arXiv subjects

David Tsiklauri

Publications and source records attributed to David Tsiklauri.

At least 19 recordsLinked to original sources

arXiv2022

The Effects of Oscillations & Collisions of Emerging Bipolar Regions on the Triggering of Solar Flares

The ability to predict the occurrence of solar flares in advance is important to humankind due to the potential damage they can cause to Earth's environment and infrastructure. It has been shown in Kusano et al. (2012) that a small-scale bipolar region (BR), with its flux reversed relative to the potential component of the overlying field, appearing near the polarity inversion line (PIL) is sufficient to effectively trigger a solar flare. In this study we perform further 3D magnetohydrodynamic simulations to study the effect that the motion of these small-scale BRs has on the effectiveness of flare triggering. The effect of two small-scale BRs colliding is also simulated. The results indicate that the strength of the triggered flare is dependent on how much of the overlying field is disrupted by the BR. Simulations of linear oscillations of the BR showed that oscillations along the PIL increase the flare strength whilst oscillations across the PIL detract from the flare strength. The flare strength is affected more by larger amplitude oscillations but is relatively insensitive to the frequency of oscillations. In the most extreme case the peak kinetic energy of the flare increased more than threefold compared to a non-oscillating BR. Simulations of torsional oscillations of the BR showed a very small effect on the flare strength. Finally, simulations of colliding BRs showed the generation of much stronger flares as the flares triggered by each individual BR coalesce. These results show that significantly stronger flares can result from motion of the BR along the PIL of a sheared field or from the presence of multiple BRs in the same region.

astro-ph.SR↗

arXiv2022

Enhanced Phase Mixing of Torsional Alfvén Waves in Stratified and Divergent Solar Coronal Structures, Paper I: Linear Solutions

We derive a corrected analytical solution for the propagation and enhanced phase mixing of torsional Alfvén waves, in a potential magnetic field with exponentially divergent field lines, embedded in a stratified solar corona. Further we develop a code named TAWAS which calculates the analytic solution describing torsional Alfvén waves using IDL software language. We then use TAWAS to demonstrate that both our correction to the analytic solution and the inclusion of wave reflection have a significant impact on Alfvén wave damping. We continue to utilise TAWAS by performing a parameter study in order to identify the conditions under which enhanced phase mixing is strongest. We find that phase mixing is the strongest for high frequency Alfvén waves in magnetic fields with highly divergent field lines and without density stratification. We then present a finite difference solver, Wigglewave, which solves the linearised evolution equations for the system directly. Comparing solutions from TAWAS and Wigglewave we see that our analytical solution is accurate within the limits of the WKB approximation but under-reports the wave damping, caused by enhanced phase mixing, beyond the WKB limit. Both TAWAS and Wigglewave solve the linearised governing equations and not the complete nonlinear MHD equations. Paper II will consider simulations that solve the full MHD equations including important nonlinear effects.

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arXiv2022

Enhanced Phase Mixing of Torsional Alfvén Waves in Stratified and Divergent Solar Coronal Structures, Paper II: Nonlinear Simulations

We use MHD simulations to detect the nonlinear effects of torsional Alfvén wave propagation in a potential magnetic field with exponentially divergent field lines, embedded in a stratified solar corona. In Paper I we considered solutions to the linearised governing equations torsional Alfvén wave propagation and showed, using a finite difference solver we developed named WiggleWave, that in certain scenarios wave damping is stronger than what would be predicted by our analytic solutions. In this paper we consider whether damping would be further enhanced by the presence of nonlinear effects. We begin by deriving the nonlinear governing equations for torsional Alfvén wave propagation and identifying the terms that cause coupling to magnetosonic perturbations. We then compare simulation outputs from an MHD solver called Lare3d, which solves the full set of nonlinear MHD equations, to the outputs from WiggleWave to detect nonlinear effects such as: the excitation of magnetosonic waves by the Alfvén wave, self-interaction of the Alfvén wave through coupling to the induced magnetosonic waves, and the formation of shock waves higher in the atmosphere caused by the steepening of these compressive perturbations. We suggest that the presence of these nonlinear effects in the solar corona would lead to Alfvén wave heating that exceeds the expectation from the phase mixing alone.

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arXiv2021

A potential magnetic field calculator for solar physics applications using staggered grids

A program has been designed for accurately generating a potential magnetic field on a staggered grid by extrapolating the magnetic field normal to the photospheric surface. The code first calculates a magnetic potential using the Green's function method and then uses a finite differencing scheme to calculate the magnetic field from the potential. A new finite differencing formula was derived which accounts for grid staggering, it is shown that this formula gives a numerical approximation that is closest to the real potential field. It is also shown that extending the region over which normal photospheric field is specified can improve the accuracy of the potential field produced. The program is a FORTRAN 90 code that can be used to generate potential magnetic field inputs for Lare3d and other MHD solvers that use a staggered grid for magnetic field components. The program can be parallelised to run quickly over multiple computing cores. The code and supporting description are provided in the appendices and can also be found at https://github.com/calboo/Potential_Field_Calculator.

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arXiv2019

The effect of a longitudinal density gradient on electron plasma wake field acceleration

Three dimensional, particle-in-cell, fully electromagnetic simulations of electron plasma wake field acceleration in the blow out regime are presented. Earlier results are extended by (i) studying the effect of longitudinal density gradient; (ii) avoiding use of co-moving simulation box; (iii) inclusion of ion motion; and (iv) studying fully electromagnetic plasma wake fields. It is established that injecting driving and trailing electron bunches into a positive density gradient of ten-fold increasing density over 10 cm long Lithium vapor plasma, results in spatially more compact and three times larger, compared to the uniform density case, electric fields ($-6.4 \times 10^{10}$ V/m), leading to acceleration of the trailing bunch up to 24.4 GeV (starting from initial 20.4 GeV), with an energy transfer efficiencies from leading to trailing bunch of 75 percent. In the uniform density case $-2.5 \times 10^{10}$ V/m wake is created leading to acceleration of the trailing bunch up to 22.4 GeV, with an energy transfer efficiencies of 65 percent. It is also established that injecting the electron bunches into a negative density gradient of ten-fold decreasing density over 10 cm long plasma, results in spatially more spread and two-and-half smaller electric fields ($-1.0 \times 10^{10}$ V/m), leading to a weaker acceleration of the trailing bunch up to 21.4 GeV, with an energy transfer efficiencies of 45 percent. Inclusion of ion motions into consideration shows that in the plasma wake ion number density can increase over few times the background value. It is also shown that transverse electromagnetic fields in plasma wake are of the same order as the longitudinal (electrostatic) ones.

physics.plasm-ph↗

arXiv2018

New regime of plasma wake field acceleration in the extreme blowout regime

Three dimensional particle in cell simulations are used for studying proton driven plasma wake-field acceleration that uses a high-energy proton bunch to drive a plasma wake-field for electron beam acceleration. A new parameter regime was found which generates essentially constant electric field that is three orders magnitudes larger than that of AWAKE design, i.e. of the order of $2 \times 10^{3}$ GV/m. This is achieved in the the extreme blowout regime, when number density of the driving proton bunch exceeds plasma electron number density 100 times.

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arXiv2018

Association between Tornadoes and Instability of Hosting Prominence

We studied the dynamics of all prominence tornadoes detected by the Solar Dynamics Observatory/Atmospheric Imaging Assembly from 2011 January 01 to December 31. In total, 361 events were identified during the whole year, but only 166 tornadoes were traced until the end of their lifetime. Out of 166 tornadoes, 80 (48%) triggered CMEs in hosting prominences, 83 (50%) caused failed coronal mass ejections (CMEs) or strong internal motion in the prominences, and only 3 (2%) finished their lifetimes without any observed activity. Therefore, almost all prominence tornadoes lead to the destabilization of their hosting prominences and half of them trigger CMEs. Consequently, prominence tornadoes may be used as precursors for CMEs and hence for space weather predictions.

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arXiv2018

Differences in 1D electron plasma wake field acceleration in MeV versus GeV and linear versus blowout regimes

In some laboratory and most astrophysical situations plasma wake-field acceleration of electrons is one dimensional, i.e. variation transverse to the beam's motion can be ignored. Thus, one dimensional (1D), particle-in-cell (PIC), fully electromagnetic simulations of electron plasma wake field acceleration are conducted in order to study the differences in electron plasma wake field acceleration in MeV versus GeV and linear versus blowout regimes. First, we show that caution needs to be taken when using fluid simulations, as PIC simulations prove that an approximation for an electron bunch not to evolve in time for few hundred plasma periods only applies when it is sufficiently relativistic. This conclusion is true irrespective of the plasma temperature. We find that in the linear regime and GeV energies, the accelerating electric field generated by the plasma wake is similar to the linear and MeV regime. However, because GeV energy driving bunch stays intact for much longer time, the final acceleration energies are much larger in the GeV energies case. In the GeV energy range and blowout regime the wake's accelerating electric field is much larger in amplitude compared to the linear case and also plasma wake geometrical size is much larger. Thus, the correct positioning of the trailing bunch is needed to achieve the efficient acceleration. For the considered case, optimally there should be approximately $(90-100) c/ω_{pe}$ distance between trailing and driving electron bunches in the GeV blowout regime.

physics.plasm-ph↗

arXiv2017

Electron plasma wake field acceleration in solar coronal and chromospheric plasmas

Three dimensional, particle-in-cell, fully electromagnetic simulations of electron plasma wake field acceleration applicable to solar atmosphere are presented. It is established that injecting driving and trailing electron bunches into solar coronal and chromospheric plasmas, results in electric fields ($-(20-5) \times 10^{6}$ V/m), leading to acceleration of the trailing bunch up to 52 MeV, starting from initial 36 MeV. The results provide one of potentially important mechanisms for the extreme energetic solar flare electrons, invoking plasma wake field acceleration.

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arXiv2016

Role of Electron Inertia and Reconnection Dynamics in a Stressed X-point Collapse with a Guide-Field

In previous simulations of collisionless 2D magnetic reconnection it was consistently found that the term in the generalised Ohm's law that breaks the frozen-in condition is the divergence of the electron pressure tensor's non-gyrotropic components. A fully relativistic particle-in-cell (PIC) code was used to model $X$-point collapse with a guide-field in two and three spatial dimensions. We show that in a 2D $X$-point collapse with a guide-field close to the strength of the in-plane field, the increased induced shear flows along the diffusion region lead to a new reconnection regime in which electron inertial terms play a dominant role at the $X$-point. This transition is marked by the emergence of a magnetic island - and hence a second reconnection site - as well as electron flow vortices moving along the current sheet. The reconnection electric field at the $X$-point is shown to exceed all lower guide-field cases for a brief period, indicating a strong burst in reconnection. By extending the simulation to three spatial dimensions it is shown that the locations of vortices along the current sheet (visualised by their $Q$-value) vary in the out-of-plane direction, producing tilted vortex tubes. The vortex tubes on opposite sides of the diffusion region are tilted in opposite directions, similarly to bifurcated current sheets in oblique tearing-mode reconnection. The tilt angles of vortex tubes were compared to a theoretical estimation and were found to be a good match. Particle velocity distribution functions for different guide-field runs, for 2.5D and 3D simulations, are analysed and compared.

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arXiv2015

The effects of ion mass variation and domain size on octupolar out-of-plane magnetic field generation in collisionless magnetic reconnection

J. Graf von der Pahlen and D. Tsiklauri, Phys. Plas. 21, 060705 (2014), established that the generation of octupolar out-of-plane magnetic field structure in a stressed $X$-point collapse is due to ion currents. The field has a central region, comprising of the well-known qaudrupolar field (quadrupolar components), as well as four additional poles of reversed polarity closer to the corners of the domain (octupolar components). In this extended work, the dependence of the octupolar structure on domain size and ion mass variation is investigated. Simulations show that the strength and spatial structure of the generated octupolar magnetic field is independent of ion to electron mass ratio. Thus showing that ion currents play a significant role in out-of-plane magnetic structure generation in physically realistic scenarios. Simulations of different system sizes show that the width of the octupolar structure remains the same and has a spacial extent of the order of the ion inertial length. The width of the structure thus appears to be independent on boundary condition effects. The length of the octupolar structure however increases for greater domain sizes, prescribed by the external system size. This was found to be a consequence of the structure of the in-plane magnetic field in the outflow region halting the particle flow and thus terminating the in-plane currents that generate the out-of-plane field. The generation of octupolar magnetic field structure is also established in a tearing-mode reconnection scenario. The differences in the generation of the octupolar field and resulting qualitative differences between $X$-point collapse and tearing-mode are discussed.

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arXiv2014

Whistler wave generation by non-gyrotropic, relativistic, electron beams

Particle-in-cell code, EPOCH, is used for studying features of the wave component evident to propagate backwards from the front of the non-gyrotropic, relativistic beam of electrons injected in the Maxwellian, magnetised background plasma with decreasing density profile. According to recent findings presented in Tsiklauri (2011), Schmitz & Tsiklauri (2013) and Pechhacker & Tsiklauri (2012), in a 1.5-dimensional magnetised plasma system, the non-gyrotropic beam generates freely escaping electromagnetic radiation with properties similar to the Type-III solar radio bursts. In this study the backwards propagating wave component evident in the perpendicular components of the elecromagnetic field in such a system is presented for the first time. Background magnetic field strength in the system is varied in order to prove that the backwards propagating wave's frequency, prescribed by the whistler wave dispersion relation, is proportional to the specified magnetic field. Moreover, the identified whistlers are shown to be generated by the normal Doppler-shifted relativistic resonance. Large fraction of the energy of the perpendicular electromagnetic field components is found to be carried away by the whistler waves, while a small but sufficient fraction is going into L- and R- electromagnetic modes.

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arXiv2013

The effect of electron beam pitch angle and density gradient on solar type III radio bursts

1.5D Particle-In-Cell simulations of a hot, low density electron beam injected into magnetized, maxwellian plasma were used to further explore the alternative non-gyrotropic beam driven electromagnetic emission mechanism, first studied in Tsiklauri (2011). Variation of beam injection angle and background density gradient showed that the emission process is caused by the perpendicular component of the beam injection current, whereas the parallel component only produces Langmuir waves, which play no role in the generation of EM waves in our mechanism. Particular emphasis was put on the case, where the beam is injected perpendicularly to the background magnetic field, as this turned off any electrostatic wave generation along the field and left a purely electromagnetic signal in the perpendicular components. The simulations establish the following key findings: i) Initially waves at a few w_ce/gamma are excited, mode converted and emitted at w_pe ii) The emission intensity along the beam axis is proportional to the respective component of the kinetic energy of the beam; iii) The frequency of the escaping EM emission is independent of the injection angle; iv) A stronger background density gradient causes earlier emission; v) The beam electron distribution function in phase space shows harmonic oscillation in the perpendicular components at the relativistic gyrofrequency; vi) The requirement for cyclotron maser emission, df/dv_perp > 0, is fulfilled; vii) The degree of linear polarization of the emission is strongly dependent on the beam injection angle; viii) The generated electromagnetic emission is left-hand elliptically polarized as the pitch angle tends to 90 deg; ix) The generated electromagnetic energy is of the order of 0.1% of the initial beam kinetic energy.

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arXiv2013

Electron cyclotron maser emission mode coupling to the z-mode on a longitudinal density gradient in the context of solar type III bursts

A beam of super-thermal, hot electrons was injected into maxwellian plasma with a density gradient along a magnetic field line. 1.5D particle-in-cell simulations were carried out which established that the EM emission is produced by the perpendicular component of the beam injection momentum. The beam has a positive slope in the distribution function in perpendicular momentum phase space, which is the characteristic feature of a cyclotron maser. The cyclotron maser in the overdense plasma generates emission at the electron cyclotron frequency. The frequencies of generated waves were too low to propagate away from the injection region, hence the wavelet transform shows a pulsating wave generation and decay process. The intensity pulsation frequency is twice the relativistic cyclotron frequency. Eventually, a stable wave packet formed and could mode couple on the density gradient to reach frequencies of the order of the plasma frequency, that allowed for propagation. The emitted wave is likely to be a z-mode wave. The total electromagnetic energy generated is of the order of 0.1% of the initial beam kinetic energy. The proposed mechanism is of relevance to solar type III radio bursts, as well as other situations, when the injected electron beam has a non-zero perpendicular momentum, e.g. magnetron.

astro-ph.SR↗

arXiv2010

Vlasov-Maxwell, self-consistent electromagnetic wave emission simulations in the solar corona

1.5D Vlasov-Maxwell simulations are employed to model electromagnetic emission generation in a fully self-consistent plasma kinetic model for the first time in the solar physics context. The simulations mimic the plasma emission mechanism and Larmor drift instability in a plasma thread that connects the Sun to Earth with the spatial scales compressed appropriately. The effects of spatial density gradients on the generation of electromagnetic radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a uniform background magnetic field directed transverse to the density gradient is aperiodically unstable to Larmor-drift instability. The latter results in a novel effect of generation of electromagnetic emission at plasma frequency. When density gradient is removed (i.e. when plasma becomes stable to Larmor-drift instability) and a $low$ density, super-thermal, hot beam is injected along the domain, in the direction perpendicular to the magnetic field, plasma emission mechanism generates non-escaping Langmuir type oscillations which in turn generate escaping electromagnetic radiation. It is found that in the spatial location where the beam is injected, the standing waves, oscillating at the plasma frequency, are excited. These can be used to interpret the horizontal strips observed in some dynamical spectra. Quasilinear theory predictions: (i) the electron free streaming and (ii) the beam long relaxation time, in accord with the analytic expressions, are corroborated via direct, fully-kinetic simulation. Finally, the interplay of Larmor-drift instability and plasma emission mechanism is studied by considering $dense$ electron beam in the Larmor-drift unstable (inhomogeneous) plasma. http://www.maths.qmul.ac.uk/~tsiklauri/movie1.mpg * http://www.maths.qmul.ac.uk/~tsiklauri/movie2.mpg * http://www.maths.qmul.ac.uk/~tsiklauri/movie3.mpg

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arXiv2009

Missing bits of the solar jigsaw puzzle: small-scale, kinetic effects in coronal studies

The solar corona, anomalously hot outer atmosphere of the Sun, is traditionally described by magnetohydrodynamic, fluid-like approach. Here we review some recent developments when, instead, a full kinetic description is used. It is shown that some of the main unsolved problems of solar physics, such as coronal heating and solar flare particle acceleration can be viewed in a new light when the small-scale, kinetic plasma description methods are used.

astro-ph.SR↗

arXiv2008

A new fast reconnection model in a collisionless regime

Based on the first principles (i.e. (i) by balancing the magnetic field advection with the term containing electron pressure tensor non-gyrotropic components in the generalised Ohm's law; (ii) using the conservation of mass; and (iii) assuming that the weak magnetic field region width, where electron meandering motion supports electron pressure tensor off-diagonal (non-gyrotropic) components, is of the order of electron Larmor radius) a simple model of magnetic reconnection in a collisionless regime is formulated. The model is general, resembling its collisional Sweet-Parker analogue in that it is not specific to any initial configuration e.g. Harris type tearing unstable current sheet, X-point collapse or otherwise. In addition to its importance from the fundamental point of view, the collisionless reconnection model offers a much faster reconnection rate (M_c'less=(c/omega_pe)^2 /(r_L,e L)) than Sweet-Parker's classical one (M_sp=S^-1/2). The width of the diffusion region (current sheet) in the collisionless regime is found to be delta_c'less=(c/omega_pe)^2/r_L,e, which is independent of global reconnection scale L and is only prescribed by micro-physics (electron inertial length, c/omega_pe, and electron Larmor radius, r_L,e). Amongst other issues, the fastness of the reconnection rate alleviates e.g. the problem of interpretation of solar flares by means of reconnection, as for the typical solar coronal parameters the obtained collisionless reconnection time can be a few minutes, as opposed to Sweet-Parker's equivalent value of < a day. The new theoretical reconnection rate is compared to the MRX device experimental data by [Yamada et al., Phys. Plasmas, 13, 052119 (2006), Ji et al. GRL, 35, 13106 (2008)] and a good agreement is obtained.

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arXiv2008

Mixing of shear Alfven wave packets

The propagation of shear Alfven wave packets in inhomogeneous magnetic fields, at the origin of their distortion, regardless of the occurrence of non-linear coupling, is considered. It is shown that the distortion mechanism can be regarded as mixing process and hence, standard "phase mixing" corresponds to the effect of an "Alfvenic" shear flow while enhanced dissipation at a magnetic X-point corresponds to mixing by an "Alfvenic" strain flow. The evolution of the global wave field is supposed to result from the dynamics of a superposition of wave packets and a kinetic equation for the wave energy is obtained following this eikonal (WKB) description. Since shear Alfven wave packets experience continuous shearing/straining while transported by an inhomogeneous Alfvenic flow $\mathbf{V}_{A}$, their mixing process, in physical space, is also a cascade of wave energy in k-space. The wave energy spectrum resulting from this linear mechanism of energy transfer is determined for the special case of waves propagating along chaotic magnetic field lines, the analog of a chaotic mixing process. The latter follows a $k^{-1}$ power-law, in the energy conserving range in $k$ space.

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