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Movies for article "Relativistic Reconnection: an Efficient Source of Nonthermal Particles"
No full textMovies of relativistic reconnection and particle acceleration in relativistic reconnection accompanying the article "Relativistic Reconnection: an Efficient Source of Nonthermal Particles" by Lorenzo Sironi and Anatoly Spitkovsky.Files: 2d.sigma10_zoomout.mov (structure of reconnection region on large scale), 2d.sigma10_zoomin.mov (zoom in to smaller section of reconnection region), 2d.sigma10_acceleration.at.xpoint.mov (particle trajectories showing acceleration near X-point), 3d.sigma10_density.mov (3D structure of the reconnection region
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Three-dimensional instability of flame fronts in type I X-ray bursts
No full textWe present the first realistic 3D simulations of flame front instabilities during type I X-ray bursts. The unperturbed front is characterized by the balance between the pressure gradient and the Coriolis force of a spinning neutron star (ν = 450 Hz in our case). This balance leads to a fast horizontal velocity field parallel to the flame front. This flow is strongly sheared in the vertical direction. When we perturb the front an instability quickly corrugates the front. We identify this instability as the baroclinic instability. Most importantly, the flame is not disrupted by the instability and there are two major consequences: the overall flame propagation speed is ~10 times faster than in the unperturbed case and distinct flame vortices appear. The speedup is due to the corrugation of the front and the dynamics of the vortices. These vortices may also be linked to the oscillations observed in the light curves of the bursts
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Preferential Heating and Acceleration of Heavy Ions in Impulsive Solar Flares
Full text linkWe simulate decaying turbulence in a homogeneous pair plasma using a three-dimensional electromagnetic particle-in-cell method. A uniform background magnetic field permeates the plasma such that the magnetic pressure is three times larger than the thermal pressure and the turbulence is generated by counter-propagating shear Alfvén waves. The energy predominately cascades transverse to the background magnetic field, rendering the turbulence anisotropic at smaller scales. We simultaneously move several ion species of varying charge to mass ratios in our simulation and show that the particles of smaller charge to mass ratios are heated and accelerated to non-thermal energies at a faster rate. This is in accordance with the enhancement of heavy ions and a non-thermal tail in their energy spectrum observed in the impulsive solar flares. We further show that the heavy ions are energized mostly in the direction perpendicular to the background magnetic field, with a rate consistent with our analytical estimate of the rate of heating due to cyclotron resonance with the Alfvén waves, of which a large fraction is due to obliquely propagating waves
Higher Order Current Deposition Schemes for Particle-in-Cell Plasma Simulations
No full textParticle-in-cell simulations provide important insight into the behavior of plasmas.
However, simulating millions or even billions of particles and their interactions through
the electric and magnetic fields over many time steps requires careful optimization
to ensure a reasonable runtime. In this thesis we examine different algorithms for
depositing current density onto an array, paying particular attention to efficiency and
charge conservation. We find that a simple first order deposition method is significantly
faster than others while still being charge conservative. We also show that
particular care must be taken in designing higher order current deposition methods so
that they obey Gauss’ law, and that higher order methods–although they cause less
noise in the simulation and therefore require less smoothing–are too time-consuming
to be practical choices
Optimizing Performance of Three-Dimensional Astrophysical Plasma Simulations
No full textParticle-in-cell simulations of collisionless shocks have been shown to produce self-consistent shock acceleration of non-thermal particles, which are an important area of astrophysical research. A challenge that remains, however, is acquiring results from these simulations without a prohibitively long runtime. In this thesis we introduce the TRISTAN-MP simulation and examine methods of optimizing its performance. These methods include replacing multiple arrays of single precision values with one array of memory-aligned structures, precomputing coefficients that may be computed multiple times, and minimizing slow double precision calculations. We then discuss the local implementations of each optimization and present the resulting change in performance for each implementation. Finally, we discuss the optimizations that were selected for use in the full simulation and the full implementation of those optimizations. We show that the output of the optimized simulation maintains the properties of the original output and we find that running time is significantly reduced
Visualizing Electron Acceleration in Simulations of Collisionless Shocks
No full textParticles accelerated to high energies, such as cosmic rays, are ubiquitous within the universe. We study electron acceleration in particle in cell simulations of quasi-perpendicular, non-relativistic collisionless shocks. We develop methods for visualizing acceleration in three dimensions using the program Paraview. We study reflectivity of the shock depending on time and correlations between location of reflection and the structure of the shock. We calculate a reflectivity of roughly 1% in the two-dimensional simulations, and find that roughly 6% of the particles are non-thermal. We visualize reflected particles undergoing shock drift acceleration and examine where they are interacting with the shock when they reflect
Using JWST to Investigate the Crab Nebula’s Synchrotron Emission
No full textThis work takes advantage of the James Webb Space Telescope’s (JWST) 1.6 − 30 μm observing
range and provides one of the first looks at the variations in the Crab Nebula’s synchrotron index in
the mid infrared. We used new, synchrotron-dominated images of the Crab Nebula taken at 4.8 μm
and 11.3 μm to search for evidence of a cooling break and multiple particle populations in the Crab
Nebula’s infrared synchrotron spectrum. We created a high resolution, spectral index map between
4.8 μm and 11.3 μm to investigate the spectral and morphological distribution of ultra-relativistic
particles in the Crab Nebula. We found that the JWST 4.8 μm to 11.3 μm spectral index, α,
steepens from ∼ 0.4 ± 0.02 in the torus and ∼ 0.5 ± 0.02 in the jet structures to ∼ 0.65 ± 0.04 in the
outer parts of the nebula, indicating the synchrotron cooling persists in the infrared. The spectral
index also steepens faster towards the southwest (bottom right) edge of the nebula compared to the
other edges, potentially indicating the existence of multiple particle populations within the nebula.
We filled in the Crab Nebula’s synchrotron spectrum in the mid-infrared by finding the total flux
through the two JWST images and using the index map to derive the synchrotron flux at 5.6 μm,
18 μm, and 21 μm. We found that the Spitzer 3.6 μm and 4.5 μm, JWST 4.8 μm and 11.3 μm, and
derived 5.6 μm, 18.0 μm, and 21.0 μm fluxes follow a power law relationship of gradually changing
slope that increases with increasing frequency within the uncertainties. This change in slope along
with synchrotron cooling indicated by the JWST 4.8 μm to 11.3 μm spectral index map indicates a
cooling break between the radio and near infrared
Particle Acceleration and Nonthermal Emission in Relativistic Astrophysical Shocks
No full textThe common observational feature of Pulsar Wind Nebulae (PWNe), gamma-ray bursts (GRBs), and AGN jets is a broad nonthermal spectrum of synchrotron and inverse Compton radiation. It is usually assumed that the emitting electrons are accelerated to a power-law distribution at relativistic shocks, via the so-called Fermi mechanism. Despite decades of research, the Fermi acceleration process is still not understood from first principles. An assessment of the micro-physics of particle acceleration in relativistic shocks is of paramount importance to unveil the properties of astrophysical nonthermal sources, and it is the subject of this dissertation. In the first part of this thesis, I explore by means of fully-kinetic first-principle particle-in-cell (PIC) simulations the properties of relativistic shocks that propagate in electron-positron and electron-proton plasmas carrying uniform magnetic fields. I find that nonthermal particle acceleration only occurs if the upstream magnetization is weak (sigma0.01) and quasi-perpendicular, yet they need to be efficient particle accelerators, in order to explain the prominent nonthermal signatures of these sources. Motivated by this discrepancy, I then relax the assumption of uniform pre-shock fields, and investigate the acceleration efficiency of perpendicular shocks that propagate in high-sigma flows with alternating magnetic fields. This is the geometry expected at the termination shock of pulsar winds, but it could also be relevant for Poynting-dominated jets in GRBs and AGNs. I show by means of PIC simulations that compression of the flow at the shock will force annihilation of nearby field lines, a process known as shock-driven reconnection. Magnetic reconnection can efficiently transfer the energy of alternating fields to the particles, generating flat power-law tails containing most of the particles. Finally, I directly relate the results of my PIC simulations to observations of nonthermal sources, by presenting a numerical technique that I have developed in order to extract ab initio photon spectra from PIC simulations of shocks. With this technique, I have modeled the emission from GRB jets, ruling out a class of models that relied on the so-called jitter radiation. This reinforces the idea that a detailed understanding of the micro-physics of particle acceleration in relativistic shocks is required in order to correctly interpret the emission signatures of astrophysical nonthermal sources
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