2020

Oct 23: Turbulence, Dynamo, and Dissipation in Magnetized, Weakly Collisional Astrophysical Plasmas

Prof. Matt Kunz (Princeton)

Abstract: The transport of energy and momentum, the amplification and sustenance of magnetic fields, and the heating of plasma particles by waves and turbulence are key ingredients in many problems at the frontiers of heliospheric and astrophysics research. This includes the heating and acceleration of the solar wind; the observational appearance of black-hole accretion flows on event-horizon scales; and the properties of the hot, dilute plasma that fills dark-matter halos. All of these plasmas are magnetized and weakly collisional, with plasma beta parameters of order unity or even much larger (“high-beta”). In this regime, deviations from local thermodynamic equilibrium ("pressure anisotropies") and the kinetic instabilities they excite can dramatically change the material properties of such plasmas and thereby influence the macroscopic evolution of their host systems. I will highlight selected results from an ongoing program of calculations aimed at elucidating from first principles the physics of waves, turbulence, heating, and transport under these conditions. Three key results will be featured. (1) Turbulent amplification of magnetic fields is possible, efficient, and likely self-accelerating in weakly collisional plasmas such as the intracluster medium (ICM). (2) Pressure anisotropies generated either by fluctuations (as in the ICM) or by global expansion (as in the solar wind) qualitatively change the properties of magnetized turbulence by triggering kinetic instabilities, reducing the plasma viscosity, and altering the so-called “critical balance”. (3) Thermal disequilibration of ions and electrons is a generic outcome of magnetized, collisionless turbulence, with ions receiving a majority of the cascaded energy for beta >~ 1. In the waning minutes of the talk, I will advertise (briefly!) our group's ongoing efforts to understand the role of magnetic reconnection in the turbulent dynamo, the nature of collisionless heat conduction and convection, and the formation and evolution of protostellar accretion disks.

Oct 16: A new look at nonlinear evolution of Alfven waves in neutron star magnetospheres

Dr. Yajie Yuan (CCA)

Abstract: Young and active neutron stars may experience star quakes that can launch kHz Alfven waves into the magnetosphere. An Alfven wave packet propagating along the closed field lines undergoes several interesting nonlinear processes: (1) the Alfven wave can convert to fast magnetosonic waves that leave the magnetosphere; (2) the Alfven wave packet gets sheared due to the different lengths of neighboring field lines, leading to enhanced current that triggers kinetic instabilities; (3) Alfven waves propagating to the outer magnetosphere may break and form "plasmoids" (closed field loops) that accelerate away from the star. Most interestingly, the third process may be a viable mechanism to produce the simultaneous X-ray bursts and fast radio bursts from the galactic magnetar SGR 1935+2154. In this talk, I'll present our systematic study of all three processes and discuss their observational consequences.

Oct 2: A Unified Picture of Fast Radio Bursts

Dr. Wenbin Lu (Caltech)

Abstract: Fast radio bursts (FRBs) are short duration (~ms), very bright, radio transients. Their detection a decade ago was a major unexpected discovery in astronomy in decades. Hunting for FRBs and measuring their physical properties have become one of the leading scientific goals in astronomy. This effort has led to a rapidly growing sample with extremely diverse properties in luminosity (10^38 to 10^45 erg/s), duration (0.1 ms to 10 ms), and repetition rate (some objects have multiple bursts in an hour and many just one burst in a few years). I will present a study of their cosmological volumetric rate density and provide evidence that these bursts all belong to the same class of transients --- most likely all are repeaters. According to my model, disturbances close to the surface of a magnetar launch Alfven waves into the magnetosphere, which propagate to a distance of a few tens of neutron star radii and then produce coherent radio emission. The coincident hard X-rays associated with the Galactic FRB 200428 can be understood in this scenario. This model provides a unified picture for weak Galactic FRBs as well as the bright bursts seen at cosmological distances. If time allows, the polarization properties of FRBs will also be addressed.

Sep 25: Turbulent energy cascade rate in the solar wind and the Earth's magnetosheath using in-situ spacecraft data

Dr. Lina Hadid (LPP Paris)

Abstract: Compressible turbulence has been a subject of active research within the space physics community for the last three decades and is actually believed to be essential for understanding the physics of the solar wind (for instance the heating of the fast wind), of the interstellar medium (in cold molecular clouds) and other astrophysical and space phenomena. In this talk I will give a review of the different studies that we have done regarding the compressible and incompressible cascade rates in the interplanetary space. Firstly, using the exact law of compressible isothermal magnetohydrodynamic (MHD) turbulence [Banerjee & Galtier, PRE, 2013], we give an estimation of the compressible energy cascade rate (|εC|) in the Eart's magnetosheath using THEMIS and CLUSTER spacecraft data and show that it is at least three orders of magnitude larger than its value in the solar wind. Moreover, we show the role of the density fluctuations in increasing the spatial anisotropy in the Earth's magnetosheath [Hadid et al., PRL, 2018]. Secondly, using the exact law of compressible Hall MHD turbulence [Andrés & Sahraoui, PRE, 2017] we give a complete estimation of |εC | at the MHD and the sub-ion scales in the Earth's magnetosheath using MMS data [Andés et al., PRL, 2019]. Finally we show the radial evolution of the turbulent cascade rate from the Sun (~0.2 A.U.) up to Mars (~1.5 A.U.), using Parker Solar Probe and Maven data [Andrés et al. in prep.].

Sep 18: Heating the ICM via cosmic ray driven instabilities

Philipp Kempski (UC Berkeley)

Abstract: Most of the baryonic mass in galaxy clusters resides in the hot and tenuous Intracluster Medium (ICM) that fills the space between individual galaxies. In the dense central regions, the ICM gas rapidly loses energy via X-ray emission. Observations show that despite the large radiative losses, the ICM plasma does not cool efficiently. This suggests that there is a heating source present that keeps the gas in approximate thermal balance. It is now widely accepted that central Supermassive Black Holes and their jets likely play an important role in providing energy to the ICM and thus prevent a "cooling catastrophe". However, how this energy is transported and thermalized throughout the ICM remains an open question.

In this talk I will argue that cosmic rays may play an important role in the heating of the ICM plasma by efficiently exciting sound waves, which subsequently travel and dissipate across the ICM.

Jul 31: A new model for the simulation of kinetic dynamics in the expanding solar wind

Dr. Maria Elena Innocenti (KU Leuven)

Abstract: Using coordinated Parker Solar Probe/ Solar Orbiter/ Earth observations, it will soon be possible to probe magnetically connected solar wind plasma parcels across large heliocentric distances. In this talk, I will present a new method and code for the simulation of the evolution of kinetic processes with heliocentric distance, in the expanding solar wind. This method can provide an interpretative framework for coordinated observations. Kinetic features are ubiquitous in the young solar wind and rarer (but still non negligible) at 1 AU [Bale et al, 2019]. During propagation, kinetic processes constrain solar wind parameters [Stverak et al, 2008; Matteini et al, 2013] and regulate heat flux [Scime et al, 1994]. We simulate the evolution of selected kinetic processes with the fully kinetic semi-implicit Expanding Box Model code EB-iPic3D [Innocenti et al, 2019a, b], which models kinetically a solar wind plasma parcel moving away radially from the Sun while expanding in the transverse direction. We investigate how plasma expansion triggers the onset and modifies the evolution of kinetic instabilities (eg, the electron firehose instability) that constrain solar wind parameters in observations. We then investigate an indirect role of solar wind expansion in regulating heat flux, through the triggering of electron scale instabilities that then, in turn, contribute to heat flux regulation [Innocenti et al, in press].

Jul 17: Mildly relativistic shock propagation

Dr. Eric Coughlin (Princeton)

Abstract: Astrophysical explosions are accompanied by the formation of a shockwave that emanates spherically from the explosion site. When the motion of this shock is sub-relativistic (shock velocity much less than the speed of light) or ultra-relativistic (shock velocity nearly equal to the speed of light), the shock properties and the behavior of the post-shock fluid are described by well-known, self-similar solutions. When the shock speed is in between these two limits, the mildly relativistic nature of the shock speed (shock velocity ~ tens of percent of the speed of light) destroys the self-similar nature of the problem, and this regime has been less thoroughly explored despite its applicability to relativistic supernovae and sub-energetic gamma-ray bursts. In this talk I will describe an approach to understanding the propagation of such mildly relativistic blastwaves. In particular, I will show that when the shock speed is on the order of tens of percent of the speed of light, one can interpret relativistic terms as ``corrections'' to non-relativistic motion. Following this perturbative route, I will show that the shock velocity in this mildly relativistic regime is constrained by a unique ``eigenvalue'' that ensures the conservation of the total energy behind the blastwave. This finding implies that mildly relativistic shocks display secular, non-self-similar evolution in their deceleration and post-shock density, velocity, and pressure profiles, and I will describe the implications of this result in the context of the recently observed transient CSS161010 -- an extremely energetic, fast-rising, explosive event that displayed marginally relativistic motion.

Feb 28: Advances in understanding relativistic plasma turbulence

Dr. Vladimir Zhdankin (Princeton)

Abstract: Many distant high-energy astrophysical systems (such as pulsar wind nebulae, black-hole accretion flows, and jets from active galactic nuclei) contain collisionless plasmas that are relativistic, radiative, and highly nonthermal. Understanding the nature of turbulence in this extreme plasma physical regime and its implications for observations is an outstanding challenge in plasma astrophysics. Particle-in-cell (PIC) simulations have recently opened this topic to detailed, first-principles numerical and theoretical scrutiny. I will describe the latest progress on understanding relativistic kinetic turbulence. PIC simulations have demonstrated that relativistic turbulence is an efficient particle accelerator, joining the ranks of shocks and magnetic reconnection as a viable source of high-energy particles (and thus broadband radiation and cosmic rays). These simulations are now giving long-awaited tests for a line of analytic theories of stochastic particle acceleration originating with Enrico Fermi in 1949. Relativistic PIC simulations are also giving new insights into two-temperature electron-ion plasmas, radiatively cooled turbulence, and kinetic dynamo. The next several years promise to bring new breakthroughs into these problems.

Feb 7: Stellar CMEs: What can we learn from historic observations?

Dr. Sofia Moschou (CfA)

Abstract: Solar coronal mass ejections (CMEs) and flares have a statistically well-defined relationship, with more energetic X-ray flares corresponding to faster and more massive CMEs. How this relationship extends to more magnetically active stars is a subject of open research. Here we study the most probable stellar CME candidates associated with flares captured in the literature to date, all of which were observed on magnetically active stars. We use a simple CME model to derive masses and kinetic energies from observed quantities and transform associated flare data to the Geostationary Operational Environmental Satellite 1-8 Å band. Derived CME masses range from ~1015 to 1022 g. Associated flare X-ray energies range from 1031 to 1037 erg. Stellar CME masses as a function of associated flare energy generally lie along or below the extrapolated mean for solar events. In contrast, CME kinetic energies lie below the analogous solar extrapolation by roughly 2 orders of magnitude, indicating approximate parity between flare X-ray and CME kinetic energies. These results suggest that the CMEs associated with very energetic flares on active stars are more limited in terms of the ejecta velocity than the ejecta mass, possibly because of the restraining influence of strong overlying magnetic fields and stellar wind drag. Lower CME kinetic energies and velocities present a more optimistic scenario for the effects of CME impacts on exoplanets in close proximity to active stellar hosts.

Jan 31: Constraining Extragalactic Magnetic Fields Using Astroparticle Physics and Cosmology

Andrey Saveliev (Immanuel Kant Baltic Federal University, Russia)

Abstract: Extragalactic Magnetic Fields (EGMFs) are one of the most important phenomena in astrophysics and cosmology, as a lot of information concerning the evolution and the present state of the Universe may be derived using them. In order to put constraints on EGMFs, one needs to consider a variety of their interactions with different environments: On the one hand they influence the propagation of charged particles, of which Ultra High Energy Cosmic Rays (UHECRs) and electromagnetic cascades initiated by TeV-gamma rays are of particular interest, since their trajectories are highly sensitive to EGMF parameters (such as their average field strength, correlation length, spectrum and helicity). On the other hand, EGMFs directly interact with the Intergalactic Medium (IGM), the understanding of which requires numerical and (semi-)analytical large-scale simulations. In case one assumes that they have been created in the Early Universe, one has also to consider cosmological aspects such as their impact on the Cosmic Microwave Background (CMB), which might be used to derive further constraints. In this talk we will discuss all these different methods for a coherent and comprehensive view on EGMFs.