172 research outputs found
Simulations of space plasma instabilities
PhD 1997 QMThis work describes computer simulations of the behaviour of plasmas similar to those
observed in the near Earth environment The work can be split into three main threads
Firstly we have developed a set of algorithms to allow the implementation of particle
type simulation models on parallel computer architectures ranging from small workstation
clusters to massively parallel supercomputers These algorithms allow large simulations
with many particles to be performed We address the problems of e cient use of available
computational resources and the scaling of algorithms as computers get larger
Secondly we use a parallel implementation of a two dimensional hybrid simulation
code with periodic boundaries to explore the evolution of ion beam distributions similar
to those observed upstream of the Earth s bow shock We follow the evolution of the
resonant instabilities of these cool tenuous proton beams both isotropic and anisotropic
in temperature into the non linear regime We examine the waves generated their e ects
on the ion distribution function the phenomenon of gyrophase bunching and describe the
life cycles of two dimensional magnetic features including oblique propagating shocklets
We suggest that such two dimensional structures may play a role in the saturation of
beam instabilities Coherence lengths of the waves are calculated We see some evidence
of anisotropy driven mirror waves late on in these simulations
Thirdly we explore the nature of parametric instabilities in two dimensions We
examine the role of parametric or wave wave instabilities in the late evolution of beam
instability generated waves We
nd little evidence of any parametric instability in this
case The two dimensional evolution of a wave known to be unstable to one dimensional
parametric instability is described We
nd that in this case the instability develops
in a manner similar to that found in one dimensional simulations although with some
angular broadening in wavevector space There is some evidence of anisotropy driven
instabilities later in the simulatio
Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection
Non-Maxwellian electron velocity space distribution functions (EVDF) are
useful signatures of plasma conditions and non-local consequences of
collisionless magnetic reconnection. In the past, EVDFs were obtained mainly
for antiparallel reconnection and under the influence of weak guide-fields in
the direction perpendicular to the reconnection plane. EVDFs are, however, not
well known, yet, for oblique (or component-) reconnection in dependence on
stronger guide-magnetic fields and for the exhaust (outflow) region of
reconnection away from the diffusion region. In view of the multi-spacecraft
Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of
collisionless magnetic reconnection in dependence on the guide-field strength
from small () to very strong () guide-fields, taking
into account the feedback of the self-generated turbulence. For this sake, we
carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM
code. We obtained anisotropic EVDFs and electron beams propagating along the
separatrices as well as in the exhaust region of reconnection. The beams are
anisotropic with a higher temperature in the direction perpendicular rather
than parallel to the local magnetic field. The beams propagate in the direction
opposite to the background electrons and cause instabilities. We also obtained
the guide-field dependence of the relative electron-beam drift speed, threshold
and properties of the resulting streaming instabilities including the strongly
non-linear saturation of the self-generated plasma turbulence. This turbulence
and its non-linear feedback cause non-adiabatic parallel electron acceleration
and EVDFs well beyond the limits of the quasi-linear approximation, producing
phase space holes and an isotropizing pitch-angle scattering.Comment: 21 pages, 8 figures. Revised to match with the version published in
Physics of Plasmas. An abridged version of the abstract is shown her
Kinetic Plasma Turbulence Generated in a 3D Current Sheet With Magnetic Islands
In this article we aim to investigate the kinetic turbulence in a reconnecting current sheet (RCS) with X- and O-nullpoints and to explore its link to the features of accelerated particles. We carry out simulations of magnetic reconnection in a thin current sheet with 3D magnetic field topology affected by tearing instability until the formation of two large magnetic islands using particle-in-cell (PIC) approach. The model utilizes a strong guiding field that leads to the separation of the particles of opposite charges, the generation of a strong polarization electric field across the RCS, and suppression of kink instability in the âout-of-planeâ direction. The accelerated particles of the same charge entering an RCS from the opposite edges are shown accelerated to different energies forming the âbump-in-tailâ velocity distributions that, in turn, can generate plasma turbulence in different locations. The turbulence-generated waves produced by either electron or proton beams can be identified from the energy spectra of electromagnetic field fluctuations in the phase and frequency domains. From the phase space analysis we gather that the kinetic turbulence may be generated by accelerated particle beams, which are later found to evolve into a phase-space hole indicating the beam breakage. This happens at some distance from the particle entrance into an RCS, e.g. about 7di (ion inertial depth) for the electron beam and 12di for the proton beam. In a wavenumber space the spectral index of the power spectrum of the turbulent magnetic field near the ion inertial length is found to be â2.7 that is consistent with other estimations. The collective turbulence power spectra are consistent with the high-frequency fluctuations of perpendicular electric field, or upper hybrid waves, to occur in a vicinity of X-nullpoints, where the Langmuir (LW) can be generated by accelerated electrons with high growth rates, while further from X-nullponts or on the edges of magnetic islands, where electrons become ejected and start moving across the magnetic field lines, Bernstein waves can be generated. The frequency spectra of high- and low-frequency waves are explored in the kinetic turbulence in the parallel and perpendicular directions to the local magnetic field, showing noticeable lower hybrid turbulence occurring between the electronâs gyro- and plasma frequencies seen also in the wavelet spectra. Fluctuation of the perpendicular electric field component of turbulence can be consistent with the oblique whistler waves generated on the ambient density fluctuations by intense electron beams. This study brings attention to a key role of particle acceleration in generation kinetic turbulence inside current sheets
Collisionless Magnetic Reconnection in Space Plasmas
Magnetic reconnection requires the violation of the frozen-in condition which
ties gyrating charged particles to the magnetic field inhibiting diffusion.
Ongoing reconnection has been identified in near-Earth space as being
responsible for the excitation of substorms, magnetic storms, generation of
field aligned currents and their consequences, the wealth of auroral phenomena.
Its theoretical understanding is now on the verge of being completed.
Reconnection takes place in thin current sheets. Analytical concepts proceeded
gradually down to the microscopic scale, the scale of the electron skin depth
or inertial length, recognizing that current layers that thin do preferentially
undergo spontaneous reconnection. Thick current layers start reconnecting when
being forced by plasma inflow to thin. For almost half a century the physical
mechanism of reconnection has remained a mystery. Spacecraft in situ
observations in combination with sophisticated numerical simulations in two and
three dimensions recently clarified the mist, finding that reconnection
produces a specific structure of the current layer inside the electron inertial
(also called electron diffusion) region around the reconnection site, the X
line. Onset of reconnection is attributed to pseudo-viscous contributions of
the electron pressure tensor aided by electron inertia and drag, creating a
complicated structured electron current sheet, electric fields, and an electron
exhaust extended along the current layer. We review the general background
theory and recent developments in numerical simulation on collisionless
reconnection. It is impossible to cover the entire field of reconnection in a
short space-limited review. The presentation necessarily remains cursory,
determined by our taste, preferences, and knowledge. Only a small amount of
observations is included in order to support the few selected numerical
simulations.Comment: Review pape
Plasma kinetics issues in an ESA study for a plasma laboratory in space
A study supported by the European Space Agency (ESA), in the context of its General Studies Programme, performed an investigation of the possible use of space for studies in pure and applied plasma physics, in areas not traditionally
covered by âspace plasma physicsâ. A set of experiments have been identified that can potentially provide access to new phenomena and to allow advances in several fields of plasma science. These experiments concern phenomena on a
spatial scale (101â104 m) intermediate between what is achievable on the ground and the usual solar system plasma observations. Detailed feasibility studies have been performed for three experiments: active magnetic experiments, largescale discharges and long tetherâplasma interactions. The perspectives opened by these experiments are discussed for magnetic reconnection, instabilities,
MHD turbulence, atomic excited states kinetics, weakly ionized plasmas,plasma diagnostics, artificial auroras and atmospheric studies. The discussion is also supported by results of numerical simulations and estimates
Order out of Randomness : Self-Organization Processes in Astrophysics
Self-organization is a property of dissipative nonlinear processes that are
governed by an internal driver and a positive feedback mechanism, which creates
regular geometric and/or temporal patterns and decreases the entropy, in
contrast to random processes. Here we investigate for the first time a
comprehensive number of 16 self-organization processes that operate in
planetary physics, solar physics, stellar physics, galactic physics, and
cosmology. Self-organizing systems create spontaneous {\sl order out of chaos},
during the evolution from an initially disordered system to an ordered
stationary system, via quasi-periodic limit-cycle dynamics, harmonic mechanical
resonances, or gyromagnetic resonances. The internal driver can be gravity,
rotation, thermal pressure, or acceleration of nonthermal particles, while the
positive feedback mechanism is often an instability, such as the
magneto-rotational instability, the Rayleigh-B\'enard convection instability,
turbulence, vortex attraction, magnetic reconnection, plasma condensation, or
loss-cone instability. Physical models of astrophysical self-organization
processes involve hydrodynamic, MHD, and N-body formulations of Lotka-Volterra
equation systems.Comment: 61 pages, 38 Figure
Electron acceleration at localized wave structures in the solar corona
Our dynamic Sun manifests its activity by different phenomena: from the 11-year cyclic sunspot pattern to the unpredictable and violent explosions in the case of solar flares. During flares, a huge amount of the stored magnetic energy is suddenly released and a substantial part of this energy is carried by the energetic electrons, considered to be the source of the nonthermal radio and X-ray radiation. One of the most important and still open question in solar physics is how the electrons are accelerated up to high energies within (the observed in the radio emission) short time scales. Because the acceleration site is extremely small in spatial extent as well (compared to the solar radius), the electron acceleration is regarded as a local process. The search for localized wave structures in the solar corona that are able to accelerate electrons together with the theoretical and numerical description of the conditions and requirements for this process, is the aim of the dissertation...thesi
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