272 research outputs found
Are Chromospheric Nanoflares a Primary Source of Coronal Plasma?
It has been suggested that the hot plasma of the solar corona comes primarily
from impulsive heating events, or nanoflares, that occur in the lower
atmosphere, either in the upper part of the ordinary chromosphere or at the
tips of type II spicules. We test this idea with a series of hydrodynamic
simulations. We find that synthetic Fe XII (195) and Fe XIV (274) line profiles
generated from the simulations disagree dramatically with actual observations.
The integrated line intensities are much too faint; the blue shifts are much
too fast; the blue-red asymmetries are much too large; and the emission is
confined to low altitudes. We conclude that chromospheric nanoflares are not a
primary source of hot coronal plasma. Such events may play an important role in
producing the chromosphere and powering its intense radiation, but they do not,
in general, raise the temperature of the plasma to coronal values. Those cases
where coronal temperatures are reached must be relatively uncommon. The
observed profiles of Fe XII and Fe XIV come primarily from plasma that is
heated in the corona itself, either by coronal nanoflares or a quasi-steady
coronal heating process. Chromospheric nanoflares might play a role in
generating waves that provide this coronal heating.Comment: 14 pages, 6 figures, accepted by Astrophysical Journa
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
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
Numerical simulations of a siphon mechanism for quiescent prominence formation
Quiescent prominences represent a significant challenge to our understanding of the flow of mass and energy in the outer layers of the solar atmosphere. A small number of quiescent prominences contain as much mass as the entire corona (Athay, 1976). The problem then is how to get that much material into the relatively small volume of a prominence and maintain it at a temperature of 10,000 K in close proximity to material at one million K. The thermal insulation to conduction provided by the magnetic field explains the disparate temperatures. The mass source problem is less well understood. One method for supplying mass to the prominence is to siphon it from the chromosphere. The siphon mechanism begins with a magnetic loop that evolves into a configuration with a gravitational well, such as that described by Kippenhahn and Schluter (1957). This could be formed, for example, by a twist in the magnetic field. A gravitational well could also be formed by a condensation induced sag in the field. This could further enhance the condensation process. Once this well has formed, or as it is forming, the material in the well area of the loop must cool and condense to the point where radiative losses exceed any heat input. Additional material must also flow into the well from the underlying chromosphere to supply the mass required to form the prominence. One example from a series of numerical simulations that were performed to study the formation of quiescent prominences is presented
Enthalpy-based Thermal Evolution of Loops: II. Improvements to the Model
This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model
"Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by Klimchuk et al
(2008), which studies the plasma response to evolving coronal heating,
especially impulsive heating events. The basis of EBTEL is the modelling of
mass exchange between the corona and transition region and chromosphere in
response to heating variations, with the key parameter being the ratio of
transition region to coronal radiation. We develop new models for this
parameter that now include gravitational stratification and a physically
motivated approach to radiative cooling. A number of examples are presented,
including nanoflares in short and long loops, and a small flare. The new
features in EBTEL are important for accurate tracking of, in particular, the
density. The 0D results are compared to a 1D hydro code (Hydrad) with generally
good agreement. EBTEL is suitable for general use as a tool for (a) quick-look
results of loop evolution in response to a given heating function, (b)
extensive parameter surveys and (c) situations where the modelling of hundreds
or thousands of elemental loops is needed. A single run takes a few seconds on
a contemporary laptop
Diagnosing the time-dependence of active region core heating from the emission measure: II. Nanoflare trains
The time-dependence of heating in solar active regions can be studied by
analyzing the slope of the emission measure distribution cool-ward of the peak.
In a previous study we showed that low-frequency heating can account for 0% to
77% of active region core emission measures. We now turn our attention to
heating by a finite succession of impulsive events for which the timescale
between events on a single magnetic strand is shorter than the cooling
timescale. We refer to this scenario as a "nanoflare train" and explore a
parameter space of heating and coronal loop properties with a hydrodynamic
model. Our conclusions are: (1) nanoflare trains are consistent with 86% to
100% of observed active region cores when uncertainties in the atomic data are
properly accounted for; (2) steeper slopes are found for larger values of the
ratio of the train duration to the post-train cooling and draining
timescale , where depends on the number of heating events,
the event duration and the time interval between successive events ();
(3) may be diagnosed from the width of the hot component of the
emission measure provided that the temperature bins are much smaller than 0.1
dex; (4) the slope of the emission measure alone is not sufficient to provide
information about any timescale associated with heating - the length and
density of the heated structure must be measured for to be uniquely
extracted from the ratio
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
Highly Efficient Modeling of Dynamic Coronal Loops
Observational and theoretical evidence suggests that coronal heating is
impulsive and occurs on very small cross-field spatial scales. A single coronal
loop could contain a hundred or more individual strands that are heated
quasi-independently by nanoflares. It is therefore an enormous undertaking to
model an entire active region or the global corona. Three-dimensional MHD codes
have inadequate spatial resolution, and 1D hydro codes are too slow to simulate
the many thousands of elemental strands that must be treated in a reasonable
representation. Fortunately, thermal conduction and flows tend to smooth out
plasma gradients along the magnetic field, so "0D models" are an acceptable
alternative. We have developed a highly efficient model called Enthalpy-Based
Thermal Evolution of Loops (EBTEL) that accurately describes the evolution of
the average temperature, pressure, and density along a coronal strand. It
improves significantly upon earlier models of this type--in accuracy,
flexibility, and capability. It treats both slowly varying and highly impulsive
coronal heating; it provides the differential emission measure distribution,
DEM(T), at the transition region footpoints; and there are options for heat
flux saturation and nonthermal electron beam heating. EBTEL gives excellent
agreement with far more sophisticated 1D hydro simulations despite using four
orders of magnitude less computing time. It promises to be a powerful new tool
for solar and stellar studies.Comment: 34 pages, 8 figures, accepted by Astrophysical Journal (minor
revisions of original submitted version
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