2018+Archive

"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 R s ) 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.

"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.

"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.

"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

"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.

"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.

"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.