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Regular and Chaotic Dynamics in 3-D Reconnecting Current Sheets
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Anastasios Anastasiadis Submitted: 2006-04-18 03:02
We consider the possibility of particles being injected at the interior of a reconnecting current sheet (RCS), and study their orbits by dynamical systems methods. As an example we consider orbits in a 3D Harris type RCS. We find that, despite the presence of a strong electric field, a mirror trapping effect persists to certain extent for orbits with appropriate initial conditions within the sheet. The mirror effect is stronger for electrons than for protons. In summary, three types of orbits are distinguished: a) chaotic orbits leading to escapes by stochastic acceleration, b) regular orbits leading to escapes along the field lines of the reconnecting magnetic component, and c) mirror-type regular orbits that are trapped in the sheet, making mirror oscillations. Dynamically, the latter orbits lie on a set of invariant KAM tori that occupy a considerable measure in the phase space of particles' motion. We also observe the phenomenon of stickiness, namely chaotic orbits that remain trapped in the sheet for a considerable time. A trapping domain, related to the boundary of mirror motions in velocity space, is calculated analytically. Analytical formulae are derived for the kinetic energy gain along regular or chaotic escaping orbits. The analytical results are compared with numerical simulations.
Authors: C. Gontikakis, C. Efthymiopoulos, A. Anastasiadis
Projects: None
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Publication Status: Mont. Not. R. Astr. Soc, 368(1), 293 - 304, 2006
Last Modified: 2006-04-19 10:18
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Electron acceleration and radiation in evolving complex active regions
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Anastasios Anastasiadis Submitted: 2004-04-06 02:23
We present a model for the acceleration and radiation of solar
energetic particles (electrons) in evolving complex active
regions. The spatio - temporal evolution of active regions is
calculated using a cellular automaton model, based on the
self-organized criticality. The acceleration of electrons is due
to the presence of randomly placed, localized electric fields
produced by the energy release process, simulated by the cellular
automaton model. We calculate the resulting kinetic energy
distributions of the particles and their emitted X-ray radiation
spectra, using the thick target approximation and we perform a
parametric study with respect to number of electric fields present
and the thermal temperature of the injected distribution. Finally,
comparing our results with the existing observations, we find that
they are in a good agreement with the observed X-ray spectra of
the energy range 100 - 1000 keV.
Authors: A. Anastasiadis, C. Gontikakis, N. Vilmer, L. Vlahos
Projects:
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Publication Status: A&A 422, 323-330, 2004.
Last Modified: 2004-07-09 02:34
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Ion heating in an auroral potential structure
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Anastasios Anastasiadis Submitted: 2004-02-27 03:12
We investigate the interaction of O$^{+}$ ions with a
one-dimensional potential well, using Hamiltonian formulation.
Heating of plasma originating in the terrestrial ionosphere plays
a catalytic role in solar-driven magnetic storms, which dissipate
energy globally within the magnetosphere of the earth. An
interesting candidate for ionospheric plasma heating is a
potential well located at auroral arcs in the high-latitude
magnetosphere. We consider a potential with an exponential form,
having a characteristic length $L_mathrm{x}$. The oxygen ions
drift towards the auroral arc in the presence of a constant
magnetic field $B_mathrm{z}$ and a constant electric field
$E_mathrm{y}$. The orbits of individual ions for different
initial conditions -- phase angle and kinetic energy -- are
traced. Our results show that, depending upon the initial
conditions, test particles can be either accelerated or
decelerated. Furthermore, we perform a parametric study for the
interactions of mono-energetic and Maxwellian type of initial ion
distribution -- using random phase angle injection of the
particles -- with respect to our main model parameter, the
characteristic length of the potential $L_mathrm{x}$. We conclude
that for characteristic lengths comparable to twice the ion
gyroradius, the O$^+$ population is accelerated.
Authors: A. Anastasiadis, I, A, Daglis and C. Tsironis
Projects:
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Publication Status: A&A, 419, 793-799 (2004)
Last Modified: 2004-05-10 05:41
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MHD consistent cellular automata (CA) models I:Basic features
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Anastasios Anastasiadis Submitted: 2003-03-21 03:26
A set-up is introduced which can be superimposed onto the existing solar flare cellular automata (CA) models, and which specifies the interpretation of the model's variables. It extends the CA models, yielding the magnetic field, the current, and an approximation to the electric field, in a way that is consistent with Maxwell's and the MHD equations. Applications to several solar flare CA models during their natural state (self-organized criticality (SOC)) show, among others, that (1) the magnetic field exhibits characteristic large-scale organization over the entire modeled volume; (2) the magnitude of the current seems spatially dis-organized, with no obvious tendency towards large-scale structures or even local organization; (3) bursts occur at sites with increased current, and after a burst the current is relaxed; (4) by estimating the energy released in individual bursts with the use of the current as Ohmic dissipation, it turns out that the power-law
distributions of the released energy persist.
The CA models, extended with the set-up, can thus be considered as models for energy-release through current-dissipation.The concepts of power-law loading and anisotropic events (bursts) in CA models are generalized to 3--D vector-field models, and their effect on the magnetic field topology is demonstrated.
Authors: H. Isliker, A, Anastasiadis, L. Vlahos
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Publication Status: A&A 363, 1134, 2000
Last Modified: 2003-03-21 03:26
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MHD consistent cellular automata (CA) models II: Aplications to solar flares
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Anastasios Anastasiadis Submitted: 2003-03-21 03:21
In Isliker et al. (2000b), an extended cellular automaton (X-CA) model for solar flares was introduced. In this model, the interpretation of the model's grid-variable is specified, and the magnetic field, the current, and an approximation to the electric field are yielded, all in a way that is consistent with Maxwell's and the MHD equations. Here, we reveal which relevant plasma physical processes are implemented by the X-CA model and in what
form, and what global physical set-up is assumed by this model when it is in its natural state (SOC). The basic results are: (1) On large-scales, all variables
show characteristic quasi-symmetries. (2) The global magnetic topology forms either (i) closed magnetic field lines, or (ii) an arcade of field lines above the
bottom plane line, if the model is slightly modified. (3) In case of the magnetic topology (ii), loading can be interpreted as if there were a plasma which flows predominantly upwards, whereas in case of the magnetic topology (i), as if there were a plasma flow expanding from the neutral line. (4) The small-scale
physics in the bursting phase represent localized diffusive processes. (5) The local diffusivity usually has a value which is effectively zero, and it turns locally
to an anomalous value if a threshold is exceeded, whereby diffusion dominates the quiet evolution (loading). (6) Flares (avalanches) are accompanied by the appearance of localized, intense electric fields. (7) In a variant on the X-CA model, the magnitude of the current is used directly in the instability criterion. First results indicate that the SOC state persists. (8) The current-dissipation during flares is spatially fragmented into a large number of dissipative current-surfaces of varying sizes, which show a highly dynamic temporal evolution.
Authors: H. Isliker, A. Anastasiadis, L. Vlahos
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Publication Status: A&A 377, 1068, 2001.
Last Modified: 2003-03-21 03:21
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Acceleration of solar energetic particles: The case of solar flares
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Anastasios Anastasiadis Submitted: 2003-03-21 03:09
Solar energetic particles (SEP) are believed to originate from two different sources, solar flares and coronal
mass ejections. These two sources are the most energetic
particle accelerators in the heliosphere, as they can
accelerate electrons from 10 keV to a few MeV and protons from a few MeV to a few GeV. In this contribution, we restrict our presentation to the case of solar flares, by reviewing the key observations of solar energetic particles, as well as the theoretical acceleration models, such as wave - particle acceleration, DC electric fields, and shock acceleration. Finally, we present a new theoretical approach connecting the acceleration with the energy release during solar flares, which might lead to a global modeling of solar flare energetics.
Authors: A. Anastasiadis
Projects:
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Publication Status: J. Atmosph. Solar-Terrestiral Pys., vol 64(5-6), 481, 2002.
Last Modified: 2003-03-21 03:11
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