2018

November 16: Neal Turner (JPL/Caltech)

Fragments from the Origins of the Solar System and our Interstellar Locale (FOSSIL): A Cometary, Asteroidal, and Interstellar Dust Mission Concept

Comets and asteroids preserve the least-altered materials remaining from the birth of the Solar System. However, the few visited by spacecraft are diverse in composition, and touring enough of these small bodies for a representative sample would be costly and time-consuming. A team of us is therefore developing a proposal for a mission to measure the makeup of large numbers of comets and asteroids via the dust they shed into the Zodiacal Cloud.

The mission concept, Fragments from the Origins of the Solar System and our Interstellar Locale (FOSSIL), would carry four identical Dust Telescopes whose combined collecting area would yield measurements of the orbits and compositions of tens of thousands of the zodiacal particles, along with about a thousand of the interstellar particles that are constantly traversing the Solar System. These measurements would enable FOSSIL to reach four science objectives: (1) determine whether Jupiter-Family Comets' fine-grained components preserve unprocessed presolar molecular cloud particles, or show signatures of processing in the Solar System; (2) determine whether the rocky composition of the contemporary local interstellar cloud's dust population is consistent with being the feedstock for the formation of the Solar System; (3) determine whether comets' and asteroids' organics are genetically related, or formed from distinct reservoirs; and (4) determine the numbers and orbits of micron-sized particles originating from each of the three categories -- comets, asteroids, and interstellar dust -- in the Solar debris disk at 1 AU.

FOSSIL's panoramic view of primitive Solar System bodies' chemical diversity, and its comprehensive measurement of interstellar dust sizes and compositions, would contribute to answering three of the major questions posed in the most recent Planetary Science Decadal Survey: What were the initial stages, conditions, and processes of Solar System formation, and the nature of the interstellar matter that was incorporated? What were the primordial sources of organic matter? And how have myriad chemical and physical processes shaped the Solar System? FOSSIL also would contribute to heliospheric science by measuring interstellar particles' deflection in the Solar wind, and to astrophysics through its measurements of the only debris disk where we can study individual particles and their parent bodies -- our own Zodiacal Cloud. Furthermore, by measuring both the particles' orbits and their compositions, FOSSIL would link decades of ground-based meteor trajectory data, lacking compositions, to decades of laboratory studies of meteorites and micrometeorites, lacking orbits. In particular, FOSSIL would tell us which families of bodies are the most likely sources of existing laboratory specimens.

Each of FOSSIL's four Dust Telescopes consists of a Dust Trajectory Sensor (DTS) and an impact ionization reflectron-type time-of-flight mass spectrometer, the Composition Analyzer (CA). The DTS and CA are derived from and would use similar technology to previous and current flight instruments, especially the Cosmic Dust Analyzer (CDA) onboard Cassini, the Lunar Dust Experiment (LDEX) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE), and the Surface Dust Analyzer (SUDA) instrument in development for Europa Clipper.

In summary, FOSSIL would be a low-risk mission to survey the composition of the Solar System's primitive material at a cost far lower than sending probes to a representative sample of small bodies. Building on in situ and returned sample measurements of cometary, asteroidal, and interstellar particles' compositions from the Giotto, Vega, Ulysses, Stardust, Rosetta, Cassini, and OSIRIS-REx missions, FOSSIL would enable breakthroughs in understanding our Solar System's largest visible structure, the Zodiacal Cloud, and reading its record of the origins of our planetary system.

Notice: Pre-Decisional Information – For Planning and Discussion Purposes Only

November 9: Paz Beniamini (George Washington University)

Observational constraints on the structure of gamma-ray burst jets and lessons from GW170817

Motivated by GW170817 we examine three independent constraints on the angular structure of gamma-ray burst (GRB) jets that are required by observations of cosmological long GRBs. We find that efficient gamma-ray emission has to be restricted to material with Gamma>50 and is most likely confined to a narrow region around the core. Comparing GRB170817 with the regular population of short GRBs (sGRBs), we show that an order unity fraction of NS mergers result in sGRB jets that breakout of the surrounding ejecta, that their luminosity function must be intrinsically peaked and that sGRB jets are typically narrow with opening angles ~ 0.1 rad. Finally, we perform Monte Carlo simulations to examine models for the structure and efficiency of the prompt emission in off-axis sGRBs. We find that only a small fraction, 0.01-0.1, of NS mergers detectable by LIGO/VIRGO in GWs is expected to be also detected in prompt gamma-rays, and GW170817-like events are very rare.

October 26: Claire Guepin (Institut d’Astrophysique de Paris)

Chasing the cosmic accelerators with multiple messengers

The advent of multi-messenger and transient astronomy is giving new insights into the most powerful particle accelerators of the Universe. Despite the recent breakthroughs in observation and modeling of transient phenomena, long-lasting mysteries still obscure the high-energy Universe, as the origins of ultra-high energy cosmic-rays (UHECR) and high-energy (HE) neutrinos are still unknown. Deciding between the various source candidates is not an easy task, which requires a precise modeling of the propagation, acceleration and interactions or cosmic-rays. We derive general criteria allowing to point the most promising transient sources for the emission of HE transient neutrino signals, which could be detected by the IceCube experiment, and future detectors such as GRAND or POEMMA. A detailed study of the fate of UHECR in the vicinity of the sources is required to refine these results and predict additional multi-messenger signatures. Using a code that we developed for this purpose, we can simulate the propagation and interaction of UHECR in any radiative background. This can be efficiently applied to tidal disruptions by massive black holes or neutron star mergers. Moreover, Pulsar magnetospheres are good laboratories to study precisely particle acceleration. Using particle in cell simulations, we show that accelerated ions can escape from these environments. This leads to observational signatures, as for instance a diffuse gamma-ray emission in the galactic center region.

October 12: Jonathan Zrake (Columbia University)

Charged particle acceleration by the helical kink mode in AGN jets

I will discuss how charged particles are accelerated in z-pinch plasma configurations by the helical kink instability (KI). This instability operates at the spine of AGN jets downstream of a recollimation shock. We have simulated the KI in pair-plasma, with and without guide magnetic field, in a 3D fully kinetic particle-in-cell setting using the OSIRIS code. Our results show that in its non-linear stage, the KI mediates efficient energy transfer from the strongly perturbed (initially toroidal) magnetic field to non-thermal particles. The particle energy spectrum dN/dE develops a power-law tail, with index p between 2 and 3 (depending on plasma magnetization), extending to the system's confinement energy. The mechanism of particle acceleration is linked directly to the geometry of kinking z-pinches, and I will argue it is distinct from those operating in reconnecting current layers or isotropic turbulence. If there is time I will discuss applications to AGN jets and their possible role as UHECR sources.

October 5: Xinyu Li (Columbia University)

Electrodynamics inside and outside magnetars

The ultra-strong magnetic fields of magnetars have profound implications for their radiative phenomena. We studied the dynamics of strong magnetic fields inside and outside magnetars. Inside the magnetar, the strong magnetic stress can break the crust and trigger plastic failures. The interaction between magnetic fields and plastic failures is studied in two scenarios: 1. Internal Hall waves launched from the core-crust interface can initiate plastic failures and lead to X-ray outbursts. 2. External Alfven waves produced by giant flares can also initiate crustal plastic failures which dissipate the waves and give rise to delayed thermal afterglow. The crustal dissipation of Alfven waves is competed by the magnetospheric dissipation outside the magnetar. Using a high order simulation of Force-Free Electrodynamics (FFE), we found that the magnetospheric dissipation of Alfven waves is generally slow and most wave energy will dissipate inside the magnetar.

June 29: Mario Riquelme (University of Chile)

"Stochastic Particle Acceleration By Pressure Anisotropy-driven Plasma Instabilities"

May 25: Anna Tenerani (UCLA)

"Alfvén waves in the solar wind and the problem of constant-B field fluctuations: theory and predictions for the Parker Solar Probe"

One of the outstanding problems in astrophysics is the origin of stellar coronae, winds, and, more generally, the ubiquitous existence in the universe of hot million degree (or more) plasmas. The solar corona and wind provide an accessible environment to understand plasma heating and acceleration, and this is one of the main goals of the upcoming NASA mission Parker Solar Probe, which will arrive closer to the Sun (10 Rs) than any previous spacecraft.

Alfvén waves, which can easily propagate along magnetic field lines from the cooler photosphere to the hot corona and above, are thought to provide a possible mechanism to supply the energy required to heat and boost the solar wind, through turbulent dissipation and pressure. Large amplitude, turbulent Alfvénic fluctuations have indeed been observed in the fast streams of the solar wind for over fifty years. Our comprehension of their nonlinear evolution in the solar wind remains however elusive.

Perhaps one of the most surprising and yet unexplained (and often forgotten) property of such Alfvénic fluctuations is that the magnitude of the total magnetic field remains remarkably constant, even in correspondence of the largest amplitudes (δB/B~1). This means that Alfvenic fluctuations must have an intrinsic degree of coherence, emerging as a specific polarization, which is required to maintain a constant-B field.

In this talk we focus on the problem of the existence and dynamical accessibility of constant-B nonlinear states in collisionless plasmas. We investigate the stability properties of Alfvénic fluctuations to both parametric decay and (dispersion-less) firehose instability, and we show that broadband, constant-B nonlinear states are a basin of attraction of the firehose instability. We discuss possible implications for Parker Solar Probe.

April 20: Alexander Tchekhovskoy (Northwestern University)

"When Pulsars Get Hungry: putting new spin on magnetospheric accretion"

April 13: Adithan Kathirgamaraju (Purdue University)

"Probing the presence of mildly relativistic components and structure in GRBs"

In the first half of this talk, I will focus on the combined emission from long GRB afterglows, their associated Supernovae and mildly relativistic components that can be present in these events. Taking into account this combined emission, I will discuss the prospects of detecting orphan afterglows and how follow up observations can reveal the presence of this mildly relativistic ejecta.

In the second half, we will examine how the structure of a GRB jet can have important consequences on the observed prompt emission, especially for off-axis observers. I will also discuss how a structured jet can help explain some of the peculiar observations of GW170817, the first detected binary neutron star merger.

April 6: Alex Chen (Princeton)

"Magnetar discharge and other numerical experiments on radiative transfer"

In many astrophysical systems, in-situ pair production from high energy radiation is an important mechanism which has intricate interactions with their electrodynamics. In the magnetosphere of magnetars, for example, a star quake can twist the field lines to launch a current that requires pair production to sustain. I will explain the radiative transfer, particle kinematics, electrodynamics, and simulations of such a twisted magnetosphere, and compare this picture with the observed hard X-ray radiation. If time permits, I will also talk about a couple of other ongoing numerical experiments on different systems where pair production is a core component of their physics.

March 16: Fabio Cruz (Instituto Superior Técnico, Lisbon)

"PIC simulations of magnetospheres: from the lunar surface to pulsars"

Particle-in-cell (PIC) is a method to solve the self-consistent interaction between plasma particles and fields with nearly no approximations for space and time scales larger than the quantum scales. Being able to resolve plasma kinetic scales, PIC simulations are an excellent tool to model the interaction between plasmas and magnetized obstacles and objects where fluid approximations break down.

In this seminar, I will present OSIRIS [1] PIC simulations of magnetospheres formed in difference space and astrophysical scenarios. In particular, I will show numerical simulations of collisionless shocks formed in miniature lunar and cometary magnetospheres [2] and their role in the X-ray emission from these objects, including a connection with recent experimental work [3].

I will also present PIC modules recently included in the OSIRIS framework and specially developed for a 2D axisymmetric spherical geometry, including a new method to accurately deposit the current carried by PIC particles while conserving charge. PIC simulations of magnetic monopoles and dipoles developed with this code will also be presented. These simulations recover the main force-free limit features of pulsar magnetospheres, and present an important benchmark for the recently developed PIC modules.

[1] R. A. Fonseca et al., Lecture Notes in Computer Science 2331 (2012)

[2] F. Cruz et al., Physics of Plasmas 24 (2017); doi: 10.1063/1.4975310

[3] A. Rigby et al., accepted for publication in Nature Physics

March 2: Daniele Gaggero (GRAPPA, University of Amsterdam)

"Learning the physics of CR transport from non-thermal Galactic emission"

The extremely accurate charged cosmic-ray data recently provided by the AMS collaboration and the gamma-ray data from Fermi-LAT and other experiments allowed to enter a new era of precision measurements in the CR field, and offer for the first time the unique opportunity to investigate different transport properties in different regions of the Galaxy. I will review the status of the field, the most relevant anomalies detected so far, possible interpretations and ways to disentangle them. I will eventually discuss future prospects in the cosmic ray field with particular emphasis on possible ways to test theoretical predictions on CR transport properties on multi-messenger data by means of comprehensive numerical frameworks.

Feb 23: Joonas Nättilä (KTH Royal Institute of Technology and Stockholm University)

"Relativistic plasma in silico - towards full 6D kinetic simulations"

Relativistic plasma is ubiquitous in astrophysics. Physically it can be studied by using the so-called Vlasov/Boltzmann equation that describes how the 6D (i.e., 3D3V) phase space of the plasma evolves. In my talk I will discuss our current efforts in building a general open source set of tools for simulating such systems known as the plasma-toolkit code. The toolkit is build on top of a new massively parallel grid infrastructure that can harness the next-generation exascale computing resources. In order to deal with the immense memory consumption of the full 6D phase space, the current Vlasov solver is designed to have aggressive adaptive mesh refinement capabilities both in configuration and momentum space. These new computational advances allow us to progress into a new era of simulating relativistic kinetic plasma from first principles in full 6D. I will end my talk by presenting some of our first physical results from running the toolkit in 1D3V.

Jan 26: Bart Ripperda and Fabio Bacchini (KU Leuven)

"Generalized, energy-conserving numerical integration of geodesics in general relativity"

The numerical integration of particle trajectories in curved spacetimes is fundamental for obtaining realistic models of the particle dynamics around massive compact objects such as black holes and neutron stars. Generalized algorithms capable of handling generic metrics are required for studies of both standard spacetimes Schwarzschild and Kerr metrics) and non-standard spacetimes (e.g. Schwarzschild metric plus non-classical perturbations or multiple black hole metrics). The most commonly employed explicit numerical schemes (e.g. Runge-Kutta) are incapable of producing highly accurate results at critical points, e.g. in the regions close to the event horizon where gravity causes extreme curvature of the spacetime, at an acceptable computational cost. Here, we describe a generalized algorithm for the numerical integration of time-like (massive particles) and null (photons) geodesics in any given 3+1 split spacetime. We introduce a new, exactly energy-conserving implicit integration scheme based on the preservation of the underlying Hamiltonian, and we compare its properties with a standard fourth-order Runge-Kutta explicit scheme and an implicit midpoint scheme. We test the numerical performance of the three schemes against analytical solutions of test particle and photon orbits in Schwarzschild and Kerr spacetimes. We also prove the versatility of our framework in handling more exotic metrics such as Morris-Thorne wormholes and quantum-perturbed Schwarzschild black holes. The generalized approach is also discussed in the perspective of future extensions to more complex particle dynamics, e.g. the addition of the Lorentz force acting on charged particles.