3,306 research outputs found
Simulating Flaring Events in Complex Active Regions Driven by Observed Magnetograms
We interpret solar flares as events originating from active regions that have
reached the Self Organized Critical state, by using a refined Cellular
Automaton model with initial conditions derived from observations. Aims: We
investigate whether the system, with its imposed physical elements,reaches a
Self Organized Critical state and whether well-known statistical properties of
flares, such as scaling laws observed in the distribution functions of
characteristic parameters, are reproduced after this state has been reached.
Results: Our results show that Self Organized Criticality is indeed reached
when applying specific loading and relaxation rules. Power law indices obtained
from the distribution functions of the modeled flaring events are in good
agreement with observations. Single power laws (peak and total flare energy) as
well as power laws with exponential cutoff and double power laws (flare
duration) are obtained. The results are also compared with observational X-ray
data from GOES satellite for our active-region sample. Conclusions: We conclude
that well-known statistical properties of flares are reproduced after the
system has reached Self Organized Criticality. A significant enhancement of our
refined Cellular Automaton model is that it commences the simulation from
observed vector magnetograms, thus facilitating energy calculation in physical
units. The model described in this study remains consistent with fundamental
physical requirements, and imposes physically meaningful driving and
redistribution rules.Comment: 14 pages; 12 figures; 6 tables - A&A, in pres
Prompt acceleration of ions by oblique turbulent shocks in solar flares
Solar flares often accelerate ions and electrons to relativistic energies. The details of the acceleration process are not well understood, but until recently the main trend was to divide the acceleration process into two phases. During the first phase elctrons and ions are heated and accelerated up to several hundreds of keV simultaneously with the energy release. These mildly relativistic electrons interact with the ambient plasma and magnetic fields and generate hard X-ray and radio radiation. The second phase, usually delayed from the first by several minutes, is responsible for accelerating ions and electrons to relativistic energies. Relativistic electrons and ions interact with the solar atmosphere or escape from the Sun and generate gamma ray continuum, gamma ray line emission, neutron emission or are detected in space by spacecraft. In several flares the second phase is coincident with the start of a type 2 radio burst that is believed to be the signature of a shock wave. Observations from the Solar Maximum Mission spacecraft have shown, for the first time, that several flares accelerate particles to all energies nearly simultaneously. These results posed a new theoretical problem: How fast are shocks and magnetohydrodynamic turbulence formed and how quickly can they accelerate ions to 50 MeV in the lower corona? This problem is discussed
Energetic ion acceleration at collisionless shocks
An example is presented from a test particle simulation designed to study ion acceleration at oblique turbulent shocks. For conditions appropriate at interplanetary shocks near 1 AU, it is found that a shock with theta sub B n = 60 deg is capable of producing an energy spectrum extending from 10 keV to approx. 1 MeV in approx 1 hour. In this case total energy gains result primarily from several separate episodes of shock drift acceleration, each of which occurs when particles are scattered back to the shock by magnetic fluctuations in the shock vicinity
A self-organized criticality model for ion temperature gradient (ITG) mode driven turbulence in confined plasma
A new Self-Organized Criticality (SOC) model is introduced in the form of a
Cellular Automaton (CA) for ion temperature gradient (ITG) mode driven
turbulence in fusion plasmas. Main characteristics of the model are that it is
constructed in terms of the actual physical variable, the ion temperature, and
that the temporal evolution of the CA, which necessarily is in the form of
rules, mimics actual physical processes as they are considered to be active in
the system, i.e. a heating process and a local diffusive process that sets on
if a threshold in the normalized ion temperature gradient R/L_T is exceeded.
The model reaches the SOC state and yields ion temperature profiles of
exponential shape, which exhibit very high stiffness, in that they basically
are independent of the loading pattern applied. This implies that there is
anomalous heat transport present in the system, despite the fact that diffusion
at the local level is imposed to be of a normal kind. The distributions of the
heat fluxes in the system and of the heat out-fluxes are of power-law shape.
The basic properties of the model are in good qualitative agreement with
experimental results.Comment: In press at Physics of Plasmas, July 2010; 11 pages, 5 figure
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