arXiv subjects

T. D. Arber

Publications and source records attributed to T. D. Arber.

At least 19 recordsLinked to original sources

arXiv2024

Non-local thermal transport impact on compressive waves in two-temperature coronal loops

Context. Observations of slow magnetoacoustic waves in solar coronal loops suggest that, in hot coronal plasma, heat conduction may be suppressed in comparison with the classical thermal transport model. Aims. We link this suppression with the effect of the non-local thermal transport that appears when the plasma temperature perturbation gradient becomes comparable to the electron mean free path. Moreover, we consider a finite time of thermalisation between electrons and ions, so that separate electron and ion temperatures can occur in the loop. Methods. We numerically compare the influence of the local and non-local thermal transport models on standing slow waves in one- and two-temperature coronal loops. To quantify our comparison, we use the period and damping time of the waves as commonly observed parameters. Results. Our study reveals that non-local thermal transport can result in either shorter or longer slow-wave damping times in comparison with the local conduction model due to the suppression of the isothermal regime. The difference in damping times can reach 80%. For hot coronal loops, we found that the finite equilibration between electron and ion temperatures results in up to 50% longer damping time compared to the one-temperature case. These results indicate that non-local transport will influence the dynamics of compressive waves across a broad range of coronal plasma parameters with Knudsen numbers (the ratio of mean-free-path to temperature scale length) larger than 1%. Conclusions. In the solar corona, the non-local thermal transport shows a significant influence on the dynamics of standing slow waves in a broad range of plasma parameters, while two-temperature effects come into play for hot and less dense loops.

astro-ph.SR↗

arXiv2018

Signatures of quiet Sun reconnection events in Ca~{\sc ii}, H$α$ and Fe~{\sc i}

We use observations of quiet Sun (QS) regions in the H$α$ 6563 Å, Ca~{\sc ii} 8542 Å\, and Fe~{\sc i} 6302 Å lines. We observe brightenings in the wings of the H$α$ and Ca~{\sc ii} combined with observations of the interacting magnetic concentrations observed in the Stokes signals of Fe~{\sc i}. These brightenings are similar to Ellerman bombs (EBs), i.e. impulsive bursts in the wings of the Balmer lines which leave the line cores unaffected. Such enhancements suggest that these events have similar formation mechanisms to the classical EBs found in active regions, with the reduced intensity enhancements found in the QS regions due to a weaker feeding magnetic flux. The observations also show that the quiet Sun Ellerman bombs (QSEBs) are formed at a higher height in the upper photosphere than the photospheric continuum level. Using simulations, we investigate the formation mechanism associated with the events and suggest that these events are driven by the interaction of magnetic field-lines in the upper photospheric regions. The results of the simulation are in agreement with observations when comparing the light-curves, and in most cases we found that the peak in the Ca~{\sc ii} 8542 Å wing occurred before the peak in H$α$ wing. Moreover, in some cases, the line profiles observed in Ca~{\sc ii} are asymmetrical with a raised core profile. The source of heating in these events is shown by the MURaM simulations and is suggested to occur 430 km above the photosphere.

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arXiv2016

Simulations of Alfven wave driving of the solar chromosphere - efficient heating and spicule launching

Two of the central problems in our understanding of the solar chromosphere are how the upper chromosphere is heated and what drives spicules. Estimates of the required chromospheric heating, based on radiative and conductive losses suggest a rate of ${\sim} 0.1 \mathrm{\:erg\:cm^{-3}\:s^{-1}}$ in the lower chromosphere dropping to ${\sim} 10^{-3} \mathrm{\:erg\:cm^{-3}\:s^{-1}}$ in the upper chromosphere (\citet{Avrett1981}). The chromosphere is also permeated by spicules, higher density plasma from the lower atmosphere propelled upwards at speeds of ${\sim} 10-20 \mathrm{\:km\:s^{-1}}$, for so called Type-I spicules (\citet{Pereira2012,Zhang2012}), reaching heights of ${\sim} 3000-5000 \mathrm{\:km}$ above the photosphere. A clearer understanding of chromospheric dynamics, its heating and the formation of spicules, is thus of central importance to solar atmospheric science. For over thirty years it has been proposed that photospheric driving of MHD waves may be responsible for both heating and spicule formation. This paper presents results from a high-resolution MHD treatment of photospheric driven Alfvén and kink waves propagating upwards into an expanding flux tube embedded in a model chromospheric atmosphere. We show that the ponderomotive coupling from Alfvén and kink waves into slow modes generates shocks which both heat the upper chromosphere and drive spicules. These simulations show that wave driving of the solar chromosphere can give a local heating rate which matches observations and drive spicules consistent with Type-I observations all within a single coherent model.

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arXiv2015

Alfvén Wave Heating of the Solar Chromosphere: 1.5D models

Physical processes which may lead to solar chromospheric heating are analyzed using high-resolution 1.5D non-ideal MHD modelling. We demonstrate that it is possible to heat the chromospheric plasma by direct resistive dissipation of high-frequency Alfvén waves through Pedersen resistivity. However this is unlikely to be sufficient to balance radiative and conductive losses unless unrealistic field strengths or photospheric velocities are used. The precise heating profile is determined by the input driving spectrum since in 1.5D there is no possibility of Alfvén wave turbulence. The inclusion of the Hall term does not affect the heating rates. If plasma compressibility is taken into account, shocks are produced through the ponderomotive coupling of Alfvén waves to slow modes and shock heating dominates the resistive dissipation. In 1.5D shock coalescence amplifies the effects of shocks and for compressible simulations with realistic driver spectra the heating rate exceeds that required to match radiative and conductive losses. Thus while the heating rates for these 1.5D simulations are an overestimate they do show that ponderomotive coupling of Alfvén waves to sound waves is more important in chromospheric heating than Pedersen dissipation through ion-neutral collisions.

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arXiv2013

Synchrotron radiation, pair production and longitudinal electron motion during 10-100PW laser solid interactions

At laser intensities above 1023W/cm2 the interaction of a laser with a plasma is qualitatively different to the interactions at lower intensities. In this intensity regime solid targets start to become relativistically underdense, gamma-ray production by synchrotron emission starts to become an important feature of the dynamics and, at even higher intensities, electron-positron pair production by the non-linear Breit-Wheeler process starts to occur. Previous work in this intensity regime has considered ion acceleration1,2, identified different mechanisms for the underlying plasma physics of laser generation of gamma-rays3,4,5 considered the effect of target parameters on gamma-ray generation6 and considered the creation of solid density positronium plasma3. However a complete linked understanding of the important new physics of this regime is still lacking. In this paper, an analysis is presented of the effects of target density, laser intensity, target preplasma properties and other parameters on the conversion efficiency, spectrum and angular distribution of gamma-rays by synchrotron emission. An analysis of the importance of Breit-Wheeler pair production is also presented.

physics.plasm-ph↗

arXiv2013

Modelling Gamma Ray Emission and Pair Production in High-Intensity Laser-Matter Interactions

In high-intensity (> 10^21W/cm^2) laser-matter interactions gamma-ray photon emission by the electrons can strongly affect the electron's dynamics and copious numbers of electron-positron pairs can be produced by the emitted photons. We show how these processes can be included in simulations by coupling a Monte-Carlo algorithm describing the emission to a particle-in-cell code. The Monte-Carlo algorithm includes quantum corrections to the photon emission, which we show must be included if the pair production rate is to be correctly determined. The accuracy, convergence and energy conservation properties of the Monte-Carlo algorithm are analysed in simple test problems.

physics.plasm-ph↗

arXiv2013

Dense electron-positron plasmas and bursts of gamma-rays from laser-generated QED plasmas

In simulations of a 12.5PW laser (focused intensity I = 4x10^23W/cm^2) striking a solid aluminium target 10% of the laser energy is converted to gamma-rays. A dense electron-positron plasma is generated with a maximum density of 10^26/m^3; seven orders of magnitude denser than pure e-e+ plasmas generated with 1PW lasers. When the laser power is increased to 320PW (I = 10^25W/cm^2) 40% of the laser energy is converted to gamma-ray photons and 10% to electron-positron pairs. In both cases there is strong feedback between the QED emission processes and the plasma physics; the defining feature of the new `QED-plasma' regime reached in these interactions.

physics.plasm-ph↗

arXiv2013

Simulation of the Observed Coronal Kink Instability and Its Implications for the SDO/AIA

Srivastava et al. (2010) have observed a highly twisted coronal loop, which was anchored in AR10960 during the period 04:43 UT-04:52 UT on 4 June 2007. The loop length and radius are approximately 80 Mm and 4 Mm, with a twist of 11.5 $π$. These observations are used as initial conditions in a three dimensional nonlinear magnetohydrodynamic simulation with parallel thermal conduction included. The initial unstable equilibrium evolves into the kink instability, from which synthetic observables are generated for various high-temperature filters of SDO/AIA. These observables include temporal and spatial averaging to account for the resolution and exposure times of SDO/AIA images. Using the simulation results, we describe the implications of coronal kink instability as observables in SDO/AIA filters.

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arXiv2012

Dense Electron-Positron Plasmas and Ultra-Intense Bursts of Gamma-Rays from Laser-Irradiated Solids

In simulations of a 10PW laser striking a solid we demonstrate the possibility of producing a pure electron-positron plasma by the same processes as those thought to operate in high-energy astrophysical environments. A maximum positron density of 10^26/m^3 is achieved, seven orders of magnitude greater than achieved in previous experiments. Additionally, 35% of the laser energy is converted to a burst of gamma-rays of intensity 10^22W/cm^2, potentially the most intense gamma-ray source available in the laboratory. This absorption results in a strong feedback between both pair and gamma-ray production and classical plasma physics in the new `QED-plasma' regime.

physics.plasm-ph↗

arXiv2012

Nonlinear wave propagation and reconnection at magnetic X-points in the Hall MHD regime

The highly dynamical, complex nature of the solar atmosphere naturally implies the presence of waves in a topologically varied magnetic environment. Here, the interaction of waves with topological features such as null points is inevitable and potentially important for energetics. The low resistivity of the solar coronal plasma implies that non-MHD effects should be considered in studies of magnetic energy release in this environment. This paper investigates the role of the Hall term in the propagation and dissipation of waves, their interaction with 2D magnetic X-points and the nature of the resulting reconnection. A Lagrangian remap shock-capturing code (Lare2d) is used to study the evolution of an initial fast magnetoacoustic wave annulus for a range of values of the ion skin depth in resistive Hall MHD. A magnetic null-point finding algorithm is also used to locate and track the evolution of the multiple null-points that are formed in the system. Depending on the ratio of ion skin depth to system size, our model demonstrates that Hall effects can play a key role in the wave-null interaction. In particular, the initial fast-wave pulse now consists of whistler and ion-cyclotron components; the dispersive nature of the whistler wave leads to (i) earlier interaction with the null, (ii) the creation of multiple additional, transient nulls and, hence, an increased number of energy release sites. In the Hall regime, the relevant timescales (such as the onset of reconnection and the period of the oscillatory relaxation) of the system are reduced significantly, and the reconnection rate is enhanced.

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arXiv2011

Observational signatures of the coronal kink instability with thermal conduction

It is known from numerical simulations that thermal conduction along magnetic field lines plays an important role in the evolution of the kink instability in coronal loops. This study presents the observational signatures of the kink instability in long coronal loops when parallel thermal conduction is included. The 3D nonlinear magnetohydrodynamic equations are solved numerically to simulate the evolution of a coronal loop that is initially in an unstable equilibrium. The loop has length 80 Mm, width 8 Mm and an initial maximum twist of Phi = 11.5 pi, where Phi is a function of the radius. The initial loop parameters are obtained from a highly twisted loop observed in the TRACE 171 A waveband. Synthetic observables are generated from the data. These observables include spatial and temporal averaging to account for the resolution and exposure times of TRACE images. Parallel thermal conduction reduces the maximum local temperature by up to an order of magnitude. This means that different spectral lines are formed and different internal loop structures are visible with or without the inclusion of thermal conduction. However, the response functions sample a broad range of temperatures. The result is that the inclusion of parallel thermal conductivity does not have as large an impact on observational signatures as the order of magnitude reduction in the maximum temperature would suggest; the net effect is a blurring of internal features of the loop structure.

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arXiv2007

Emergence of a flux tube through a partially ionised solar atmosphere

For a magnetic flux tube, or indeed any flux, to emerge into the Solar corona from the convection zone it must pass through the partially ionised layers of the lower atmosphere: the photosphere and the chromosphere. In such regions the ion-neutral collisions lead to an increased resistivity for currents flowing across magnetic field lines. This Cowling resistivity can exceed the Spitzer resistivity by orders of magnitude and in 2.5D simulations has been shown to be sufficient to remove all cross field current from emerging flux. Here we extend this modelling into 3D. Once again it is found that the Cowling resistivity removes perpendicular current. However the presence of 3D structure prevents the simple comparison possible in 2.5D simulations. With a fully ionised atmosphere the flux emergence leads to an unphysically low temperature region in the overlying corona, lifting of chromospheric material and the subsequent onset of the Rayleigh-Taylor instability. Including neutrals removes the low temperature region, lifts less chromospheric matter and shows no signs of the Rayleigh-Taylor instability. Simulations of flux emergence therefore should include such a neutral layer in order to obtain the correct perpendicular current, remove the Rayleigh-Taylor instability and get the correct temperature profile. In situations when the temperature is not important, i.e. when no simulated spectral emission is required, a simple model for the neutral layer is demonstrated to adequately reproduce the results of fully consistent simulations.

astro-ph↗

arXiv2006

Aspects of Electron Acoustic Wave Physics in Laser Backscatter from Plasmas

Recent experimental results from the Trident laser confirm the importance of kinetic effects in determining laser reflectivities at high intensities. Examples observed include scattering from low frequency electron acoustic waves (EAWs), and the first few stages of a cascade towards turbulence through the Langmuir decay instability. Interpretive and predictive computational capability in this area is assisted by the development of Vlasov codes, which offer high velocity space resolution in high energy regions of particle phase space, and do not require analytical pre-processing of the fundamental equations. A direct Vlasov solver, capable of resolving these kinetic processes, is used here to address fundamental aspects of the existence and stability of the electron acoustic wave, together with its collective scattering properties. These simulations are extended to realistic laser and plasma parameters characteristic of single hot-spot experiments. Results are in qualitative agreement with experiments displaying both stimulated Raman and stimulated electron acoustic scattering. The amplitude of simulated EAWs is greater than that observed experimentally, and is accompanied by a higher phase velocity. These minor differences can be attributed to the limitations of a one-dimensional collisionless model.

physics.plasm-ph↗

arXiv2006

Stabilisation of BGK modes by relativistic effects

We investigate the acceleration of electrons via their interaction with electrostatic waves, driven by the relativistic Buneman instability, in a system dominated by counter-propagating proton beams. We observe the growth of these waves and their subsequent saturation via electron trapping for a range of proton beam velocities, from 0.15c to 0.9c. We can report a reduced stability of the electrostatic wave (ESW) with increasing non-relativistic beam velocities and an improved wave stability for increasing relativistic beam velocities, both in accordance with previous findings. At the highest beam speeds, we find the system to be stable again for a period of ~160 plasma periods. Furthermore we observe a, to our knowledge, previously unreported secondary electron acceleration mechanism at low beam speeds. We believe that it is the result of parametric couplings to produce high phase velocity ESW's which then trap electrons, accelerating them to higher energies. This allows electrons in our simulation study to achieve the injection energy required for Fermi acceleration, for beam speeds as low as 0.15c in unmagnetised plasma.

physics.plasm-ph↗

arXiv2005

Collisional dissipation of Alfvén waves in a partially ionised solar chromosphere

Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised. The lower chromosphere contains neutral atoms, the existence of which greatly increases the efficiency of the damping of waves due to collisional friction momentum transfer. More specifically the Cowling conductivity can be up to 12 orders of magnitude smaller than the Spitzer value, so that the main damping mechanism in this region is due to the collisions between neutrals and positive ions. Using values for the gas density and temperature as functions of height taken from the VAL C model of the quiet Sun, an estimate is made for the dependance of the Cowling conductivity on height and strength of magnetic field. Using both analytic and numerical approaches the passage of Alfven waves over a wide spectrum through this partially ionised region is investigated. Estimates of the efficiency of this region in the damping of Alfven waves are made and compared for both approaches. We find that Alfven waves with frequencies above 0.6Hz are completely damped and frequencies below 0.01 Hz unaffected.

astro-ph↗

arXiv2004

Accelerated electron populations formed by Langmuir wave-caviton interactions

Direct numerical simulations of electron dynamics in externally driven electrostatic waves have been carried out using a relativistic two-fluid one-dimensional Vlasov-Poisson code. When the driver wave has sufficiently large amplitude, ion density holes (cavitons) form. The interaction between these cavitons and other incoming Langmuir waves gives rise to substantial local acceleration of groups of electrons, and fine jet-like structures arise in electron phase space. We show that these jets are caused by wave-breaking when finite amplitude Langmuir waves experience the ion density gradient at the leading edge of the holes, and are not caused by caviton burn-out. An analytical two-fluid model gives the critical density gradient and caviton depth for which this process can occur. In particular, the density gradient critically affects the rate at which a Langmuir wave, moving into the caviton, undergoes Landau damping. This treatment also enables us to derive analytical estimates for the maximum energy of accelerated electrons, and for the energy spectrum along a phase-space jet. These are confirmed by direct numerical simulations.

physics.plasm-ph↗

arXiv2004

The Nature and Excitation Mechanisms of Acoustic Oscillations in Solar and Stellar Coronal Loops

In the recent work of Nakariakov et al. (2004), it has been shown that the time dependences of density and velocity in a flaring loop contain pronounced quasi-harmonic oscillations associated with the 2nd harmonic of a standing slow magnetoacoustic wave. That model used a symmetric heating function (heat deposition was strictly at the apex). This left outstanding questions: A) is the generation of the 2nd harmonic a consequence of the fact that the heating function was symmetric? B) Would the generation of these oscillations occur if we break symmetry? C) What is the spectrum of these oscillations? Is it consistent with a 2nd spatial harmonic? The present work (and partly Tsiklauri et al. (2004b)) attempts to answer these important outstanding questions. Namely, we investigate the physical nature of these oscillations in greater detail: we study their spectrum (using periodogram technique) and how heat positioning affects the mode excitation. We found that excitation of such oscillations is practically independent of location of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly a quarter of the period, it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating. We also were able to model successfully SUMER oscillations observed in hot coronal loops.

astro-ph↗

arXiv2004

Flare Generated Acoustic Oscillations in Solar and Stellar Coronal Loops

Low-frequency longitudinal oscillations of a flaring coronal loop are studied numerically. In the recent work of Nakariakov et al., Astron. and Astrophys., 414, L25-L28 (2004) it has been shown that the time dependences of density and velocity in a flaring loop contain well-pronounced quasi-harmonic oscillations associated with a 2nd harmonic of a standing slow magnetoacoustic wave. In this work we investigate physical nature of these oscillations in greater detail, namely, we study their spectrum (using periodogram technique) and how does heat positioning affects the mode excitation. We found that excitation of such oscillations practically independent of the positioning of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly equal to the quarter of the period, it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating. KEYWORDS -- Sun: flares -- Sun: oscillations -- Sun: Corona -- Stars: flare -- Stars: oscillations -- Stars: coronae

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