radiation

The dynamics and energy gain of an electron in the field of a transverse electric wave propagating inside an elliptical waveguide is analytically investigated by considering the existence of a helical magnet in which the field is perpendicular to the axis of the waveguide and rotating as a function of position along the magnet. Besides, by solving the relativistic momentum and energy equations, the deflection angle and the acceleration gradient of the electron in the waveguide are obtained. It is shown that the electron is deflected due to the field components of the transverse electric mode of this microwave, and at the same time, it is accelerated by these fields. Furthermore, the expressions of the acceleration gradient and deflection angle for an electron in the transverse electric mode inside the plasma elliptical waveguide without a static helical magnet are presented, which was injected initially along the propagation direction of the microwave. The results are graphically presented.     

Double layers and ion phase-space holes in the auroral upward-current region

The dynamic evolution of the boundary between the ionosphere and auroral cavity is studied using 1D and 2D kinetic Vlasov simulations. The initial distributions of three singly ionized species (H+, O+, e-) are determined from space-based observations on both sides of an inferred strong double layer. The kinetic simulations reproduce features of parallel electric fields, electron distributions, ion distributions, and wave turbulence seen in satellite observations in the auroral upward-current region and, for the first time, demonstrate that auroral acceleration can be driven by a parallel electric field supported, in part, by a quasistable, strong double layer. In addition, the simulations verify that the streaming interaction between accelerated O+ and H+ populations continuously replenished by the double layer provides the free energy for the persistent formation of ion phase-space holes.     

洛伦兹光谱线型的高层大气风场被动探测原理分析

通过探测高层大气中气辉(极光)辐射线的多普勒频移,可以反演出高层大气中的速度、温度和压强等物理量。以高层大气中的极光(原子氧跃迁所辐射的两条主要谱线)为被探测源,对于洛伦兹光谱线型高层大气风场的探测原理和方法进行了研究;给出了基于洛伦兹光谱线型高层大气的速度场、温度场和压力场的分布规律和理论计算公式;采用计算机模拟,描绘了洛伦兹光谱线型风场的误差曲线,表明了洛伦兹光谱线型在高层大气风场探测中占有相当重要的地位。     

The turbulent Alfvénic aurora

It is demonstrated from observations that the Alfvénic aurora may be powered by a turbulent cascade transverse to the geomagnetic field from large MHD scales to small Alfvén wave scales of several electron skin depths and less. We show that the energy transport through the cascade is sufficient to drive the observed acceleration of electrons from near-Earth space to form the aurora. We find that regions of Alfvén wave dissipation, and particle acceleration, are localized or intermittent and embedded within a near-homogeneous background of large-scale MHD structures.     

O( Ⅰ ,Ⅱ) 禁戒线极光及其光化学反应

利用干涉成像光谱技术被动探测上层大气风场所使用的光源主要是氧禁戒线极光。讨论了禁戒跃迁出现较强谱线需要的两个条件,对满足上述条件的上层大气中的原子氧(OⅠ)和离子氧(OⅡ)的可见光波段禁戒线进行了详细的计算,并对应指出产生这些O(Ⅰ,Ⅱ)禁戒线的光化学反应。得出的结论是被动探测上层大气风场所用极光源可以使用所指认的10条O(Ⅰ,Ⅱ)禁戒线和允许线:557.7nm,630.0/636.4/639.3 nm,672.8nm,732.2/733.2nm,777.7/777.6/777.4nm,比加拿大的风成像光谱干涉仪(WINDII)使用的极光谱线增添了5条:639.3nm,672.8nm,777.7/777.6/777.4nm,扩展了成像光谱干涉仪的波段范围。     

Collisionless magnetic reconnection in the magnetosphere

Magnetic reconnection underlies the physical mechanism of explosive phenomena in the solar atmosphere and planetary magnetospheres, where plasma is usually collisionless. In the standard model of collisionless magnetic reconnection, the diffusion region consists of two substructures: an electron diffusion region is embedded in an ion diffusion region, in which their scales are based on the electron and ion inertial lengths. In the ion diffusion region, ions are unfrozen in the magnetic fields while electrons are magnetized. The resulted Hall effect from the different motions between ions and electrons leads to the production of the in-plane currents, and then generates the quadrupolar structure of out-of-plane magnetic field. In the electron diffusion region, even electrons become unfrozen in the magnetic fields, and the reconnection electric field is contributed by the off-diagonal electron pressure terms in the generalized Ohm's law. The reconnection rate is insensitive to the specific mechanism to break the frozen-in condition, and is on the order of 0.1. In recent years, the launching of Cluster, THEMIS, MMS, and other spacecraft has provided us opportunities to study collisionless magnetic reconnection in the Earth's magnetosphere, and to verify and extend more insights on the standard model of collisionless magnetic reconnection. In this paper, we will review what we have learned beyond the standard model with the help of observations from these spacecraft as well as kinetic simulations.     

Electric field in a double layer and the imparted momentum

It is shown that the net momentum delivered by the large electric field inside a one-dimensional double layer is zero. This is demonstrated through an analysis of the momentum balance in the double layer at the boundary between the ionosphere and the aurora cavity. For the recently observed double layer in a current-free plasma expanding along a divergent magnetic field, an analysis of the evolution of the radially averaged variables shows that the increase of plasma thrust results from the magnetic-field pressure balancing the plasma pressure in the direction of acceleration, rather than from electrostatic pressure.     

Confinement physics for thermal, neutral, high-charge-state plasmas in nested-well solenoidal traps

A theoretical study is presented which indicates that it is possible to confine a neutral plasma using static electric and solenoidal magnetic fields. The plasma consists of equal temperature electrons and highly stripped ions. The solenoidal magnetic field provides radial confinement, while the electric field, which produces an axial nested-well potential profile, provides axial confinement. A self-consistent, multidimensional numerical solution for the electric potential is obtained, and a fully kinetic theoretical treatment on axial transport is used to determine an axial confinement time scale. The effect on confinement of the presence of a radial electric field is explored with the use of ion trajectory calculations. A thermal, neutral, high-charge-state plasma confined in a nested-well trap opens new possibilities for fundamental studies on plasma recombination and cross-field transport processes under highly controlled conditions.     

Stark spectrum of barium in highly excited Rydberg states

We present observations of Stark spectra of barium in highly excited Rydberg states in the energy region around n = 35. The one-photon excitation concerns the π transition. The observed Stark spectra at electric fields ranging from 0 to 60 V·cm-1 are well explained by the diagonalization of the Hamiltonian incorporating the core effects. From the Stark maps, the anti-crossings between energy levels are identified experimentally and theoretically. The time of flight spectra at the specified Stark states are recorded, where the deceleration and acceleration of barium atoms are observed. This is very consistent with the prediction derived from the Stark maps from the point of view of energy conservation.     

A new static calculation of the streamer region for long spark gaps

Different electrostatic approximations have been proposed to calculate the streamer region without going in deep details of the behavior of density of particles under the effect of high electric fields; this kind of approximations have been used in numerical calculations of long spark gaps and lightning attachment. The simplifications of the streamer region are achieved by considering it to be a geometrical region with a constant geometrical shape. Different geometrical shapes have been used, such as cones or several parallel filaments. Afterward, to simplify the procedures, the streamer region was approximated by two constants, one denoted K_Q, called the geometrical constant and in other cases K named as geometrical factor.However, when a voltage that varies with time is applied to an arrangement of electrodes (high voltage and grounded electrodes), the background electric field will change with time. Thus, if the background electric field is modified, the streamer zone could cover a larger or smaller area.With the aim of reducing the number of assumptions required in the calculation of long gap discharges, a new electrostatic model to calculate the streamer region is presented. This model considers a variable streamer zone that changes with the electric field variations. The three-dimensional region that fulfills the minimum electric field to sustain a streamer is identified for each time step, and the charge accumulated in that region is then calculated. The only parameter that is being used in the calculation is the minimum electric field necessary for the propagation of streamers.     

What Laboratory-Produced Plasma Structures Can Contribute to the Understanding of Cosmic Structures Both Large and Small

The tradition of the classical 1901 work by Birkeland [1] on aurora phenomena by laboratory terrella experiments was resumed by Alfvén [2], Cowling [3], Ferraro et al. [4], and by Bennett [5] in his terrella experiments. In 1954 [6] when experimenters accidentally produced in the laboratory structures later identified as diamagnetic vortex filaments, and in 1961 [7] when filaments, later identified as current-carrying paramagnetic plasma vortex structures (which are both electric motors and dynamos), were observed in the Z and theta-pinch experiments, this tradition was being further reestablished. It has been successfully argued [6], [8], [11], [20] that both of these types of vortices are force-free minimum-free-energy structures that spontaneously spring to life as readily as do thousands of spherical bubbles and water droplets during the splash of a breaking water wave. The Birkeland aurora filaments are a hybrid combination of these two basic types (paramagnetic and diamagnetic) of plasma vortices. It is to be expected that such structures on a cosmic scale play an important role in the cosmos, and indeed they do in the formation of galaxies, stars, binary stars, solar systems, solar prominences, solar flares, solar wind, comet tails, cosmic "strings" in the Crab nebula, string-like galactic clusters, expansion of the Universe, giant galactic jets, cosmic rays, sunspots, vortex rolls in sunspot penumbra, twinkling of radio stars by the density fluctuations in the ionosphere, turbulence at the interface between the solar wind and the earth's magnetosphere, etc.     

Harmonic mixing in a cosine potential for arbitrary damping

Harmonic mixing of two alternating electric fields due to a Brownian charged particle in a nonlinear one-dimensional potential of cosine shape is investigated. The dynamics of the system are described by a time dependent Fokker-Planck equation. The appropriate distribution function is obtained by a matrix continued fraction expansion method, which is treated numerically. The dc signal due to mixing is computed for strong thermal fluctuations in all relevant parameter ranges of the pinning potential strength, damping and frequency. The dc signal without fluctuations is discussed separately. Resonance effects are shown in the electric dc field and the additional phase shift, caused by intrinsic relaxation processes.     

Polarization kinetics of a photosensitive relaxor ferroelectric

Polarization switching in alternating quasi-static electric fields of frequency 10^?4 Hz and polarization relaxation in dc fields were studied in a photosensitive La-and Ce-doped barium-strontium niobate relaxor ferroelectric. Experimental data obtained in the thermal activation stage of the relaxation were used to reconstruct the relaxation time distribution spectrum. The characteristics of the polarization kinetics of an illuminated and a dark crystal are compared. It is shown that, in the crystal illuminated by light, the photoconductivity compensates for random electric and depolarization fields, thereby giving rise to a growth in amplitude of the dielectric hysteresis loops in the polarization versus field relation and to longer polarization relaxation times or increased heights of the potential barriers separating stable states from metastable states.