2016

29 January: Yuri Levin (Monash University)

"Magnetic toys in the sky"

In terrestrial conditions, magnetic fields usually play second fiddle to matter. However, in space magnetic fields can play a decisive role in dynamics of many astrophysical objects. In my talk I will describe several interesting magnetic structures that do not have counterparts in terrestrial conditions: 1. Accretion discs around supermassive black holes that are in a state of magnetic levitation 2. Magnetically driven hurricane-like deflagration fronts that are explosively consuming oceans on rapidly spinning neutron stars. 3. Avalanches of magnetically-induced thermoplastic failures in magnetar crusts that my be connected to the observed activity of magnetars.

19 February: Wei Deng (UNLV-NANL)

"MHD simulations of collision-induced magnetic reconnection in Poynting-flux-dominated jets"

The energy composition in the jet/outflow of astrophysical systems is an important and fundamental question. Recently, the collision-induced magnetic reconnection and dissipation models in the Poynting-flux-dominated (PFD) environment become more attractive to overcome some criticisms in the traditional matter-flux-dominated (MFD) models and interpret new observations. Here, we perform 3D relativistic MHD simulations to study the collisions between high-sigma (PFD) blobs to mimic the above considerations. Our results strongly support these considerations from the energy dissipation efficiency (> 35%) and the mini-jets generation point of view. On the other hand, polarization observation is another important and independent information to constrain the energy environment and physical models. Here we study the polarization feature based on the magnetic field evolution from our above simulations, and find that the same numerical model with different input parameters can reproduce well the observational data of both GRBs and blazars, especially the 90 degrees polarization angle (PA) change in GRB 100826A and the 180 degrees PA swing in Blazar 3C279. This supports a unified model for GRB and blazar jets, suggesting that collision-induced magnetic reconnection is a common physical mechanism to power the relativistic jet emission from events with very different black hole masses.

26 February: Luca Comisso (PPPL)

"Plasmoid Instability and Fast Magnetic Reconnection"

During the last few years there have been great advances in understanding the nature of the plasmoid instability and its implications on the magnetic reconnection processes. In particular, it is now well established that the plasmoid instability has a key role in allowing very fast magnetic energy conversion rates in astrophysical plasmas. In this talk, we will review the main results about the linear phase of this instability and its effects on the magnetic reconnection rate. We will then see how plasma viscosity modifies the linear growth of the plasmoid instability and the magnetic reconnection rate. Finally, we will discuss also the nonlinear phase of the plasmoid instability and how it can be used to evaluate the time scale of the current sheet disruption.

11 March: Nicholas Senno (Penn State University)

"A story of choked jets and other hidden neutrino sources"

The Antarctic neutrino observatory IceCube has detected a robust diffuse flux signal consistent with neutrinos of extragalactic origin. To date, none of the observed neutrinos have been associated with either point sources or transient events. While a very high-energy (VHE) gamma-ray signal was theorized to accompany the observation of astrophysical neutrinos (if produced by e.g. Gamma Ray Bursts, starburst galaxies, and active galactic nuclei), new analyses from the IceCube and Fermi collaborations indicate that the sources of VHE neutrinos may be either gamma-ray dark or located at large cosmological distances. Previous work attempted to explain the diffuse neutrino signal using hypernovae (hyper-energetic supernovae) as sources located in galaxies with unusually high star forming activity, but such scenarios are currently disfavored. Recent work on choked jet and low-luminosity (LL) GRBs has renewed interest in the study of hidden neutrino sources. Under-powered jets have been shown to be more efficient cosmic-ray accelerators, and are therefore attractive neutrino factory candidates. I will show that if LL GRBs are caused by conventional GRB jets that are smothered by massive, optically-thick winds – as in the Nakar scenario, local sub-luminous bursts (e.g., GRB 060218, 100316D) should produce an observable precursor neutrino signal in IceCube.

15 April: Lynn Wilson III (NASA GSFC)

"Relativistic electrons produced by foreshock disturbances"

Charged particles reflected and accelerated to suprathermal or even ultra relativistic energies by strong (i.e., high Mach number) astrophysical collisionless shock waves can stream away forming a foreshock region in communication with the shock. Foreshocks -- observed since the 1960s -- are primarily populated by suprathermal ions (some contain relativistic particles) energized at or near the shock. The drift between foreshock ions and incident upstream flow generates electromagnetic waves and large-scale, transient structures -- called foreshock disturbances or FDs --, which have also been recently shown to energize ions, causing most work to focus on ion acceleration. Relativistic electrons were found at the high Mach number Kronian bow shock but no research has ever proposed Earth's lower Mach number bow shock, let alone the FDs, as a relativistic electron source. We present THEMIS satellite observations of relativistic electrons (>200 keV) locally produced by terrestrial FDs. Previous work attributed such electrons to escaping magnetospheric particles or a solar source. However, the enhancements do not coincide with any solar activity and are localized to FDs, supporting the conclusion that they are locally energized. The discovery of relativistic electrons generated by FDs could fundamentally change our understanding of shock acceleration, given the ubiquity of foreshocks in collisionless plasmas.

22 April: Fiorenza Donato (University of Turin, Italy)

"Antimatter in the Galaxy"

The physics of the galactic cosmic rays is entering a precision era, which touches also the rare species as the cosmic antimatter. We inspect the effect of close sources -such as supernovae and pulsars - on the positron and electron fluxes measured by AMS-02.

We discuss how the interpretation of antiproton data could improve with fixed target experiments at colliders, and why the still undetected cosmic antideuterons have the best signal-to-noise ration for indirect particle dark matter detection.

13 May: Vasily Beskin (LPI, MIPT)

"Radio pulsars (wind, statistics, and propagation of waves)."

Different aspects of the physics of radio pulsars are considered together trying to clarify the key properties of the electrodynamics of the pulsar magnetosphere. First, using two-fluid MHD approximation we investigate the internal region of the current sheet in the pulsar wind. Passing into comoving reference frame (where the current sheet is essentially time-dependent) we determine intrinsic electric and magnetic fields. It allows us to estimate the efficiency of the particle acceleration, the width of the sheet and its time evolution. Further, we analyse the statistics of interpulse pulsars which can give us the key to solve alignment/counter-alignment problem (which can be connected with the value of the north-south anti-symmetric bulk current circulating in the pulsar magnetosphere). Indeed, the number of interpulse pulsars, both, axisymmetric and orthogonal, drastically depends on the evolution of inclination angle. But it is shown that the observable distribution of interpulse pulsars slightly depends on the evolution law. Finally, it is shown that the existence of the high amplitude anti-symmetric current can change drastically the polarization properties of the mean radio profiles.

27 May: Bin Chen (NJ Institute of Technology)

"Particle Acceleration in Solar Flares: New Insights from Radio Observations"

Particle acceleration is an outstanding topic in many space, astrophysical, and laboratory contexts that involve magnetized plasma. Solar flares—the most powerful explosions in the solar system—are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. Thanks to the Sun’s proximity, solar flares serve as an outstanding laboratory to study fundamental physical processes of particle acceleration in great detail. In this talk, I will briefly review the current understanding of particle acceleration in solar flares. I will then present our new results, which have been recently published in Science, using radio observations from the Karl G. Jansky Very Large Array combined with extreme ultraviolet and X-ray data from space-based telescopes.

15 July: Mallory Roberts (NYU Abu Dhabi)

"Eclipsing Millisecond Pulsars: An Arachnophiliac’s Guide to Working in Mirkwood and the Forbidden Forest"

The life of a pulsar born in a binary system has been studied in many ways. The recent proliferation of eclipsing millisecond pulsars, split into two classes called Redbacks and Black Widows, lets us study a variety of pulsars in short orbits around low-mass stars during the accreting phase and the post-accreting phase. In addition to studying their evolution, these new systems also allow measurements of the upper range of neutron star masses and give us new ways of studying pulsar winds. But the observational and theoretical challenges are proliferating as fast as the spiders themselves. I will review where we are observationally with these systems, and discuss old and new theoretical challenges.

29 July: Dong Zhang (University of Virginia)

"On the Theory of Galactic Winds"

Galactic winds are ubiquitous in most rapidly star-forming galaxies in both the local and high-redshift universe. They shape the galaxy luminosity function, flattening its faint-end slope compared to the halo mass function, and affect the chemical evolution of galaxies, determining the mass-metallicity relation, regulating star formation over cosmic time, and polluting the intergalactic medium (IGM) with metals. Although important, the physics of galactic winds is still unclear. Many theoretical mechanisms have been proposed. Winds may be driven by the heating of the interstellar medium by overlapping supernovae, cosmic rays, the radiation pressure by continuum absorption and scattering of starlight on dust grains, or the momentum input from SNe. However, the comparison between theory and observation is incomplete. The growing observations of emission and absorption of cold molecular, cool atomic, and ionized gas in galactic outflows in a large number of galaxies have not been well explained by any models over a vast range of galaxy parameters. A full understanding of these issues requires both better theoretical explorations and comparisons with new and existing observations. In this presentation, I will be taking about the theoretical models of both radiation pressure- and supernova-driven galactic winds, and compared these models with observations. I will also be talking about my recent numerical simulations on momentum coupling between dust and gas in strong radiation field in starbursts and rapidly star-forming galaxies.

9 September: Pallavi Bhat (PPPL)

"Origin of large-scale magnetic fields in MRI simulations"

Magneto-rotational instability (MRI) drives turbulence in accretion disks. Large-scale fields in such systems are important for facilitating jets, nonlocal transport of angular momentum and understanding the MRI saturation.

The origin of such large-scale fields in shearing box simulations of MRI will be discussed. Of particular interest is whether the large scale dynamo can be modelled using mean field theory.

A novel result regarding MRI, that it can even be seeded by more realistic random/turbulent fields, will also be presented.

30 September: Almog Yalinewich (The Hebrew University of Jerusalem)

"Analytic solutions to strong shocks: Bow shocks and relativistic breakout"

In this talk I will present two of my recent projects. The first is the analytic study of a bow shock with infinite mach number. We show that in this case the shock will not converge to a mach cone, but rather to a parabolic solid of revolution. We calculate the hydrodynamic profiles inside the bow shock, and verify our results using a numerical simulation. We use our model to characterise a bow shock around an isolated neutron star.

The second project is the analysis of spherical relativistic shock breakout. Previous works studied the case where an outward moving shock accelerates matter in the stellar atmosphere, but the post shock acceleration ends before a fluid element travels a distance comparable to the stellar radius. Unfortunately, this formalism is only valid for low values of the Lorentz factor. In this work, we extend the formalism to higher values of the Lorentz factor, where the acceleration ends at distances much larger than the initial radius. We verify our results with a numerical simulation, and make predictions about the lightcurves and spectra from such breakouts. Finally, we discuss the question whether such events can occur in an astrophysical explosions.

14 October: Yuran Chen (Columbia University)

"PIC simulations of the twisted magnetosphere of magnetars"

Magnetars are rotating neutron stars with X-ray luminosity far exceeding their spin-down power. The radiation is believed to come from magnetospheric activity, powered by crustal motion caused by internal stress, which launches twists into the external magnetic field. Electric currents associated with the magnetic twists are sustained by electron-positron discharge. We study the mechanism of magnetar activity with plasma simulations using the particle-in-cell (PIC) technique. The simulations show how the twisted magnetosphere self-organizes to conduct the electric current and how the twist gradually dissipates over time. We are able to draw conclusions regarding the lifetime of the twist, locations of electric gaps accelerating particles, and the mechanisms of untwisting.

4 November: Slavko Bogdanov (Columbia University)

"Transitional Millisecond Pulsars"

The prevailing theory for the formation of radio millisecond pulsars posits that they acquire rapid spins via accretion of mass and angular momentum from a close companion in low-mass X-ray binaries. This long suspected evolutionary connection was confirmed only recently with the discovery of three binary millisecond pulsars that switch between clearly distinguishable accretion and rotation powered states. These so-called transitional millisecond pulsars are already offering valuable insight into the poorly understood details of compact binary evolution, as well as low luminosity accretion and jet formation processes. In this talk, I will present a summary of the recent deluge of radio, optical, UV, X-ray and gamma-ray data of these peculiar systems.

11 November

Joint meeting of the AstroPlasmas seminar group and the Princeton Center for Heliophysics

18 November: Rahul Kannan (MIT)

"Quenching, blackhole feedback and anisotropic thermal conduction"

I present high resolution cosmological magneto-hydrodynamical simulations of a galaxy cluster combined with anisotropic thermal conduction. We quantify the effect of the coupling between active galactic nucleai (AGN) feedback, magnetic fields and thermal conduction on both the integrated and small-scale properties of galaxy clusters. We show that conduction leads to an earlier termination of a cool core, reduces the star formation rates by more than an order of magnitude and leads to earlier quenching. The efficient coupling of the AGN feedback energy can be attributed to lowering of the stabilising buoyancy forces due to anisotropic conduction, which leads to rapid mixing of thermal plasma. These results indicate that thermal conduction plays a significant role in the efficient isotropisation of the injected AGN feedback energy thereby being important for the quenching and quiescence of massive galaxies.

2 December: Eric Zirnstein (Princeton)

"IBEX Observations and Simulations of Energetic Neutral Hydrogen from the Heliosphere"

The heliosphere is a dynamic region of space spanning hundreds to possibly thousands of astronomical units in size, created by the outflow of solar wind plasma and its interaction with the partially-ionized local interstellar medium. The solar and interstellar plasmas are separated by a tangential discontinuity known as the heliopause, but interstellar neutral atoms permeate the boundaries of the heliosphere and interact with the solar wind, generating energetic neutral atoms (ENAs). Hydrogen ENAs are created when relatively energetic protons from the heliospheric plasma charge-exchange with interstellar hydrogen atoms. Due to their large mean free paths, ENAs can propagate large distances before ionizing and can be detected at 1 AU. NASA’s Interstellar Boundary Explorer (IBEX) mission is aimed at improving our understanding of the heliospheric interface. Launched in 2008, IBEX continues to measure fluxes of hydrogen ENAs at 1 AU that are created from the solar-interstellar interaction, as well as interstellar neutral atoms that permeate the heliospheric boundary. In this talk I will briefly discuss IBEX observations of ENAs, results from simulating ENA fluxes at 1 AU derived from a three-dimensional MHD-plasma/kinetic-neutral simulation of the heliosphere, and some key properties we have learned about the solar-interstellar interaction.

9 December: Illya Plotnikov (The Research Institute in Astrophysics and Planetology, France)

"Relativistic shocks in magnetised pair plasmas: 2D kinetic structure, formation time and particle acceleration efficiency"

We study the structure and particle acceleration properties of perpendicular relativistic shocks for a large set of upstream pair plasma magnetizations using two dimensionnal Particle-In-Cell simulations. A systematic survey is done from non-magnetized up to highly magnetized shocks in order to accurately capture the transition from the filamentation dominated structure to magnetic reflection shaped shocks. For shock Lorentz factors equal to 17 we find that this transition occurs at magnetizations between 1e-3 and 1e-2 in agreement with Sironi et 2013. Particle populated precursor penetrating the upstream flow disappears leaving place to a strong emission of the extraordinary mode wave. A slight 10% deviation in shock jump conditions compared to the ideal MHD prediction is observed for mildly magnetized shock while a very good agreement is found to lower or higher magnetizations. Particle acceleration is efficient in weakly magnetized perpendicular shocks in agreement with previously published works. The non-thermal tail dissapears at the transition and the particle spectrum remains Maxwellian at higher magnetizations. We derive systematically the particle precursor length, amplitude of the electromagnetic precursor wave as well as particle spatial and energy diffusion coefficients in the downstream dacaying micro-turbulence. These results have implications for the properties of particle acceleration efficiency in the termination shocks of the Pulsar Wind Nebulae and in the internal shocks of the AGN jets.

20 December: Nuno Loureiro (MIT)

"The onset of magnetic reconnection"

Magnetic reconnection is a two-timescale process: an initial, slow, energy build-up stage is suddenly followed by an explosive stage of energy release. Understanding this transition is usually referred as the reconnection onset, or trigger problem. While there have been many studies of the reconnection stage proper, the onset has remained largely unexplored. Recent results, however, have convincingly demonstrated that the initial configurations (current sheets) typically employed in the study of reconnection are strongly unstable, implying that they cannot form in the first place. As a result, the process of current sheet formation, throughout which energy accumulates, cannot be decoupled from the reconnection process itself, where such energy is released, and suggests that the onset problem can be addressed by inquiring about the current sheet stability as it forms.

I will present a recent analytical model (Uzdensky and Loureiro, Phys. Rev. Lett. 2016) which addresses these issues, and then discuss how to apply it to predict the onset of reconnection in MHD turbulence.