32 research outputs found
Ion acceleration in non-relativistic astrophysical shocks
We explore the physics of shock evolution and particle acceleration in
non-relativistic collisionless shocks using multidimensional hybrid
simulations. We analyze a wide range of physical parameters relevant to the
acceleration of cosmic rays (CRs) in astrophysical non-relativistic shock
scenarios, such as in supernova remnant (SNR) shocks. We explore the evolution
of the shock structure and particle acceleration efficiency as a function of
Alfv\'enic Mach number and magnetic field inclination angle . We show
that there are fundamental differences between high and low Mach number shocks
in terms of the electromagnetic turbulence generated in the pre-shock zone and
downstream; dominant modes are resonant with the streaming CRs in the low Mach
number regime, while both resonant and non-resonant modes are present for high
Mach numbers. Energetic power law tails for ions in the downstream plasma can
account for up to 15% of the incoming upstream flow energy, distributed over
of the particles in a power law with slope in energy. The
energy conversion efficiency (for CRs) peaks at to
and , and decreases for higher Mach numbers, down to for
. Accelerated particles are produced by Diffusive Shock Acceleration
(DSA) and by Shock Drift Acceleration (SDA) mechanisms, with the SDA
contribution to the overall energy gain increasing with magnetic inclination.
We also present a direct comparison between hybrid and fully kinetic
particle-in-cell results at early times; the agreement between the two models
justifies the use of hybrid simulations for longer-term shock evolution. In SNR
shocks, particle acceleration will be significant for low Mach number
quasi-parallel flows (, ). This finding underscores the
need for effective magnetic amplification mechanism in SNR shocks
Sep acceleration in CME driven shocks using a hybrid code
WOS:000341172100009 (Nº de Acesso Web of Science)We perform hybrid simulations of a super-Alfvénic quasi-parallel shock, driven by a coronal mass ejection (CME), propagating in the outer coronal/solar wind at distances of between 3 to 6 solar radii. The hybrid treatment of the problem enables the study of the shock propagation on the ion timescale, preserving ion kinetics and allowing for a self-consistent treatment of the shock propagation and particle acceleration. The CME plasma drags the embedded magnetic field lines stretching from the sun, and propagates out into interplanetary space at a greater velocity than the in situ solar wind, driving the shock, and producing very energetic particles. Our results show that electromagnetic Alfvén waves are generated at the shock front. The waves propagate upstream of the shock and are produced by the counter-streaming ions of the solar wind plasma being reflected at the shock. A significant fraction of the particles are accelerated in two distinct phases: first, particles drift from the shock and are accelerated in the upstream region, and second, particles arriving at the shock get trapped and are accelerated at the shock front. A fraction of the particles diffused back to the shock, which is consistent with the Fermi acceleration mechanism
Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons
We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks
The IST Cluster: an integrated infrastructure for parallel applications in Physics and Engineering
WOS:000283531600008 (Nº de Acesso Web of Science)The infrastructure to support advanced computing applications at Instituto Superior T´ecnico is presented, including a detailed description of the hardware, system software, and benchmarks, which show an HPL performance of 1.6 Tflops. Due to its decentralized administrative basis, a discussion of the usage policy and administration is also given. The in-house codes running in production are also presented
Mini-magnetospheres above the lunar surface and the formation of lunar swirls
In this paper we present in-situ satellite data, theory and laboratory
validation that show how small scale collisionless shocks and
mini-magnetospheres can form on the electron inertial scale length. The
resulting retardation and deflection of the solar wind ions could be
responsible for the unusual "lunar swirl" patterns seen on the surface of the
Moon.Comment: 5 pages, 5 figure
ION ACCELERATION AT THE QUASI-PARALLEL BOW SHOCK: DECODING THE SIGNATURE OF INJECTION
Collisionless shocks are efficient particle accelerators. At Earth, ions with
energies exceeding 100 keV are seen upstream of the bow shock when the magnetic
geometry is quasi-parallel, and large-scale supernova remnant shocks can
accelerate ions into cosmic rays energies. This energization is attributed to
diffusive shock acceleration, however, for this process to become active the
ions must first be sufficiently energized. How and where this initial
acceleration takes place has been one of the key unresolved issues in shock
acceleration theory. Using Cluster spacecraft observations, we study the
signatures of ion reflection events in the turbulent transition layer upstream
of the terrestrial bow shock, and with the support of a hybrid simulation of
the shock, we show that these reflection signatures are characteristic of the
first step in the ion injection process. These reflection events develop in
particular in the region where the trailing edge of large-amplitude upstream
waves intercept the local shock ramp and the upstream magnetic field changes
from quasi-perpendicular to quasi-parallel. The dispersed ion velocity
signature observed can be attributed to a rapid succession of ion reflections
at this wave boundary. After the ions' initial interaction with the shock, they
flow upstream along the quasi-parallel magnetic field. Each subsequent wave
front in the upstream region will sweep the ions back toward the shock, where
they gain energy with each transition between the upstream and the shock wave
frames. Within three to five gyroperiods, some ions have gained enough parallel
velocity to escape upstream, thus completing the injection process.Comment: 30 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
Routine screening of harmful microorganisms in beach sands: implications to public health
Beaches worldwide provide recreational opportunities to hundreds of millions of people and serve as important components of coastal economies. Beach water is often monitored for microbiological quality to detect the presence of indicators of human sewage contamination so as to prevent public health outbreaks associated with water contact. However, growing evidence suggests that beach sand can harbor microbes harmful to human health, often in concentrations greater than the beach water. Currently, there are no standards for monitoring, sampling, analyzing, or managing beach sand quality. In addition to indicator microbes, growing evidence has identified pathogenic bacteria, viruses, and fungi in a variety of beach sands worldwide. The public health threat associated with these populations through direct and indirect contact is unknown because so little research has been conducted relating to health outcomes associated with sand quality. In this manuscript, we present the consensus findings of a workshop of experts convened in Lisbon, Portugal to discuss the current state of knowledge on beach sand microbiological quality and to develop suggestions for standardizing the evaluation of sand at coastal beaches. The expert group at the "Microareias 2012" workshop recommends that 1) beach sand should be screened for a variety of pathogens harmful to human health, and sand monitoring should then be initiated alongside regular water monitoring; 2) sampling and analysis protocols should be standardized to allow proper comparisons among beach locations; and 3) further studies are needed to estimate human health risk with exposure to contaminated beach sand. Much of the manuscript is focused on research specific to Portugal, but similar results have been found elsewhere, and the findings have worldwide implications
Routine screening of harmful microorganisms in beach sands: implications to public health
Beaches worldwide provide recreational opportunities to hundreds of millions of people and serve as important components of coastal economies. Beach water is often monitored for microbiological quality to detect the presence of indicators of human sewage contamination so as to prevent public health outbreaks associated with water contact. However, growing evidence suggests that beach sand can harbor microbes harmful to human health, often in concentrations greater than the beach water. Currently, there are no standards for monitoring, sampling, analyzing, or managing beach sand quality. In addition to indicator microbes, growing evidence has identified pathogenic bacteria, viruses, and fungi in a variety of beach sands worldwide. The public health threat associated with these populations through direct and indirect contact is unknown because so little research has been conducted relating to health outcomes associated with sand quality. In this manuscript, we present the consensus findings of a workshop of experts convened in Lisbon, Portugal to discuss the current state of knowledge on beach sand microbiological quality and to develop suggestions for standardizing the evaluation of sand at coastal beaches. The expert group at the "Microareias 2012" workshop recommends that 1) beach sand should be screened for a variety of pathogens harmful to human health, and sand monitoring should then be initiated alongside regular water monitoring; 2) sampling and analysis protocols should be standardized to allow proper comparisons among beach locations; and 3) further studies are needed to estimate human health risk with exposure to contaminated beach sand. Much of the manuscript is focused on research specific to Portugal, but similar results have been found elsewhere, and the findings have worldwide implications
3D pic simulations of collisionless shocks at lunar magnetic anomalies and their role in forming lunar swirls
The authors would like to thank the Science and Technology Facilities Council for fundamental physics and computing resources that were provided by funding from STFC’s Scientific Computing Department, and would like to thank the European Research Council (ERC 2010 AdG Grant 267841) and FCT (Portugal) grants SFRH/BD/75558/2010 for support.Investigation of the lunar crustal magnetic anomalies offers a comprehensive long-term data set of observations of small-scale magnetic fields and their interaction with the solar wind. In this paper a review of the observations of lunar mini-magnetospheres is compared quantifiably with theoretical kinetic-scale plasma physics and 3D particle-in-cell simulations. The aim of this paper is to provide a complete picture of all the aspects of the phenomena and to show how the observations from all the different and international missions interrelate. The analysis shows that the simulations are consistent with the formation of miniature (smaller than the ion Larmor orbit) collisionless shocks and miniature magnetospheric cavities, which has not been demonstrated previously. The simulations reproduce the finesse and form of the differential proton patterns that are believed to be responsible for the creation of both the "lunar swirls" and "dark lanes." Using a mature plasma physics code like OSIRIS allows us, for the first time, to make a side-by-side comparison between model and space observations. This is shown for all of the key plasma parameters observed to date by spacecraft, including the spectral imaging data of the lunar swirls. The analysis of miniature magnetic structures offers insight into multi-scale mechanisms and kinetic-scale aspects of planetary magnetospheres.Publisher PDFPeer reviewe