Oct 28: Experimental observation of the standard magnetorotational instability in a modified Taylor-Couette cell

Yin Wang (PPPL)

Abstract: The standard magnetorotational instability (SMRI) has been regarded as the most promising instability responsible for the turbulence required to explain the fast accretion observed across the Universe. However, unlike other fundamental plasma processes such as Alfvén waves and magnetic reconnection which have been subsequently detected and studied in space and in the laboratory, SMRI remains unconfirmed even for its existence long after its proposal, despite its widespread applications in modeling including recent black hole imaging. Its direct detection has been hindered in observations due to its microscopic nature at astronomical distances, and in the laboratory due to stringent requirements and interferences from other processes. Here we report the first direct evidence showing that SMRI indeed exists in a novel laboratory setup where a uniform magnetic field is imposed along the axis of a differentially rotating flow of liquid metal confined radially between concentric cylinders and axially by copper endrings. Through in situ measurement of the radial magnetic field Br at the inner cylinder, onset of the axisymmetric SMRI is identified from the nonlinear increase of Br beyond a critical magnetic Reynolds number. Further analysis reveals that the SMRI is accompanied by a nonaxisymmetric m=1 mode, which is a linear instability having an exponential growth at its onset. Further analysis excludes the possibility that the m=1 mode is the conventional Rayleigh instability or the Stewartson-Shercliff layer instability, implying that it could be a non-axisymmetric version of SMRI that breaks the rotational symmetry of the system. The experimental results are reproduced by nonlinear three-dimensional numerical simulations, which further show that SMRI causes the velocity and magnetic fields to contribute an outward flux of axial angular momentum in the bulk region, just as it does in accretion disks.

Oct 26: Understanding the multi-wavelength emission from astrophysical shocks

Rebecca Diesing (University of Chicago)

Abstract: Interpreting observations of the universe’s most energetic phenomena requires a detailed understanding of particle acceleration in astrophysical environments. In particular, these accelerated particles, or cosmic rays, are responsible for non-thermal emission observed in supernova remnants, novae, AGN winds, and a host of other astrophysical shocks. In this talk I will review the current paradigm of shock acceleration and present a fast, multi-zone modeling framework that self-consistently incorporates findings from state-of-the-art kinetic simulations. This model has been used to reproduce the multi-wavelength emission from a variety of astrophysical objects, including the steep radio and gamma-ray spectra inferred from Galactic supernova remnants, radio and X-ray observations from extragalactic supernovae (“radio supernovae”), the GeV and TeV emission detected from the recent outburst of recurrent nova RS Ophiuchi, and the gamma-rays detected from fast AGN winds.

Sep 30: Shock acceleration in the inhomogeneous interstellar medium

Siyao Xu (Institute of Advanced Studies)

Abstract: With the development of multi-messenger astronomy and the detection of astrophysical neutrinos, we are in a golden age of new cosmic ray (CR) measurements and studies. Supernova remnants are believed as the leading candidate for the source of Galactic CRs. Recent observations, however, challenge the well-developed theory of diffusive shock acceleration of CRs in supernova remnants. I will reexamine the assumptions in the standard shock acceleration model and discuss the key missing physics for shocks propagating in the turbulent and inhomogeneous interstellar medium. A more realistic shock acceleration model holds the promise to understand the observational puzzles and the origin of Galactic CRs.

Sep 16: Thermodynamics and collisionality in firehose-susceptible high-beta plasmas

Archie F. Bott (Princeton University)

Abstract: In this talk, I will discuss the evolution of collisionless plasmas which, due to their macroscopic evolution, are susceptible to the firehose instability. Using both analytic theory and hybrid-PIC simulations of expanding plasmas, we establish that, depending on the relative magnitude of the plasma beta (viz., the ratio of the thermal to magnetic pressure), the characteristic timescale of macroscopic evolution, and the ion Larmor frequency, the saturation of the firehose instability in high-beta plasma can result in two qualitatively distinct thermodynamic and dynamic states. By contrast with the previously identified ‘Alfvén-inhibiting’ state, the newly identified ‘Alfvén-enabling’ state, which is realised when the macroscopic evolution time exceeds the ion Larmor frequency by a beta-dependent critical parameter, can support Alfvén waves and Alfvén turbulence because the magnetic tension associated with the plasma's macroscopic magnetic field is never completely negated by anisotropic pressure forces. We characterise both states in detail, including saturated magnetic-energy spectra. The effective collision operator associated with the firehose fluctuations is also described. Our findings are essential for understanding high-beta plasmas in astrophysical environments such as the intracluster medium of galaxy clusters or black-hole accretion flows, on account of the characteristically large separations between macroscopic and plasma timescales in such systems engendering the Alfvén-enabling state.

Sep 02: Instabilities and plasma heating in collisionless high-beta turbulence

Lev Arzamasskiy (Institute of Advanced Studies)

Abstract: Many space and astrophysical plasmas are hot and dilute, which makes them weakly collisional or even collisionless. Particle distribution functions in such plasmas are often out of thermodynamic equilibrium, which can make them susceptible to a number of kinetic micro-instabilities (such as firehose, mirror, ion-cyclotron). In this talk, I will cover how kinetic instabilities interact with the turbulent cascade, how kinetic turbulence is dissipated in the presence of the instabilities, and what kind of distribution functions are produced as a result. Particular attention will be paid to transport properties of collisionless high-beta plasmas, such as their effective collisionality and viscosity, and the potential implications to astrophysical and space systems such as the intracluster medium, low-luminosity black-hole accretion flows, and the solar wind.

Jul 15: Collisionless shocks in plasmas: when the density jump and the shock front do not obey MHD (at all)

Antoine Bret (Universidad de Castilla-La Mancha)

Abstract: According to MHD, a strong parallel shock is a few mean-free-paths thick, with a density jump of 4. The talk will comment on these 2 aspects for the case of a collisionless shock. First, I’ll review recent analytical works explaining how, as the plasma becomes collisionless, the front continuously switches from a few mean-free-path to a distance much shorter, in terms of the upstream plasma parameter. Second, I will also show how the density jump of a strong parallel collisionless shock can shrink to 2 instead of 4. This is due to the ability of a collisionless plasma to sustain a stable anisotropy in the presence of a magnetic field. PIC simulations will be presented confirming this conclusion.

Apr 11: Characterization of quasi-Keplerian, Differentially Rotating, Free-Boundary Laboratory Plasmas

Vicente Valenzuela-Villaseca (Imperial College London)

Abstract: In this talk I will present results from the Rotating Plasma Experiment (RPX), a novel laboratory platform developed on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise-time). RPX drives differentially rotating high-energy-density plasma flows using the slightly off-radial inward-convergence of 8 magnetized plasma jets. The goal is to interpret and model the rotation profile and pressure balance of differentially rotating plasmas, and study their stability and overall evolution, which are relevant to laboratory modeling of astrophysical accretion disks and protostellar jets.

The data shows that rotating plasmas have a hollow density structure and are radially confined by the ram pressure of the ablation flows. A combination of axial thermal and magnetic pressure launches an axial, highly collimated, supersonic jet with a velocity ~ 100 km/s (M > 5). The axial jet also rotates, transporting angular momentum, as it remains collimated by a hot (Ti ~ 250 eV) surrounding plasma halo. It is inferred that the plasma halo is magnetized by a ~ 3 T magnetic field. I will discuss the thermal and possible magnetic structure of the halo and its impact on jet collimation. Finally, the rotation velocity radial distribution such that angular frequency decreases with radius, as the opposite happens to specific angular momentum. The calculated squared epicyclic frequency (Rayleigh determinant) of the flow is estimated to be κ2 ~ r-2.8 > 0. This implies that the flows at RPX are quasi-Keplerian and share stability properties of gravitationally driven accretion disks.

Mar 30: Electron acceleration at supernova remnants

Artem Bohdan (DESY)

Abstract: Supernova remnants (SNRs) are believed to produce the most part of the galactic cosmic rays (CRs). SNRs harbor non-relativistic collisionless shocks responsible for acceleration of CRs via diffusive shock acceleration (DSA), in which particles gain their energies in repetitive interactions with the shock front. As the DSA theory involves pre-existing mildly energetic particles, a means of pre-acceleration is required, especially for electrons. Electron injection remains one of the most troublesome and still unresolved issues and our physical understanding of it is essential to fully comprehend the physics of SNRs. To study any electron-scale phenomena responsible for pre-acceleration, we require a method capable of resolving these small kinetic scales and Particle-in-cell (PIC) simulations fulfill this criterion. Here I report about the latest achievements on kinetic simulations of non-relativistic high Mach number shocks. I discuss how the physics of SNR shocks depends on the shock parameters (e.g., the shock obliquity, Mach numbers, the ion-to-electron mass ratio), which processes are responsible for the electron pre-acceleration and how these shocks can be studied using in-situ satellite measurements. Finally, I outline future perspectives of the electron injection problem and other complementary ways to solve it.

Mar 18: Extreme QED and the Magnetar Circuit

Christopher Thompson (CITA)

Abstract: In contrast with radio pulsars, the electric current flowing outside a magnetar can approach in strength the internal current supporting its magnetic field. Obtaining a self-consistent description of this circuit is a pre-requisite for understanding the unusual X-ray, infrared and radio emission and extreme torque variability of Galactic magnetars, and the plasma state at the onset of a giant flare or fast radio burst. We show that the interactions of electrons, positrons and photons are modified in some significant ways in the presence of a super-QED magnetic field. Collisionality of a pair plasma is strongly enhanced by single-photon pair annihilation and re-conversion; two-photon pair annihilation develops a soft, bremsstrahlung-like spectral tail; electron-photon scattering shows a new resonance in the recoil-dominated regime; and the cross section for photon collisions is dramatically enhanced. A Monte Carlo implementation of these processes reveals a novel collisional state in which hard X-ray emission similar to that observed is balanced by ohmic heating in narrow flux bundles that may be connected with fault-like features in the magnetar crust.

Feb 25: Polarized Radiation Signals from Highly Magnetized Neutron Star Surfaces

Kun Hu (Rice University)

Abstract: The surfaces of neutron stars are likely sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Scattering transport in the surface layers is critical to the determination of the emergent anisotropy of light intensity, and is strongly influenced by the complicated interplay between linear and circular polarization information. We have developed a magnetic Thomson scattering simulation to model the outer layers of fully-ionized atmospheres in such compact objects. Here we summarize emergent intensities and polarizations from extended atmospheric simulations, spanning considerable ranges of magnetic colatitudes. General relativistic propagation of light from the surface to infinity is fully included. The net polarization degrees are moderate and not very small when summing over a variety of field directions. These results provide an important foundation for future X-ray polarimetry observations of neutron stars with a variety of magnetizations from CCOs to magnetars.

Jan 28: Aftermath of White Dwarf Tidal Capture

Wenbin Lu (Princeton University)

Abstract: Tidal capture of a white dwarf (WD) by a stellar-mass compact object occurs due to close encounters (with periastron separation of a few WD radii) in dense stellar clusters as well as in triple systems. We consider the subsequent evolution of the binary system after the tidal capture, under the influence of dynamical tides and gravitational waves (GWs). We find that the post-capture orbit rapidly shrinks to an eccentricity of about 0.9 in about 10 years as a result of diffusive growth of the amplitude of the prograde f-mode oscillation. Afterwards, the mode amplitude stops growing diffusively, and the orbital decay is dominated by GW emission with a merger time of a few kyr. The observational appearance of the system in the GW and electromagnetic sky will be discussed.

Jan 14: The Importance of Distribution Function Structure: Magnetic Pumping and Predictions of Plasma Stability in a Marginally Unstable Plasma

Emily Lichko (University of Arizona)

Abstract: Due to low collisionality in space and astrophysical plasmas, distributions of ions and electrons observed by spacecraft exist in a state far from thermodynamic equilibrium. The non-Maxwellian features in these distribution functions can significantly impact the evolution of the plasma. First we will examine how the fidelity of the model to the observed distribution function affects our predictions for the linear stability of the plasma. To do this, we use marginally stable one-dimensional, electrostatic simulations of the electron two-stream instability. For these simulations, there is significantly better agreement between the behavior of the plasma and the predictions of linear theory when a higher-fidelity representation of the distribution function is used. We will also look at a model of magnetic pumping, a process where particles are heated by the largest-scale turbulent fluctuations, naturally producing power law distributions. When the magnetic pumping model is extended to a spatially-varying flux tube, magnetic trapping renders magnetic pumping an effective Fermi heating process for particles with v >> w/k, a regime where few other heating mechanisms are effective.