Stochastic Heating Mechanism Research Articles

How Hall electric fields intrinsically chaotize and heat ions during collisionless magnetic reconnection

The origin of extreme and anomalous ion heating during magnetic reconnection has been a long-standing mystery. It is shown in this paper that, at the core of the process, the stochastic heating mechanism is responsible for this energization. The in-plane Hall electric fields that intrinsically develop during collisionless reconnection not only accelerate ions but also thermalize them by inducing chaotic motion. The mechanism explains various observed features of ion heating, e.g., its relation to the guide field strength and the ion mass-to-charge ratio.

New features of ion acoustic waves in inhomogeneous and permeating plasmas

It is generally accepted that the ion acoustic (IA) wave in plasmas containing ions and electrons with the same temperature is of minor importance due to strong damping of the wave by hot resonant ions. In this work it will be shown that the IA wave is susceptible to excitation even in plasmas with hot ions when both an electromagnetic transverse wave and a background density gradient are present in the plasma, and in addition the wave is in fact unstable (i.e., growing) in the case of permeating homogeneous plasmas. The multi-component fluid theory is used to describe the IA wave susceptibility for excitation in inhomogeneous plasmas and its coupling with electromagnetic waves. The growing IA wave in permeating homogeneous plasmas is described by the kinetic theory. In plasmas with density and temperature gradients the IA wave is effectively coupled with the electromagnetic waves. In comparison to ordinary IA wave in homogeneous plasma, the Landau damping of the present wave is much smaller, and to demonstrate this effect a simple but accurate fluid model is presented for the Landau damping. In the case of permeating plasmas, a kinetic mechanism for the current-less IA wave instability is presented, with a very low threshold for excitation as compared with ordinary electron-current-driven kinetic instability. Such growing IA waves can effectively heat plasma in the upper solar atmosphere by a stochastic heating mechanism presented in the work. The results of this work suggest that the IA wave role in the heating of the solar atmosphere (chromosphere and corona) should be reexamined.

A stochastic mechanism of electron heating

Due to Landau resonant interaction with lower hybrid waves in the lower hybrid current drive scheme part of electrons are accelerated and, as a result of this, a tail of energetic electrons is formed on the electron distribution function. The same situation takes place in the problem of type III radio bursts when the suprathermal burst electrons acquire a plateau distribution due to excitation of plasma waves in the solar wind plasma. These distributions are unstable with respect to the cyclotron excitation of waves at anomalous Doppler resonance (“fan” instability). In this case, the tail electrons interact simultaneously with both (i) waves that accelerate or decelerate them (Čerenkov resonance) and (ii) waves excited in the process of the fan instability that led to their pitch angle diffusion. Because velocity diffusion lines of electrons formed due to heir interaction with each type of waves intersect, this interaction can lead not only to pitch angle diffusion but also to heating of electrons mainly in perpendicular direction. We investigated this mechanism of electron heating and studied the temporal evolution of the electron temperature and the energy of excited waves. Our results show significant enhancement of the electron perpendicular temperature T⊥ due to this stochastic heating mechanism.

Growing electric field parallel to magnetic field due to transverse kinetic drift waves in inhomogeneous corona

The transverse drift wave, which is unstable due to purely kinetic effects, and driven by the density and magnetic field gradients, is discussed in context of its application to the solar corona. The gradients of the two quantities are opposite to each other, as required by the equilibrium pressure balance, and they are in the plane perpendicular to the magnetic field vector. The transverse drift wave has such properties that it propagates strictly perpendicularly to both the magnetic field vector and the mentioned gradients. It is electromagnetic, with the perturbed electric field in the direction of the equilibrium magnetic field, while the perturbed magnetic field vector is in the direction of the equilibrium gradients. Such an orientation of the electric field implies a possibility of acceleration of coronal plasma particles along the background magnetic field, in both directions, outward and inward. In the case of locally open magnetic structures, the outwardly moving particles should contribute to the solar wind. Those moving inwards eventually arrive in the lower solar atmosphere where the mean free path is short and, due to collisions, they should disperse their energy to the surrounding plasma and contribute to heating. It is also shown that accelerated particles can additionally be stochastically heated by the wave. This completely new stochastic heating mechanism is found here for the first time. It takes place provided that the particles are simultaneously accelerated by the wave to large enough velocities in the parallel direction. The model is applicable to any inhomogeneous coronal environment, like magnetic loops, coronal holes and the so-called EIT waves, named after the Extreme-ultraviolet Imaging Telescope (EIT) used for their first detection.

Electron stochastic heating in a capacitively coupled low-pressure argon rf-discharge

This paper reports on the kinetic treatment of the electron stochastic heating mechanism in rf-discharges, which can be described by the diffusion coefficient in energy space. The influence of electron stochastic heating on the formation of the electron distribution function (EDF) at low gas pressures has been analyzed by the numerical solution of the Boltzmann kinetic equation. Comparison of the modeling results with the experimentally measured EDFs under similar conditions reveals a good agreement between theoretical and experimental EDFs.

The electron diffusion coefficient in energy in bounded collisional plasmas

The electron energies in typical gas discharge plasmas do not exceed significantly the first ionization potential. This being the case, the momentum relaxation in collisions with neutrals is significantly faster than the energy relaxation due to collisions. It follows that the main part of the electron distribution function (EDF) is isotropic. So the interaction of an electron with an electric field is predominantly stochastic random walk process and can be described by a diffusion coefficient in energy Depsiv. Both collisional and stochastic heating mechanisms can be incorporated in it. By the proper choice of variables, the electron Boltzmann equation can be reduced to the standard diffusion one, both in space and in energy. This approach is very efficient in solution of the problems of the electron kinetics in bounded nonuniform plasmas. Some paradoxical effects, such as the formation of a cold electron population in discharges with peripheral energy input, and nonmonotonic radial profiles of the excitation rates, are explained within this framework. The expressions for Depsiv in different discharges are presented. The history of the EDF nonlocality concept is discussed for stationary gas discharges

Collisionless electron heating by capacitive radio-frequencyplasma sheaths

Low-pressure capacitive rf plasmas can be maintained chiefly bycollisionless heating in the rf modulated sheaths adjacent to the electrodes.Theoretical models dealing with this mechanism are often based on a `hardwall' approximation where the electrons are considered to collide elasticallywith the oscillating sheath edge. The power transfer is then calculated byaveraging forward and reverse power fluxes over an rf period. There are,however, several drawbacks to this approach: the models are sensitive toassumptions regarding the incident electron distribution, transit time effectsin the sheath electric field are neglected, electron loss is not consideredand current conservation is not satisfied. In order to examine the validity ofthe theoretical models, we use a Monte Carlo approach to study electroninteractions with the model and self-consistent fields providing modificationsthat can lead to a more consistent treatment of the electron dynamics insidethe sheath. Of particular importance is the presence of a small field behindthe moving electron sheath edge which maintains quasi-neutrality between theelectron sheath position and the bulk plasma. In addition, a semi-infiniteparticle-in-cell (PIC) simulation is used to investigate in detail sheathdynamics. The errors that the `hard wall' approximation gives are calculatedand power deposition scalings with current drive, frequency and electrontemperature are provided. Our results indicate that collisionless heatingcannot be attributed to the stochastic heating mechanism based on the `hardwall' approximation and that in contrast electron inertia plays a dominantrole as far as collisionless heating is concerned.

The operation of stochastic heating mechanisms in an electromagnetic standing wave configuration

The possibility of the operation of stochastic heating mechanisms of charged particles in a configuration consisting of a left-handed circularly polarized standing electromagnetic wave and a uniform magnetic field, has been studied numerically and theoretically. It is found that such a configuration induces stochasticity, the threshold of which is dependent on two independent parameters, determined by the frequency and the amplitude of the wave and the strength of the magnetic field. From the theoretical analysis, it emerges that the origin of onset of large-scale stochasticity is the destabilization of fixed points associated with an equation describing the motion of the particles in an electrostatic-type potential having standing wave characteristics. The comparison of the theoretical predictions with the numerical results is found to be quite satisfactory. Possible applications to realistic plasmas have been discussed.

Random fluctuations versus Poynting–Robertson drag on interplanetary dust grains

A particle moving through a sea of stochastic impulses gains energy from the sea; this is a long-established property of brownian motion, given theoretical backing in the fluctuation–dissipation theorem1. However, this energy gain is generally neglected in describing circumstellar dust grains as slowly spiralling into the central star under Poynting–Robertson (P–R) drag. Stochastic forces on circumsolar grains arise from low-frequency fluctuations in surface charge and interplanetary fields, as has long been recognized2,3, but previous studies have treated the mean orbital energy as constant4,5 or slowly decreasing6. I show here that stochastic heating can balance and indeed overwhelm the dissipative P–R drag for small grains (≲1 µm scale radius). Furthermore, the consequent drift to the outer Solar System may cancel the strong collisional source inferred7 for micrometre and sub-micrometre fragments, explaining how their present distribution is stable in time. More generally, the stochastic heating mechanism combined with collisional fragmentation sets limits on dust accretion onto cool stars.