56,705 research outputs found
A simple model for the evolution of multi-stranded coronal loops
We develop and analyze a simple cellular automaton (CA) model that reproduces
the main properties of the evolution of soft X-ray coronal loops. We are
motivated by the observation that these loops evolve in three distinguishable
phases that suggest the development, maintainance, and decay of a
self-organized system. The model is based on the idea that loops are made of
elemental strands that are heated by the relaxation of magnetic stress in the
form of nanoflares. In this vision, usually called "the Parker conjecture"
(Parker 1988), the origin of stress is the displacement of the strand
footpoints due to photospheric convective motions. Modeling the response and
evolution of the plasma we obtain synthetic light curves that have the same
characteristic properties (intensity, fluctuations, and timescales) as the
observed cases. We study the dependence of these properties on the model
parameters and find scaling laws that can be used as observational predictions
of the model. We discuss the implications of our results for the interpretation
of recent loop observations in different wavelengths.Comment: 2010, accepted for publication in Ap
Are constant loop widths an artifact of the background and the spatial resolution?
We study the effect of the coronal background in the determination of the
diameter of EUV loops, and we analyze the suitability of the procedure followed
in a previous paper (L\'opez Fuentes, Klimchuk & D\'emoulin 2006) for
characterizing their expansion properties. For the analysis we create different
synthetic loops and we place them on real backgrounds from data obtained with
the Transition Region and Coronal Explorer (\textit{TRACE}). We apply to these
loops the same procedure followed in our previous works, and we compare the
results with real loop observations. We demonstrate that the procedure allows
us to distinguish constant width loops from loops that expand appreciably with
height, as predicted by simple force-free field models. This holds even for
loops near the resolution limit. The procedure can easily determine when loops
are below resolution limit and therefore not reliably measured. We find that
small-scale variations in the measured loop width are likely due to
imperfections in the background subtraction. The greatest errors occur in
especially narrow loops and in places where the background is especially bright
relative to the loop. We stress, however, that these effects do not impact the
ability to measure large-scale variations. The result that observed loops do
not expand systematically with height is robust.Comment: Accepted for publication in Ap
The Berry-Tabor conjecture for spin chains of Haldane-Shastry type
According to a long-standing conjecture of Berry and Tabor, the distribution
of the spacings between consecutive levels of a "generic'' integrable model
should follow Poisson's law. In contrast, the spacings distribution of chaotic
systems typically follows Wigner's law. An important exception to the
Berry-Tabor conjecture is the integrable spin chain with long-range
interactions introduced by Haldane and Shastry in 1988, whose spacings
distribution is neither Poissonian nor of Wigner's type. In this letter we
argue that the cumulative spacings distribution of this chain should follow the
"square root of a logarithm'' law recently proposed by us as a characteristic
feature of all spin chains of Haldane-Shastry type. We also show in detail that
the latter law is valid for the rational counterpart of the Haldane-Shastry
chain introduced by Polychronakos.Comment: LaTeX with revtex4, 6 pages, 6 figure
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