59 research outputs found
Melting-induced stratification above the Earth's inner core due to convective translation
In addition to its global North-South anisotropy(1), there are two other
enigmatic seismological observations related to the Earth's inner core:
asymmetry between its eastern and western hemispheres(2-6) and the presence of
a layer of reduced seismic velocity at the base of the outer core(6-12). This
250-km-thick layer has been interpreted as a stably stratified region of
reduced composition in light elements(13). Here we show that this layer can be
generated by simultaneous crystallization and melting at the surface of the
inner core, and that a translational mode of thermal convection in the inner
core can produce enough melting and crystallization on each hemisphere
respectively for the dense layer to develop. The dynamical model we propose
introduces a clear asymmetry between a melting and a crystallizing hemisphere
which forms a basis for also explaining the East-West asymmetry. The present
translation rate is found to be typically 100 million years for the inner core
to be entirely renewed, which is one to two orders of magnitude faster than the
growth rate of the inner core's radius. The resulting strong asymmetry of
buoyancy flux caused by light elements is anticipated to have an impact on the
dynamics of the outer core and on the geodynamo
Relativistic magnetic reconnection in collisionless ion-electron plasmas explored with particle-in-cell simulations
Magnetic reconnection is a leading mechanism for magnetic energy conversion
and high-energy non-thermal particle production in a variety of high-energy
astrophysical objects, including ones with relativistic ion-electron plasmas
(e.g., microquasars or AGNs) - a regime where first principle studies are
scarce. We present 2D particle-in-cell (PIC) simulations of low
ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic
energy exceeding the plasma rest-mass energy. We identify outstanding
properties: (i) For relativistic inflow magnetizations (here ), the reconnection outflows are dominated by thermal agitation instead of
bulk kinetic energy. (ii) At large inflow electron magnetization (), the reconnection electric field is sustained more by bulk inertia than by
thermal inertia. It challenges the thermal-inertia-paradigm and its
implications. (iii) The inflows feature sharp transitions at the entrance of
the diffusion zones. These are not shocks but results from particle ballistic
motions, all bouncing at the same location, provided that the thermal velocity
in the inflow is far smaller than the inflow E cross B bulk velocity. (iv)
Island centers are magnetically isolated from the rest of the flow, and can
present a density depletion at their center. (v) The reconnection rates are
slightly larger than in non-relativistic studies. They are best normalized by
the inflow relativistic Alfv\'en speed projected in the outflow direction,
which then leads to rates in a close range (0.14-0.25) thus allowing for an
easy estimation of the reconnection electric field.Comment: Submitted to A&
The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness
Collisionless magnetic reconnection is a prime candidate to account for
flare-like or steady emission, outflow launching, or plasma heating, in a
variety of high-energy astrophysical objects, including ones with relativistic
ion-electron plasmas. But the fate of the initial magnetic energy in a
reconnection event remains poorly known: what is the amount given to kinetic
energy, the ion/electron repartition, and the hardness of the particle
distributions? We explore these questions with 2D particle-in-cell simulations
of ion-electron plasmas. We find that 45 to 75% of the total initial magnetic
energy ends up in kinetic energy, this fraction increasing with the inflow
magnetization. Depending on the guide field strength, ions get from 30 to 60%
of the total kinetic energy. Particles can be separated into two populations
that only weakly mix: (i) particles initially in the current sheet, heated by
its initial tearing and subsequent contraction of the islands; and (ii)
particles from the background plasma that primarily gain energy via the
reconnection electric field when passing near the X-point. Particles (ii) tend
to form a power-law with an index , that
depends mostly on the inflow Alfv\'en speed and magnetization
of species , with for electrons to for increasing .
The highest particle Lorentz factor, for ions or electrons, increases roughly
linearly with time for all the relativistic simulations. This is faster, and
the spectra can be harder, than for collisionless shock acceleration. We
discuss applications to microquasar and AGN coronae, to extragalactic jets, and
to radio lobes. We point out situations where effects such as Compton drag or
pair creation are important.Comment: 15 pages, submitted to A&
Apar-T: code, validation, and physical interpretation of particle-in-cell results
We present the parallel particle-in-cell (PIC) code Apar-T and, more
importantly, address the fundamental question of the relations between the PIC
model, the Vlasov-Maxwell theory, and real plasmas.
First, we present four validation tests: spectra from simulations of thermal
plasmas, linear growth rates of the relativistic tearing instability and of the
filamentation instability, and non-linear filamentation merging phase. For the
filamentation instability we show that the effective growth rates measured on
the total energy can differ by more than 50% from the linear cold predictions
and from the fastest modes of the simulation.
Second, we detail a new method for initial loading of Maxwell-J\"uttner
particle distributions with relativistic bulk velocity and relativistic
temperature, and explain why the traditional method with individual particle
boosting fails.
Third, we scrutinize the question of what description of physical plasmas is
obtained by PIC models. These models rely on two building blocks:
coarse-graining, i.e., grouping of the order of p~10^10 real particles into a
single computer superparticle, and field storage on a grid with its subsequent
finite superparticle size. We introduce the notion of coarse-graining dependent
quantities, i.e., quantities depending on p. They derive from the PIC plasma
parameter Lambda^{PIC}, which we show to scale as 1/p. We explore two
implications. One is that PIC collision- and fluctuation-induced thermalization
times are expected to scale with the number of superparticles per grid cell,
and thus to be a factor p~10^10 smaller than in real plasmas. The other is that
the level of electric field fluctuations scales as 1/Lambda^{PIC} ~ p. We
provide a corresponding exact expression.
Fourth, we compare the Vlasov-Maxwell theory, which describes a phase-space
fluid with infinite Lambda, to the PIC model and its relatively small Lambda.Comment: 24 pages, 14 figures, accepted in Astronomy & Astrophysic
Simulations of Optical Emissions for Attacking AES and Masked AES
In this paper we present a novel attack based on photonic
emission analysis targeting software implementations of AES. We focus on the particular case in which the attacker can collect the photonic emission of a limited number of sense amplifiers (e.g. only one) of the SRAM storing the S-Box. The attack consists in doing hypothesis on the secret key based on the knowledge of the partial output of the SubBytes operation. We also consider the possibility to attack a masked implementation of AES using the photonic emission analysis. In the case of masking, the attacker needs 2 leakages of the same encryption to overcome the randomization of the masks. For our analysis, we assume the same physical setup described in other previous works. Reported results are based on simulations with some hypothesis on the probability of photonic emission of a single transistor
Particle Acceleration in Pulsar Wind Nebulae: PIC modelling
We discuss the role of particle-in-cell (PIC) simulations in unveiling the
origin of the emitting particles in PWNe. After describing the basics of the
PIC technique, we summarize its implications for the quiescent and the flaring
emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be
emerging that, in addition to the standard scenario of particle acceleration
via the Fermi process at the termination shock of the pulsar wind, magnetic
reconnection in the wind, at the termination shock and in the Nebula plays a
major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae"
edited by D. Torres for Springer, based on the invited contributions to the
workshop held in Sant Cugat (Barcelona), June 14-17, 201
Role of electron inertia and reconnection dynamics in a stressed X-point collapse with a guide-field
In previous simulations of collisionless 2D magnetic reconnection it was
consistently found that the term in the generalised Ohm's law that breaks the
frozen-in condition is the divergence of the electron pressure tensor's
non-gyrotropic components. A fully relativistic particle-in-cell (PIC) code was
used to model -point collapse with a guide-field in two and three spatial
dimensions. We show that in a 2D -point collapse with a guide-field close to
the strength of the in-plane field, the increased induced shear flows along the
diffusion region lead to a new reconnection regime in which electron inertial
terms play a dominant role at the -point. This transition is marked by the
emergence of a magnetic island - and hence a second reconnection site - as well
as electron flow vortices moving along the current sheet. The reconnection
electric field at the -point is shown to exceed all lower guide-field cases
for a brief period, indicating a strong burst in reconnection. By extending the
simulation to three spatial dimensions it is shown that the locations of
vortices along the current sheet (visualised by their -value) vary in the
out-of-plane direction, producing tilted vortex tubes. The vortex tubes on
opposite sides of the diffusion region are tilted in opposite directions,
similarly to bifurcated current sheets in oblique tearing-mode reconnection.
The tilt angles of vortex tubes were compared to a theoretical estimation and
were found to be a good match. Particle velocity distribution functions for
different guide-field runs, for 2.5D and 3D simulations, are analysed and
compared.Comment: A&A, Forthcoming articl
Collisionless magnetic reconnection in relativistic plasmas with particle-in-cell simulations
L'objectif de cette thèse est l'étude de la reconnexion magnétique dans les plasmas non-collisionels et relativistes. De tels plasmas sont présents dans divers objets astrophysiques (MQs, AGNs, GRBs...), où la reconnexion pourrait expliquer la production de particules et de radiation de haute énergie, un chauffage, ou des jets. Une compréhension fondamentale de la reconnexion n'est cependant toujours pas acquise, en particulier dans les plasmas relativistes ion-électron. Nous présentons d'abord les bases de la reconnexion magnétique. Nous démontrons des résultats particuliers à la physique des plasmas relativistes, concernant par exemple la distribution de Maxwell-Jüttner. Ensuite, nous réalisons une étude détaillée de l'outil numérique utilisé : les simulations particle-in-cell (PIC). Le fait que le plasma réel contienne beaucoup plus de particules que le plasma PIC a des conséquences importantes (collisionalité, relaxation, bruit) que nous décrivons. Enfin, nous étudions la reconnexion magnétique dans les plasmas ion-électron et relativistes à l'aide de simulations PIC. Nous soulignons des points spécifiques : loi d'Ohm (l'inertie de bulk dominante), zone de diffusion, taux de reconnexion (et sa normalisation relativiste). Les ions et les électrons produisent des lois de puissance, avec un index qui dépend de la vitesse d'Alfvén et de la magnétisation, et qui peut être plus dur que dans le cas des chocs non-collisionels. De plus, les ions peuvent avoir plus ou moins d'énergie que les électrons selon la valeur du champ guide. Ces résultats fournissent une base solide à des modèles d'objets astrophysiques qui, jusque là, supposaient a priori ces résultats.The purpose of this thesis is to study magnetic reconnection in collisionless and relativistic plasmas. Such plasmas can be encountered in various astrophysical objects (microquasars, AGNs, GRBs...), where reconnection could explain high-energy particle and photon production, plasma heating, or transient large-scale outflows. However, a first principle understanding of reconnection is still lacking, especially in relativistic ion-electron plasmas. We first present the basis of reconnection physics. We derive results relevant to relativistic plasma physics, including properties of the Maxwell-Jüttner distribution. Then, we provide a detailed study of our numerical tool, particle-in-cell simulations (PIC). The fact that the real plasma contains far less particles than the PIC plasma has important consequences concerning relaxation times or noise, that we describe. Finally, we study relativistic reconnection in ion-electron plasmas with PIC simulations. We stress outstanding properties: Ohm's law (dominated by bulk inertia), structure of the diffusion zone, energy content of the outflows (thermally dominated), reconnection rate (and its relativistic normalization). Ions and electrons produce power law distributions, with indexes that depend on the inflow Alfvén speed and on the magnetization of the corresponding species. They can be harder than those produced by collisionless shocks. Also, ions can get more or less energy than the electrons, depending on the guide field strength. These results provide a solid ground for astrophysical models that, up to now, assumed with no prior justification the existence of such distributions or of such ion/electron energy repartition
The Hera space mission to the binary asteroid Didymos
reservedHera è una missione di difesa planetaria in fase di sviluppo nel programma Space Safety and Security dell'Agenzia spaziale europea per il lancio nell'ottobre 2024. Si incontrerà alla fine di dicembre 2026 con l'asteroide binario Didymos e in particolare la sua luna, Dimorphos, che è stata colpita dalla navicella spaziale DART della NASA il 26 settembre 2022 come primo test di deflessione dell'asteroide. Gli obiettivi principali di Hera sono la caratterizzazione dettagliata delle proprietà fisiche di Didymos e Dimorphos e del cratere prodotto dalla missione DART, nonché la misurazione dell'efficienza del trasferimento del momento derivante dall'impatto di DART.Hera is a planetary defense mission under development in the European Space Agency's Space Safety and Security program for launch in October 2024. It will rendezvous in late December 2026 with the binary asteroid Didymos and specifically its moon, Dimorphos, which was hit by NASA's DART spacecraft on Sept. 26, 2022 as the first deflection test of the asteroid. Hera's main objectives are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater produced by the DART mission, as well as the measurement of the momentum transfer efficiency resulting from the DART impact
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