Session JO03: Astrophysical Plasma: Accretion Disks, Dynamos, Supernovae, and Cosmic Rays

Live

We perform fully kinetic simulations of flows known to produce dynamo in magnetohydrodynamics (MHD), considering scenarios with low Reynolds number and high magnetic Prandtl number, with relevance to fluctuation dynamos in galaxy clusters. We find that Landau damping on the electrons leads to a rapid decay of magnetic perturbations (apart from those corresponding to a current caused by the forcing of the flows), impeding the dynamo. The effect of the magnetic Landau damping is similar to that of a magnetic diffusivity that scales with the wave number of the perturbation. This collisionless damping process operates on spatial scales where electrons are nonmagnetized, reducing the range of scales where the magnetic field grows in high magnetic Prandtl number fluctuation dynamos. When electrons are not magnetized down to the resistive scale, such as galaxy clusters at typical Biermann battery seed fields, the magnetic energy spectrum is expected to be limited by the scale corresponding to magnetic Landau damping or, if smaller, the electron gyroradius scale, instead of the resistive scale, potentially reducing the total energy in magnetic fluctuations. In simulations we thus observe decaying magnetic fields where resistive MHD predicts a dynamo.   

Live

We report a new kind of magneto-hydrodynamic (MHD) instability in a modified Taylor-Couette experiment using Galinstan as the working fluid. In the experiment, the inner cylinder, outer cylinder and upper (lower) endcaps corotate independently at an angular speed of $W_{\mathrm{1}}$, $W_{\mathrm{2}}$ and $W_{\mathrm{3}}$. A uniform magnetic field $B_{\mathrm{z}}$ is applied along the central axis. By using high-precision Hall probes installed at the inner cylinder surface, we obtained the radial magnetic Br at various azimuths. The new MHD instability is identified from the measured time sequence of $B_{\mathrm{r}}$, which is non-axisymmetric with an azimuthal mode number $m=$1 and has a moderate frequency between $W_{\mathrm{1}}$-$W_{\mathrm{3}}$ and $W_{\mathrm{1}}$-$W_{\mathrm{2}}$. The new-found instability only exists at sufficiently large $W_{\mathrm{1}}$ and moderate $B_{\mathrm{z}}$, consistent with typical requirements for the magnetorotational instability (MRI), and detailed quantitative comparisons are underway with theoretical analysis and numerical simulations. Further analysis shows it is not the Rayleigh instability or the Shercliff layer instability. Our results therefore shed light on the direction for finding a non-axisymmetric MRI.   

Live

Intrinsic gravitational ballooning modes, and the associated fields and currents, are found to be sustained by the time dependent tridimensional gravitational fields of Black Hole binaries. The modes are ballooning in the "vertical" direction (referring to the binary angular momentum), rippled in the radial direction and propagating in the toroidal direction with a frequency (of the main mode component) equal to twice the binary rotation frequency. Characteristic mode particle resonances [1] provide a means to transfer energy from high to low energy populations \newline offering an explanation for the absence of high energy radiation emission as the collapse of Black Hole binaries is observed. \newline [1] B.Coppi, Pl. Phys. Rep. \textbf{45} , 438 (2019).   

Live

Turbulence in a conducting plasma can amplify seed magnetic fields in what is known as the small-scale, or turbulent, dynamo. The associated growth rate and emergent magnetic-field geometry depends sensitively on the material properties of the plasma, in particular the magnetic Prandtl number Pm. For ${\rm Pm > 1}$, the amplified magnetic field is gradually arranged into a folded structure, with direction reversals at the resistive scale and field lines curved at the larger scale of the flow. As the magnetic energy grows to come into approximate equipartition with the fluid motions, this folded structure persists. Using analytical theory and high-resolution MHD simulations with the Athena++ code, we investigate the conditions under which these magnetic folds may become unstable to tearing instability, and ask how the resulting disrupted current sheets affect the energy spectrum, geometry, and statistics of the magnetic field in its saturated state.   

Live

Turbulence energizes electrons and ions in collisionless astrophysical plasmas, such as hot accretion flows onto black holes, and thus shapes their radiative signatures (luminosity, spectra, and variability). To understand the kinetic properties of a collisionless radiative plasma subject to externally driven turbulence, we investigate particle-in-cell simulations of relativistic plasma turbulence with external inverse Compton cooling acting on electrons. We find that ions continuously heat up while electrons gradually cool down (due to the net effect of radiation). Thus, the ion-to-electron temperature ratio $T_i/T_e$ grows in time and is limited only by the size and duration of the simulations, indicating the absence of efficient collisionless mechanisms of electron-ion thermal coupling. Electrons acquire a quasi-thermal distribution (dictated by the competition of turbulent energization and radiative cooling), while ions undergo efficient nonthermal acceleration. There is a modest nonthermal population of high-energy electrons that are beamed intermittently, which may explain rapid high-energy flares in certain astrophysical systems. These results show that extreme two-temperature plasmas are produced and maintained by relativistic radiative turbulence.   

Live

Relativistic jets from active galaxies are among the most powerful cosmic particle accelerators. It is thought that MHD instabilities, such as kink modes, can play an important role in the dissipation of the jet's copious magnetic energy. By using large-scale 3D particle-in-cell simulations we explore particle acceleration for extreme magnetizations associated with force-free equilibria. We find that magnetic reconnection is important in pre-accelerating thermal particles to gyroradius comparable to the plasma skin depth. This allows particles to experience the magnetic field curvature associated with the nonlinear development of the kink instability. We find that once particles are injected they can be efficiently accelerated by the combination of a large-scale coherent electric field and turbulence magnetic fields, with curvature drifts mediating the acceleration, similarly to what was observed in pressure-supported jets [1]. The non-thermal particles develop a power-law energy spectrum with an index that approaches \textasciitilde 1.75 for high-magnetizations and carry away the majority of the initial toroidal magnetic field energy, establishing the kink instability as an efficient mechanism to trigger the conversion of the jet's magnetic energy in accelerated particles. [1] E. P. Alves, J. Zrake, F. Fiuza, Phys. Rev. Lett. \textbf{121}, 245101 (2018)   

Live

The typical photon spectrum of accreting X-ray binaries in the hard state is modelled with a non-thermal power-law component. This component is usually interpreted as thermal Comptonization of disk photons by a cloud of trans-relativistic electrons in the disk ``corona''. However, the electron energization mechanism needed to balance the inverse Compton (IC) cooling remains uncertain. We perform first-principle 2D particle-in-cell simulations of magnetic reconnection---with a wide range of magnetizations ($0.3 \le \sigma \le 40$)---in electron-positron and electron-proton plasma, subject to different levels of IC cooling. We find that, for all the magnetizations we explored, the electrons' energy spectra are comprised of a high-energy peak dominated by particles with Lorentz factors of $\gamma\sim\sigma/4$, and a low-energy component populated by cold particles residing inside plasmoids, which move as a bulk at trans-relativistic speeds. For $\sigma \ge 1$, the latter can be fit with a Maxwellian distribution with an effective temperature of $T_{\rm eff}\sim 100$ keV, and so it could play the role of the electron distribution used in thermal Comptonization models. In summary, bulk Comptonization in reconnection may explain the hard state spectrum of accreting X-ray binaries.   

Live

The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming large-scale cosmic magnetic fields with dynamical strengths. However, how these seed fields are generated and how the turbulent dynamo operates in the weakly collisional intergalactic/intracluster medium remains a mystery. We employ first-principles 3D particle-in-cell simulations of driven non-helical turbulence to study the generation and amplification of magnetic fields in an initially unmagnetized collisionless plasma. For computational feasibility and physical clarity, we consider a relativistically hot pair plasma. Pressure anisotropy develops due to the local shear flows caused by the forcing and triggers the Weibel instability, which generates the kinetic microscale seed magnetic fields with a filamentary morphology. During the subsequent nonlinear Weibel stage, the coherence length of the magnetic field grows via the coalescence of filaments until the turbulent dynamo develops and starts governing the magnetic field evolution. The turbulent dynamo saturates when the bulk kinetic and magnetic energies come into equipartition. This work demonstrates self-consistently the operation of dynamo in collisionless plasma and informs our understanding of cosmic magnetogenesis.   

Live

Turbulent high-energy astrophysical systems often feature asymmetric mechanical energy injection, for instance Alfv\'{e}n waves propagating from an accretion disk into its corona. Such systems (relativistic analogs of the solar wind) are ``imbalanced": the energy fluxes parallel and anti-parallel to the large-scale magnetic field are unequal and the plasma possesses net cross-helicity. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In the present study, we investigate externally-driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using three-dimensional particle-in-cell simulations. We examine how statistical properties of the turbulence as well as kinetic aspects such as plasma heating, momentum anisotropy, and nonthermal particle acceleration depend on the degree of imbalance, and compare the results to the balanced case. We also investigate the efficiency of converting injected Poynting flux into net momentum of the plasma, and discuss subsequent implications for the launching of a disk wind. These results will better characterize properties of imbalanced turbulence in a collisionless plasma and may have ramifications for black hole accretion disk coronae, winds, and jets.   

Live

Extreme astrophysical objects such as relativistic jets and supernova remnants exhibit nonthermal radiative spectra highlighting the presence and the generation of suprathermal particle distributions -- $i.e.$ cosmic rays. The maximum energy reached through diffusive shock acceleration in such environments is directly conditioned by the energy injected~by the cosmic rays~in the self-generated turbulent electromagnetic field at~large scales comparable to~the Larmor radius of the most energetic particles.~Capturing the self-consistent, multiscale feedback of the turbulence on the nonthermal distribution, taking into account kinetic effects, has been a significant challenge. We will present the results of large-scale particle-in-cell kinetic simulations that capture~the generation and the nonlinear evolution of current- and pressure-driven instabilities relevant to the context of relativistic and sub-relativistic shocks.~~We will discuss how the coupling between different instabilities affects the nonlinear evolution of the system and shapes~the magnetic field energy injected at the~largest~scales.   

Live

Cosmic ray acceleration in supernova remnants is characterized by turbulence and magnetic field amplification created by streaming cosmic rays. Physics upstream of the shock is most important for acceleration; both the scale and the magnitude of the magnetic field have to be large enough to ensure sufficiently rapid acceleration to observed energies. The physics of small-scale dynamo, driven by cosmic ray pressure can provide such scales and magnitudes, however, the critical question remains regarding the dynamics of particles. I report the results of PIC-MHD modeling of nonlinear streaming and dynamo of cosmic rays having broad energy distribution. It has been often assumed that cosmic ray dynamics is diffusive. In fact, in this cosmic ray driven turbulence the dynamics is super-diffusive and the index as well as the coefficient of diffusion depends on energy. We discuss implications of this to the standard shock acceleration picture.   

Live

This talk reports on a study where material samples are exposed to DIII-D tokamak plasmas with the goal of examining the processes occurring during spacecraft atmospheric re-entries. Due to inherent properties of the tokamak plasma -- rotation of the core and edge plasma and fast flow in the scrape-off layer -- any object launched radially from the tokamak wall with zero toroidal speed incurs velocity (relative to the plasma) which is comparable to the entry velocity of the Galileo probe to Jupiter. Thus, this project presents a unique opportunity for examining plasma-materials interactions at space-relevant enthalpy and heat fluxes. Here we discuss scaling between laboratory and space conditions, specifics of the experimental design, and calculations of the heat flux and ablation of the material samples. The possibility of heat and particle flux detachment in front of the sample is also explored.   

Live

The emission from supernovae is produced as the energetic shock produced in a stellar core bursts out of its star, allowing the energy from the explosion to be emitted through photons. This initial shock breakout produces a burst of UV and X-ray photons that can be used to probe the stellar and explosion properties. But most calculations to date assume a smooth transition of the material surrounding the star. Radiatively-driven instabilities, mass eruptions, convective instabilities, and other mechanisms can produce large scale inhomogenous structures such as dense shells and clumps in the wind. Supernova explosions that propagate through such winds can produce widely varying emission. In this work, we present results of 1D and 2D multi-group radiation hydrodynamic simulations of supernova explosions through dense porous shells, in which the structure in the shell is clumpy, irregular, and optically porous.   

On Demand

Relativistic collisionless shocks are considered responsible for particle energization mechanisms leading to particle acceleration. While particle acceleration in shock transition region has been extensively investigated, aspects and mechanism of electron energization in upstream region of the shock is still unclear. We study electron energization mechanism in this area using two dimensional particle-in-cell simulations. We show that electron energization happens due to interaction of electrons with induced electric ?elds of self-generated magnetic vortices (MV) in upstream region of the shock. Electrons gain signi?cant amount of energy during interaction.