5,411 research outputs found
Unstable coronal loops : numerical simulations with predicted observational signatures
We present numerical studies of the nonlinear, resistive magnetohydrodynamic
(MHD) evolution of coronal loops. For these simulations we assume that the
loops carry no net current, as might be expected if the loop had evolved due to
vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical
flux tube with line-tied ends. For a given amount of twist in the magnetic
field it is well known that once such a loop exceeds a critical length it
becomes unstableto ideal MHD instabilities. The early evolution of these
instabilities generates large current concentrations. Firstly we show that
these current concentrations are consistent with the formation of a current
sheet. Magnetic reconnection can only occur in the vicinity of these current
concentrations and we therefore couple the resistivity to the local current
density. This has the advantage of avoiding resistive diffusion in regions
where it should be negligible. We demonstrate the importance of this procedure
by comparison with simulations based on a uniform resistivity. From our
numerical experiments we are able to estimate some observational signatures for
unstable coronal loops. These signatures include: the timescale of the loop
brightening; the temperature increase; the energy released and the predicted
observable flow speeds. Finally we discuss to what extent these observational
signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure
Dichotomous decisions based on dichotomously scored items: a case study
In a course in elementary statistics for psychology students using criterion-referenced achievement tests, the total test score, based on dichotomously scored items, was used for classifying students into those who passed and those who failed. The score on a test is considered as depending on a latent variable; it is assumed that the students can be dichotomized into the categories "mastery" (with scores on the latent variable above a cutting score), and "no mastery" (with scores below the cutting score on the latent variable). Two problems are considered: (a) How many students are classified incorrectly? Using the binomial error model a procedure is described for computing the classification proportions: p(mastery, passed), p(mastery, failed), p(no mastery, passed), and p(no mastery, failed), (b) What is the optimal cutting score on a test? Using a loss function a procedure for computing the optimal curring score is described
Alternative model of the Antonov problem
Astrophysical systems will never be in a real Thermodynamic equilibrium: they
undergo an evaporation process due to the fact that the gravity is not able to
confine the particles. Ordinarily, this difficulty is overcome by enclosing the
system in a rigid container which avoids the evaporation. We proposed an
energetic prescription which is able to confine the particles, leading in this
way to an alternative version of the Antonov isothermal model which unifies the
well-known isothermal and polytropic profiles. Besides of the main features of
the isothermal sphere model: the existence of the gravitational collapse and
the energetic region with a negative specific heat, this alternative model has
the advantage that the system size naturally appears as a consequence of the
particles evaporation.Comment: RevTex4, 9 pages, 10 figures, Version Submitted to PR
Shotgun Cloning of Transposon Insertions in the Genome of Caenorhabditis elegans
We present a strategy to identify and map large numbers of transposon insertions in
the genome of Caenorhabditis elegans. Our approach makes use of the mutator strain
mut-7, which has germline-transposition activity of the Tc1/mariner family of transposons,
a display protocol to detect new transposon insertions, and the availability of
the genomic sequence of C. elegans. From a pilot insertional mutagenesis screen, we
have obtained 351 new Tc1 transposons inserted in or near 219 predicted C. elegans
genes. The strategy presented provides an approach to isolate insertions of natural
transposable elements in many C. elegans genes and to create a large-scale collection
of C. elegans mutants
Symptom complexes in patients with seropositive arthralgia and in patients newly diagnosed with rheumatoid arthritis: a qualitative exploration of symptom development
Objective: The aim of this study was to explore symptoms and symptom development during the earliest phases of rheumatoid arthritis (RA) in patients with seropositive arthralgia and patients newly diagnosed with RA
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