Excitation Of Plasma Instabilities Research Articles

Force-free magnetic flux ropes: String confinement of super-strong magnetic fields and flare energy release

ABSTRACT A new class of force-free solutions for a horizontal magnetic filament with a circular cross-section is found, in which the magnetic field strength on the axis significantly (up to 2–3 orders of magnitude and more) exceeds the strength of the longitudinal external field that keeps the rope from lateral expansion. A weak transverse field leads to a small deviation from the force-free field structure and results in a density deficit and an increase in temperature on the rope axis. The possibility of a flare release of magnetic energy is shown when critical values ​​of the longitudinal electric current density in the filament are reached at which ‘anomalous resistance’ occurs, caused by the development of the current ion-sound plasma instability. It turns out to be much larger than the usual Coulomb resistance. The following values are determined: the scale of the current dissipation region, the electric field strength in it, which significantly exceeds the Dreicer value, and the possible energy of accelerated charged particles (up to hundreds of MeV). The critical density of the longitudinal current at which the plasma turbulence is excited does not depend on the presence of a super-strong field on the flux-rope axis, so that the current density depends only on the electron concentration, temperature, and anomalous conductivity. However, super-strong magnetic fields can manifest themselves in the fact that, in their presence, the excitation of plasma instabilities can occur at sufficiently high electron concentrations. This effect may explain the large number of accelerated particles sometimes observed in solar flares.

On a force balance and role of cathode plasma in Hall effect thrusters

The cathode plasma is a specific transition region in the Hall effect thruster (HET) discharge that localizes between the strongly magnetized acceleration layer (magnetic layer or B-layer) and non-magnetized exhaust plume. Cathode plasma provides a flow of electron current that supplies losses in the magnetic layer (due to ionization, excitation, electron-wall interactions, etc). The electrons’ transport in this region occurs in collisionless mode through the excitation of plasma instabilities. This effect is also known as ‘anomalous transport/conductivity’. In this work, we present the results of a 2D (drift-plane) kinetic simulation of the HET discharge, including the outside region that contains cathode plasma. We discuss the process of cathode plasma formation and the mechanisms of ‘anomalous transport’ inside it. We also analyze how fluid force balance emerges from collisionless kinetic approach. The acceleration mechanism in HETs is commonly described in terms of force balance. Namely, the reactive force produced by accelerated ions has the same value as Ampère’s force acting on a drift current loop. This balance written in integral form provides the basis for quantitative estimations of HETs’ parameters and scaling models.

Kinetic effects of the dust charge fluctuations on the instability of dust ion-acoustic waves

In this paper, a kinetic description is used to derive the perturbed dust grain currents carried by the plasma particles, taking into account dust charge fluctuations. The longitudinal dielectric permittivity and the dispersion relation of dust ion-acoustic waves in an unmagnetized dusty plasma are obtained. It is shown that the dust charge fluctuations effectively modify the damping rate of these waves, which can lead to an excitation of instability in plasma. This instability is due to the thermal velocity of the plasma particles and dust charge fluctuations. It is different from the instability due to the drift speed of the plasma particles. It is found that there is a critical wave number above which these waves are unstable. In addition, the growth rate of these waves is numerically investigated for different plasma parameters. The present theory is applicable in astronomers and space scientists working on dusty plasmas, especially planetary ring systems and cometary tails, where dust charge fluctuations are important.

Modeling of flare filaments with a helical structure of the magnetic field

An exact solution to magnetohydrostatic equations has been obtained, which describes the magnetic and thermodynamic structure of two long parallel magnetic ropes located in the chromosphere one above the other. The plasma density in the upper filament is much higher than the density of the environment, and it is much lower in the lower filament. The structure provides the basis for a comprehensive description of a complex active phenomenon (coronal mass ejection and flare), which took place in the southwestern sector of the solar disk on June 7, 2011. It is assumed that the physical mechanism of energy release in the flare was not the reconnection of magnetic field lines, but was provided by the excitation of plasma instabilities in the lower rarified magnetic rope, when the current velocity of electrons reached a critical high level in some part of its volume.

Solar eclipse‐induced E‐region plasma irregularities observed by the Gadanki radar

We investigate the generation of low latitude E‐region plasma density irregularities that were triggered during a solar eclipse on 11 August 1999. The observations were made using the Gadanki radar. The present study shows that a solar eclipse can provide night‐like ionospheric conditions which allow the excitation of plasma instabilities and the generation of irregularities in the E‐region at multiple height regions, as happens routinely during post‐sunset hours. We surmise that the echo layers were associated with long‐lived metallic ion layers that were made manifest when the more abundant ordinary molecular ions disappeared through recombination in the absence of photoionization. We propose that the layers turned unstable via the gradient‐drift instability mechanism. This is the first time that a solar eclipse has been conclusively shown to generate E‐region plasma irregularities.

Excitation of plasma waves by unstable photoelectron and thermal electron populations on closed magnetic field lines in the Martian ionosphere

Abstract. It is argued that anisotropic electron pitch angle distributions in the closed magnetic field regions of the Martian ionosphere gives rise to excitation of plasma instabilities. We discuss two types of instabilities that are excited by two different populations of electrons. First, the generation of Langmuir waves by photoelectrons with energies of the order of 10eV is investigated. It is predicted that the measured anisotropy of their pitch angle distribution at the heights z≈400km causes excitation of waves with frequencies f~30kHz and wavelengths λ~30m. Near the terminators the instability of the electrostatic waves with frequencies of the order of or less than the electron gyrofrequency exited by thermal electrons is predicted. The typical frequencies of these waves depend on the local magnitude of the magnetic field and can achieve values f~3-5kHz above strong crustal magnetic fields.

Electron and Ion Runaway in a Fully Ionized Gas. II

The treatment presented in an earlier paper is extended to give a more exact estimate of the particle runaway rate in a fully ionized gas under the action of a weak applied electric field. By analyzing the motion of particles in various regions of velocity space, it is shown that in any weak applied electric field some particles will always run away. The rate at which this occurs is determined by the flow of particles from the collision-dominated to the electric-field-dominated region of velocity space. The probability, $Q(\ensuremath{\tau})$, of electron runaway as a function of time is calculated with the help of the Boltzmann-Fokker-Planck equation and can be expressed in the form $Q(\ensuremath{\tau})=1\ensuremath{-}\mathrm{exp}(\ensuremath{-}{\ensuremath{\lambda}}_{1}\ensuremath{\tau})$. The runaway rate, ${\ensuremath{\lambda}}_{1}$, is presented as a function of applied electric field, and the plasma temperature and density. It exceeds by several orders of magnitude the rate recently proposed by Harrison. The runaway rate for positive ions is shown to be exceedingly small compared to ${\ensuremath{\lambda}}_{1}$, in the circumstances usually encountered.A brief discussion is devoted to the experimental evidence, the effects of magnetic fields, and the excitation of plasma instabilities. The correction which particle runaway introduces in the equation of pressure balance is presented for the case of a static pinched discharge.