88 research outputs found
Particle-in-cell simulations of circularly polarised Alfvén wave phase mixing: A new mechanism for electron acceleration in collisionless plasmas
In this work we used Particle-In-Cell simulations to study the interaction of circularly polarised Alhén waves with one dimensional plasma density inhomogeneities transverse to the uniform magnetic field (phase mixing) in collisionless plasmas. In our preliminary work we reported discovery of a new electron acceleration mechanism, in which progressive distortion of the Alfvén wave front, due to the differences in local Alfvén speed, generates an oblique (nearly parallel to the magnetic field) electrostatic field. The latter accelerates electrons through the Landau resonance. Here we report a detailed study of this novel mechanism, including: (i) analysis of broadening of the ion distribution function due to the presence of Alfvén waves; and (ii) the generation of compressive perturbations due to both weak non-linearity and plasma density inhomogeneity. The amplitude decay law in the inhomogeneous regions, in the kinetic regime, is demonstrated to be the same as in the MHD approximation described by Heyvaerts & Priest (1983, A&A, 117, 220)
Phase mixing of shear Alfvén waves as a new mechanism for electron acceleration in collisionless, kinetic plasmas
Particle-in-cell (kinetic) simulations of shear Alfv´en wave (AW) interaction with one-dimensional, across the uniform-magnetic field, density inhomogeneity (phase mixing) in collisionless plasma were performed for the first time. As a result, a new electron acceleration mechanism is discovered. Progressive distortion of the AW front, due to the differences in local Alfv´en speed, generates electrostatic fields nearly parallel to the magnetic field, which accelerate electrons via Landau damping. Surprisingly, the amplitude decay law in the inhomogeneous regions, in the kinetic regime, is the same as in the MHD approximation described by Heyvaerts and Priest (1983 Astron. Astrophys. 117 220)
Phase mixing of Alfvén waves propagating in non-reflective magnetic plasma configurations
The ability of phase mixing to provide efficient damping of Alfvén waves even in weakly dissipative plasmas made it a popular mechanism for explaining the solar coronal heating. Initially it was studied in the equilibrium configurations with the straight magnetic field lines and the Alfvén speed only varying in the direction perpendicular to the magnetic field. Later the analysis of the Alfvén wave phase mixing was extended in various directions. In particular it was studied in two-dimensional planar magnetic plasma equilibria. Analytical investigation was carried out under the assumption that the wavelength is much smaller than the characteristic scale of the background quantity variation. This assumption enabled using the Wentzel, Kramers, and Brillouin (WKB) method. When it is not satisfied the study was only carried out numerically. In general, even the wave propagation in a one-dimensional inhomogeneous equilibrium can be only studied numerically. However there is one important exception, so-called non-reflective equilibria. In these equilibria the wave equation with the variable phase speed reduces to the Klein-Gordon equation with constant coefficients. In this paper we apply the theory of non-reflective wave propagation to studying the Alfvén wave phase mixing in two-dimensional planar magnetic plasma equilibria. Using curvilinear coordinates we reduce the equation describing the Alfvén wave phase mixing to the equation that becomes a one-dimensional wave equation in the absence of dissipation. This equation is further reduced to the equation which is the one-dimensional Klein-Gordon equation in the absence of dissipation. Then we show that this equation has constant coefficients when a particular relation between the plasma density and magnetic field magnitude is satisfied. Using the derived Klein-Gordon-type equation we study the phase mixing in various non-reflective equilibria. We emphasise that our analysis is valid even when the wavelength is comparable with the characteristic scale of the background quantity variation. In particular, we study the Alfvén wave damping due to phase mixing in an equilibrium with constant plasma density and exponentially divergent magnetic field lines. We confirm the result previously obtained in the WKB approximation that there is enhanced Alfvén wave damping in this equilibrium with the damping length proportional to ln(Re), where Re is the Reynolds number. Our theoretical results are applied to heating of coronal plumes. We show that, in spite of enhanced damping, Alfvén waves with periods of the order of one minute can be efficiently damped in the lower corona, at the height about 200 Mm, only if the shear viscosity is increased by about 6 orders of magnitude in comparison with its value given by the classical plasma theory. We believe that such increase of the shear viscosity can be provided by the turbulence
The Effect of Particle Gas Composition and Boundary Conditions on Triboplasma Generation: A Computational Study Using the Particle-in-Cell Method
Two dimensional particle in cell simulations of free charge creation by
collisional ionization of C12 and C60 molecules immersed in plasma for the
parameters of relevance to plasma gasification are presented. Our main findings
are that (i) in uniform plasmas with smooth walls two optimal values which
emerge for free electron production by collisional ionization (i.e. a most
efficient discharge condition creation) are fractions of and
, (ii) in plasmas with rough walls, modelled by comb-like electric field
at the boundary, the case of tangential electric field creates significant
charge localization in C12+ and C60+ species, again creating most favorable
discharge condition for tribo-electrically generated plasma. The numerical
simulation results are discussed with reference to recent triboelectric plasma
experiments and are corroborated by suitable analytical models.Comment: The final version accepted for publication in IEEE Trans. Plasm. Sc
Magnetic reconnection during collisionless, stressed, X-point collapse using Particle-in-Cell simulation
Two cases of weakly and strongly stressed X-point collapse were considered.
Here descriptors weakly and strongly refer to 20 % and 124 % unidirectional
spatial compression of the X-point, respectively. In the weakly stressed case,
the reconnection rate, defined as the out-of-plane electric field in the
X-point (the magnetic null) normalised by the product of external magnetic
field and Alfv\'en speeds, peaks at 0.11, with its average over 1.25 Alfv\'en
times being 0.04. Electron energy distribution in the current sheet, at the
high energy end of the spectrum, shows a power law distribution with the index
varying in time, attaining a maximal value of -4.1 at the final simulation time
step (1.25 Alfv\'en times). In the strongly stressed case, magnetic
reconnection peak occurs 3.4 times faster and is more efficient. The peak
reconnection rate now attains value 2.5, with the average reconnection rate
over 1.25 Alfv\'en times being 0.5. The power law energy spectrum for the
electrons in the current sheet attains now a steeper index of -5.5, a value
close to the ones observed in the vicinity of X-type region in the Earth's
magneto-tail. Within about one Alfv\'en time, 2% and 20% of the initial
magnteic energy is converted into heat and accelerated particle energy in the
case of weak and strong stress, respectively. In the both cases, during the
peak of the reconnection, the quadruple out-of-plane magnetic field is
generated, hinting possibly to the Hall regime of the reconnection. These
results strongly suggest the importance of the collionless, stressed X-point
collapse as a possible contributing factor to the solution of the solar coronal
heating problem or more generally, as an efficient mechanism of converting
magnetic energy into heat and super-thermal particle energy.Comment: Final Accepted Version (Physics of Plasmas in Press 2007
Non-Newtonian effects in the peristaltic flow of a Maxwell fluid
We analyzed the effect of viscoelasticity on the dynamics of fluids in porous
media by studying the flow of a Maxwell fluid in a circular tube, in which the
flow is induced by a wave traveling on the tube wall. The present study
investigates novelties brought about into the classic peristaltic mechanism by
inclusion of non-Newtonian effects that are important, for example, for
hydrocarbons. This problem has numerous applications in various branches of
science, including stimulation of fluid flow in porous media under the effect
of elastic waves. We have found that in the extreme non-Newtonian regime there
is a possibility of a fluid flow in the direction {\it opposite} to the
propagation of the wave traveling on the tube wall.Comment: to Appear in Phys. Rev. E., 01 September 2001 issu
An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts
1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to
model EM wave emission generation in the context of solar type III radio
bursts. The model studies generation of EM waves by a super-thermal, hot beam
of electrons injected into a plasma thread that contains uniform longitudinal
magnetic field and a parabolic density gradient. In effect, a single magnetic
line connecting Sun to earth is considered, for which several cases are
studied. (i) We find that the physical system without a beam is stable and only
low amplitude level EM drift waves (noise) are excited. (ii) The beam injection
direction is controlled by setting either longitudinal or oblique electron
initial drift speed, i.e. by setting the beam pitch angle. In the case of zero
pitch angle, the beam excites only electrostatic, standing waves, oscillating
at plasma frequency, in the beam injection spatial location, and only low level
EM drift wave noise is also generated. (iii) In the case of oblique beam pitch
angles, again electrostatic waves with same properties are excited. However,
now the beam also generates EM waves with the properties commensurate to type
III radio bursts. The latter is evidenced by the wavelet analysis of transverse
electric field component, which shows that as the beam moves to the regions of
lower density, frequency of the EM waves drops accordingly. (iv) When the
density gradient is removed, electron beam with an oblique pitch angle still
generates the EM radiation. However, in the latter case no frequency decrease
is seen. Within the limitations of the model, the study presents the first
attempt to produce simulated dynamical spectrum of type III radio bursts in
fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle
(non-gyrotropic) electron beam, that is an alternative to the plasma emission
classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version
Numerical simulation of the internal plasma dynamics of post-flare loops
We integrate the MHD ideal equations of a slender flux tube to simulate the
internal plasma dynamics of coronal post-flare loops. We study the onset and
evolution of the internal plasma instability to compare with observations and
to gain insight into physical processes and characteristic parameters
associated with flaring events. The numerical approach uses a finite-volume
Harten-Yee TVD scheme to integrate the 1D1/2 MHD equations specially designed
to capture supersonic flow discontinuities. We could reproduce the
observational sliding down and upwardly propagating of brightening features
along magnetic threads of an event occurred on October 1st, 2001. We show that
high--speed downflow perturbations, usually interpreted as slow magnetoacoustic
waves, could be better interpreted as slow magnetoacoustic shock waves. This
result was obtained considering adiabaticity in the energy balance equation.
However, a time--dependent forcing from the basis is needed to reproduce the
reiteration of the event which resembles observational patterns -commonly known
as quasi--periodic pulsations (QPPs)- which are related with large scale
characteristic longitudes of coherence. This result reinforces the
interpretation that the QPPs are a response to the pulsational flaring
activity.Comment: Accepted MNRAS, 10 pages, 14 figures, 1 tabl
Experimental Observation of Differences in the Dynamic Response of Newtonian and Viscoelastic Fluids
In this paper we present an experimental study of the dynamic responses of a
Newtonian fluid and a Maxwellian fluid under an oscillating pressure gradient.
We use laser Doppler anemometry in order to determine the velocity of each
fluid inside a cylindrical tube. In the case of the Newtonian fluid, the
dissipative nature is observed and the response obeys the Zhou and Sheng
universality (PRB 39, 12027 (1989)). In the dynamic response of the Maxwellian
fluid an enhancement at the frequencies predicted by the corresponding theory
(PRE 58, 6323 (1998)) is observed.Comment: 5 pages, 4 Figures, paper to be published in Phys. Rev.
The periodic variations of a white-light flare observed with ULTRACAM
High time resolution observations of a white-light flare on the active star EQ PegB show evidence of intensity variations with a period of ≈10 s. The period drifts to longer values during the decay phase of the flare. If the oscillation is interpreted as an impulsively-excited,
standing-acoustic wave in a flare loop, the period implies a loop length of ≈3.4 Mm and ≈6.8 Mm for the case of the fundamental mode and the second harmonic, respectively. However, the small loop lengths imply a very high modulation depth making the acoustic interpretation unlikely. A more realistic interpretation may be that of a fast-MHD wave, with the modulation of the emission being
due to the magnetic field. Alternatively, the variations could be due to a series of reconnection events. The periodic signature may then arise as a result of the lateral separation of individual flare loops or current sheets with oscillatory dynamics (i.e., periodic reconnection)
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