625 research outputs found
Diagnosa Penyakit Kulit Wajah Menggunakan Metode Decession Tree Dan Algoritma C4.5
Today's face is something that is very much considered by both women and men. Women and men do facial treatments so often because the face is the first thing to see when meeting someone. However, there are those that often interfere with the face, one of which is a skin disease that is very diverse ranging from acne, dullness, blackheads to cancer. In fact, to overcome this, many people always consult with doctors, especially face problems. One thing that can be done is to diagnose facial skin diseases using the decession tree method and the c 4.5 algorithm. The existence of this system is expected to be a solution in conducting consultations for women and men for facial problems. By using the decession tree method and the c 4.5 algorithm and by using a number of data mining, it will give results that can be used as guidelines in treating facial skin diseases. In addition, the existence of this system will also be very helpful in the field of services to consumers, both women and men, especially for facial care to avoid facial skin diseases, which has been a problem so far
Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating
It has been established that cold plasma condensations can form in a magnetic
loop subject to localized heating of the footpoints. In this paper, we use
grid-adaptive numerical simulations of the radiative hydrodynamic equations to
parametrically investigate the filament formation process in a pre-shaped loop
with both steady and finite-time chromospheric heating. Compared to previous
works, we consider low-lying loops with shallow dips, and use a more realistic
description for the radiative losses. We demonstrate for the first time that
the onset of thermal instability satisfies the linear instability criterion.
The onset time of the condensation is roughly \sim 2 hr or more after the
localized heating at the footpoint is effective, and the growth rate of the
thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the
amplitude and the decay length scale characterizing this localized
chromospheric heating. We show how single or multiple condensation segments may
form in the coronal portion. In the asymmetric heating case, when two segments
form, they approach and coalesce, and the coalesced condensation later drains
down into the chromosphere. With a steady heating, this process repeats with a
periodicity of several hours. While our parametric survey confirms and augments
earlier findings, we also point out that steady heating is not necessary to
sustain the condensation. Once the condensation is formed, it can keep growing
also when the localized heating ceases. Finally, we show that the condensation
can survive continuous buffeting by perturbations resulting from the
photospheric p-mode waves.Comment: 43 pages, 18 figure
Can Thermal Nonequilibrium Explain Coronal Loops?
Any successful model of coronal loops must explain a number of observed
properties. For warm (~ 1 MK) loops, these include: 1. excess density, 2. flat
temperature profile, 3. super-hydrostatic scale height, 4. unstructured
intensity profile, and 5. 1000--5000 s lifetime. We examine whether thermal
nonequilibrium can reproduce the observations by performing hydrodynamic
simulations based on steady coronal heating that decreases exponentially with
height. We consider both monolithic and multi-stranded loops. The simulations
successfully reproduce certain aspects of the observations, including the
excess density, but each of them fails in at least one critical way. Monolithic
models have far too much intensity structure, while multi-strand models are
either too structured or too long-lived. Our results appear to rule out the
widespread existence of heating that is both highly concentrated low in the
corona and steady or quasi-steady (slowly varying or impulsive with a rapid
cadence). Active regions would have a very different appearance if the dominant
heating mechanism had these properties. Thermal nonequilibrium may nonetheless
play an important role in prominences and catastrophic cooling events (e.g.,
coronal rain) that occupy a small fraction of the coronal volume. However,
apparent inconsistencies between the models and observations of cooling events
have yet to be understood.Comment: 40 pages, 10 figures, accepted by the Astrophysical Journal (vol.
714
Signatures of impulsive localized heating in the temperature distribution of multi-stranded coronal loops
We study the signatures of different coronal heating regimes on the
differential emission measure (DEM) of multi-stranded coronal loops by means of
hydrodynamic simulations. We consider heating either uniformly distributed
along the loops or localized close to the chromospheric footpoints, in both
steady and impulsive conditions. Our simulations show that condensation at the
top of the loop forms when the localized heating is impulsive with a pulse
cadence time shorter than the plasma cooling time, and the pulse energy is
below a certain threshold. A condensation does not produce observable
signatures in the global DEM structure. Conversely, the DEM coronal peak is
found sensitive to the pulse cadence time. Our simulations can also give an
explanation of the warm overdense and hot underdense loops observed by TRACE,
SOHO and Yohkoh. However, they are unable to reproduce both the transition
region and the coronal DEM structure with a unique set of parameters, which
outlines the need for a more realistic description of the transition region.Comment: 31 pages, 7 figure
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences: Cross-sectional area variation for thin tubes
Solar prominences are subject to both field-aligned (longitudinal) and
transverse oscillatory motions, as evidenced by an increasing number of
observations. Large-amplitude longitudinal motions provide valuable information
on the geometry of the filament-channel magnetic structure that supports the
cool prominence plasma against gravity. Our pendulum model, in which the
restoring force is the gravity projected along the dipped field lines of the
magnetic structure, best explains these oscillations. However, several factors
can influence the longitudinal oscillations, potentially invalidating the
pendulum model. The aim of this work is to study the influence of large-scale
variations in the magnetic field strength along the field lines, i.e.,
variations of the cross-sectional area along the flux tubes supporting
prominence threads. We studied the normal modes of several flux tube
configurations, using linear perturbation analysis, to assess the influence of
different geometrical parameters on the oscillation properties. We found that
the influence of the symmetric and asymmetric expansion factors on longitudinal
oscillations is small.}{We conclude that the longitudinal oscillations are not
significantly influenced by variations of the cross-section of the flux tubes,
validating the pendulum model in this context.Comment: Accepted for publication in Astronomy & Astrophysic
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences
We investigate the influence of the geometry of the solar filament magnetic
structure on the large-amplitude longitudinal oscillations. A representative
filament flux tube is modeled as composed of a cool thread centered in a dipped
part with hot coronal regions on either side. We have found the normal modes of
the system, and establish that the observed longitudinal oscillations are well
described with the fundamental mode. For small and intermediate curvature radii
and moderate to large density contrast between the prominence and the corona,
the main restoring force is the solar gravity. In this full wave description of
the oscillation a simple expression for the oscillation frequencies is derived
in which the pressure-driven term introduces a small correction. We have also
found that the normal modes are almost independent of the geometry of the hot
regions of the tube. We conclude that observed large-amplitude longitudinal
oscillations are driven by the projected gravity along the flux tubes, and are
strongly influenced by the curvature of the dips of the magnetic field in which
the threads reside
Tangled Magnetic Fields in Solar Prominences
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called "hedgerow"
prominences, which consist of thin vertical threads. We explore the possibility
that such prominences are supported by tangled magnetic fields. A variety of
different approaches are used. First, the dynamics of plasma within a tangled
field is considered. We find that the contorted shape of the flux tubes
significantly reduces the flow velocity compared to the supersonic free fall
that would occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such fields to
gravitational forces is considered. We demonstrate that the prominence plasma
can be supported by the magnetic pressure of a tangled field that pervades not
only the observed dense threads but also their local surroundings. Tangled
fields with field strengths of about 10 G are able to support prominence
threads with observed hydrogen density of the order of 10^(11) cm^(-3).
Finally, we suggest that the observed vertical threads are the result of
Rayleigh-Taylor instability. Simulations of the density distribution within a
prominence thread indicate that the peak density is much larger than the
average density. We conclude that tangled fields provide a viable mechanism for
magnetic support of hedgerow prominences.Comment: 14 pages (emulateapj style), 10 figures, ApJ, in pres
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