Adiabatic Motion Of Electrons Research Articles

Ninety degrees pitch angle enhancements of suprathermal electrons associated with interplanetary shocks

AbstractWe report the results of the first systematic analysis of 90° pitch angle (PA) enhancements or the ring distributions of suprathermal (E ∼70 eV–2 keV) electrons at interplanetary (IP) shocks. We analyze 2 h time intervals around 232 IP shocks observed by the two STEREO spacecraft between 2007 and 2011. The ring distributions were detected downstream of 114 events (49%). In 52 (22.4%) cases they were detected at the shock ramp. We also found 90° enhancements upstream of 11 (4.7%) events. Statistical analysis of basic shock properties did not reveal substantial differences between the shocks that are associated with the enhancements and those that are not. The data from the STEREO/WAVES instruments revealed that the 90° PA enhancements tend to be associated with magnetic and electric field fluctuations. Although at this point we do not have a satisfactory explanation for the mechanism that produces these distributions, our findings suggest that wave‐particle interactions play a role, while pure focusing and mirroring effects due to adiabatic motion of electrons across the shock fronts cannot fully account for the observations.

Inhomogeneous microstrain in cylindrical semiconductor heterostructures and its influence on the adiabatic motion of electrons

We analyze fluctuation of the layer thicknesses and its influence on the strain state of (In,Ga)As/(Al,Ga)As micro-tubes containing quantum well structures. In those structures a curved high-mobility two-dimensional electron gas (HM2DEG) is established. The layer thickness fluctuation studied by atomic force microscopy, x-ray scattering, and spatially resolved cathodoluminescence spectroscopy occurs on two different lateral length scales. On the shorter length scale of about 0.01~$\mu$m, we found from atomic force micrographs and the broadening of the satellite maxima in x-ray diffraction curves a very small value of the mean square roughness of 0.1~nm. However, on a longer length scale of about 1.0~$\mu$m, step bunching during epitaxial growth resulted in layer thickness inhomogeneities of up to 2~nm. The resulting fluctuation of the strain in the micro-tubes leads to a local variation of the chemical potential, which results in the fluctuation of the carrier density as well. This leads to a phase cancelation of the Shubnikov-de-Haas oscillations in the curved HM2DEG and a reduction of the single-electron scattering time, while the electron mobility in the structures remains high. The estimated fluctuation of the carrier density agrees well with the energy fluctuation measured in the cathodoluminescence spectra of the free-electron transition of the quantum well.

Reduced kinetic description of weakly-driven plasma waves

A model of kinetic effects in Langmuir wave dynamics is presented using a nonlinear distribution function that includes particle separatrix crossing and self-consistent electrostatic evolution. This model is based on the adiabatic motion of electrons in the wave to describe Bernstein–Greene–Kruskal-like Langmuir waves over a wide range of temperatures (0.1⩽kλD⩽0.4). The asymptotic distribution function yields a nonlinear frequency shift of the Langmuir wave that agrees well with Vlasov simulations, and can furthermore be used to determine the electrostatic energy required to develop the phase-mixed, asymptotic state. From this incoherent energy, energy conservation is employed to determine a simplified model of nonlinear Landau damping. The resulting nonlinear, dynamic frequency shift and damping are then used in an extended three-wave-type model of driven Langmuir waves and compared to Vlasov simulations in the context of backward Raman scattering.

Electrostatic potentials in the downward auroral current region

Assuming a fixed ion density, adiabatic electron motion, and quasi‐neutrality, we use the stationary Vlasov equation to derive the self‐consistent potential in an auroral flux tube that carries downward current. Our model predicts downward electric fields ∼5 mV/m at an altitude near 2000 km, and around 4000 km the potential reaches ∼2.5 kV. A weak upward electric field at high altitudes reduces the potential, and the potential difference between the ionosphere and magnetosphere is much smaller.

Dusty plasma effects in Saturn's magnetosphere

Comets, planetary rings, exposed dusty surfaces, and the zodiacal dust cloud are all examples of environments where dusty plasma effects establish the size and spatial distributions of small grains. Simultaneously, dust often influences the composition, density, and temperature of the plasma surrounding it. The dynamics of charged dust particles can be surprisingly complex and fundamentally different from the well‐understood limits of gravitationally dominated motions of neutral particles or the adiabatic motion of electrons and ions in electromagnetic fields that dominate gravity. In this review we focus on observations that are best explained by theories concerning dusty plasma effects at Saturn. In addition to presenting our current models we also discuss our expectations for new discoveries based on existing observations at Jupiter or on purely theoretical considerations. Our intent is to give an up‐to‐date overview of dusty plasma effects in Saturn's magnetosphere and to draw attention to several outstanding problems that could be resolved by the Cassini mission.

Testing loss mechanisms capable of rapidly depleting relativistic electron flux in the Earth's outer radiation belt

We investigate how relativistic electrons are lost from the Earth's magnetosphere in order to better understand the dynamic variability of the radiation belts. We identify 52 events where the >2 MeV electron flux at geostationary orbit decreases rapidly and use a superposed epoch analysis of multispacecraft data to characterize the accompanying solar wind and geomagnetic conditions and examine the relevance of potential loss mechanisms. The results show that the flux decrease events follow a common sequence. The electron flux is reduced first in the dusk sector concurrent with the stretching of the magnetic field to a more tail‐like configuration. The extreme stretching at dusk is caused by the formation of a partial ring current driven by changing solar wind conditions. We investigate three possible causes of the ensuing flux decrease: adiabatic electron motion in response to the changing magnetic field topology, drift out the magnetopause boundary, and precipitation into the atmosphere. The analysis reveals that the flux depletion is likely due to enhanced precipitation into the atmosphere, but the exact cause of the enhanced precipitation is still uncertain.

Low‐altitude distribution of radiation belt electrons

A numerical simulation of the low‐altitude electron radiation belt is described. It includes dependences on the electrons' bounce and drift phases, equatorial pitch angle, and kinetic energy in the range of ∼1 to several MeV at L = 3.5. Physical processes in addition to the adiabatic electron motion are pitch angle diffusion and backscattering from a realistic model atmosphere. Quasi‐linear diffusion coefficients are calculated from a model of the whistler mode plasmaspheric hiss wave intensity. Comparisons of the simulation results with electron data from a low‐altitude satellite show that the model accounts for the main features of the electron spatial distribution during selected periods of differing geomagnetic activity.

Electrons in a curvilinear quantum wire

The adiabatic motion of electrons in curvilinear quantum wires was studied. It was assumed that the cross section of a wire was constant along its length. The potential that limited electron motion across a wire and the shape of the cross section of the wire were considered arbitrary, while the curvature and the torsion (defined as the derivative of the cross section rotation angle with respect to the length) were assumed to be small. An effective nonrelativistic Hamiltonian for the motion of electrons along a wire with the conservation of transverse quantum numbers was obtained. The spin-orbit coupling Hamiltonian related to the curvature and torsion of a wire was found. Particular cases of a rectilinear twisted quantum wire with a noncircular cross section and a curvilinear quantum wire on a plane were studied. Various transverse potential models limiting the motion of electrons were considered. In particular, the coefficients of the effective Hamiltonian for quantum wires with rectangular and circular cross sections and hard walls and for wires with a parabolic potential were found.

Observations of electron conies by the Viking satellite

Electron angular distributions peaked at oblique angles to the magnetic field, electron conies, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conies have been observed by Viking as well as by other satellites. One type is frequently seen below the acceleration region. It is caused by adiabatic motion of electrons with a field‐aligned distribution that has a broader angular width than the loss cone. The atmospheric loss of particles will result in a distribution peaked at pitch angles close to the loss cone. Another type is due to heating or acceleration processes at low altitudes and is seen above the acceleration region. Particle and wave characteristics and possible generation mechanisms for this type of electron conic are discussed. The ion arid electron angular distributions observed by Viking indicate that a parallel electric field is present below Viking during this type of event. Ion observations give important information on the processes that produce the electron conies. Almost 200 electron conic events have been detected in the Viking data, that is, in about every third orbit that was studied. Their frequency of occurrence in altitude maximizes in the upper part of the acceleration region, between 10,000 and 11,000 km. The number of events are found to decrease at higher altitudes. This might indicate that diffusion processes, associated with wave generation, remove the anisotropy in the electron distribution function in and above the acceleration region. Wave observations show the presence of both low‐frequency waves and waves close to the electron gyro frequency. Acceleration in a fluctuating (approximately 1 Hz) parallel electric field is suggested as a likely mechanism to create the electron conies observed by Viking. The period of the fluctuations is comparable to the travel time, below the acceleration region, of the electrons forming the conic distribution.

Ballistic electron transport in quasi-one-dimensional systems

Results of low-temperature resistance measurements performed upon short, narrow channels defined in the two-dimensional electron gas (2DEG) of GaAs-AlGaAs heterostructures are presented. The observed quantisation of resistance has been rigorously derived in recent theoretical models, notably the adiabatic model of electronic transport and the wide-narrow-wide geometry. The predictions of these two models are compared and their application to the experimental results is discussed. The application of a transverse magnetic field gives rise to the phenomenon of magnetic depopulation. A simple variational model has been used to model this behaviour and the derived results are contrasted with previous calculations. The additional spin-split plateaux observed in a parallel magnetic field have been used to determine the Landé g-factor and suggest that there is little or no enhancement. Results are also presented for a device which manifested oscillatory behaviour as a function of applied gate voltage and the observed Aharonov-Bohm oscillations have been used to characterise the device. The non-ohmic addition of resistance which has been observed in structures with two defined constrictions is discussed within the context of a recent theoretical model and is explained in terms of the adiabatic motion of electrons through the channel constriction.

Latitudinal structures of discrete arcs resulting from viscous interaction between sheared plasma flows

Latitudinal structures of discrete arcs are modelled as a consequence of the quasi-steady magnetosphere-ionosphere coupling involving viscous interaction between sunward and anti-sunward plasma flows in the magnetosphere. The quasi-steady state in the magnetosphere and ionosphere coupling is described by the magnetospheric and ionospheric current conservation and the field-aligned currentpotential relation assuming adiabatic electron motion along field lines. The upward and downward fieldaligned currents are assumed to be stably maintained by vorticity-induced space charges in the region of plasma flow reversal, where divergence of the magnetospheric electric field E ⊥ is negative and positive, respectively. By introducing the effective conductance Σ dc arising from the anomalous viscosity, a specific relation between the dc field-aligned current density J ∥ and the magnetospheric electric field E ⊥ is derived as J ∥=−Σ dcdiv E ⊥ . Sufficiently large potential drops to accelerate auroral electrons are shown to exist along the auroral field lines originating from the flow reversal region with div E ⊥ < 0 . It is shown that the latitudinal structure of a discrete arc is primarily determined by the magnetospheric potential structure and the characteristic width is on the order of 10 km at the ionospheric altitude.

Stability of a thick two-dimensional quasineutral sheet

The stability of a two-dimensional magnetic model of a thick quasineutral sheet including a small but finite normal magnetic field component is studied in a collisionless plasma. Such a magnetic configuration allows an adiabatic motion for electrons which are trapped in good agreement with the thick sheet model. A detailed kinetic study of the energy balance shows that the ion linear tearing mode instability is suppressed by the interaction between the perturbed electromagnetic field and the adiabatic electron motion; this interaction leads to a ‘‘compressibility’’ effect which prevents the instability onset. The connection with the magnetohydrodynamic energy principle for stability is demonstrated. A general formalism is developed for any magnetic configuration and applied to a parabolic and a magnetic island topology. Some conclusions concerning the nonlinear tearing mode behavior (explosive phase) are drawn.

Plasma heating from upper-hybrid mode conversion in an inhomogeneous magnetic field

Large-amplitude short-wavelength electrostatic fields can be generated at the upper-hybrid resonance layer due to the mode conversion process when an incident wave of appropriate frequency falls on a plasma. As a consequence, a host of nonlinear and kinetic phenomena can take place. Among them, the generation of energetic electrons due to the breaking of adiabatic electron motion in the wave field, the enhancement of electron second harmonic cyclotron heating at the double resonance (ωuh = 2ωce), and parametric instabilities in which the upper-hybrid mode decays into another upper-hybrid mode and a low-frequency mode. Investigations of these phenomena through the use of computer simulations are presented. The parametric process is observed to produce a substantial number of energetic ions localized at the resonance layer.