74 research outputs found
Catastrophic cooling and cessation of heating in the solar corona
Condensations in the more than 10^6 K hot corona of the Sun are commonly
observed in the extreme ultraviolet (EUV). While their contribution to the
total solar EUV radiation is still a matter of debate, these condensations
certainly provide a valuable tool for studying the dynamic response of the
corona to the heating processes. We investigate different distributions of
energy input in time and space to investigate which process is most relevant
for understanding these coronal condensations. For a comparison to observations
we synthesize EUV emission from a time-dependent, one-dimensional model for
coronal loops, where we employ two heating scenarios: simply shutting down the
heating and a model where the heating is very concentrated at the loop
footpoints, while keeping the total heat input constant. The heating off/on
model does not lead to significant EUV count rates that one observes with
SDO/AIA. In contrast, the concentration of the heating near the footpoints
leads to thermal non-equilibrium near the loop top resulting in the well-known
catastrophic cooling. This process gives a good match to observations of
coronal condensations. This shows that the corona needs a steady supply of
energy to support the coronal plasma, even during coronal condensations.
Otherwise the corona would drain very fast, too fast to even form a
condensation.Comment: Astronomy & Astrophysics, in press, 10 pages, 5 figure
Parametrization of coronal heating: spatial distribution and observable consequences
We investigate the difference in the spatial distribution of the energy input
for parametrizations of different mechanisms to heat the corona of the Sun and
possible impacts on the coronal emission. We use a 3D MHD model of a solar
active region as a reference and compare the Ohmic-type heating in this model
to parametrizations for alternating current (AC) and direct current (DC)
heating models, in particular, we use Alfven wave and MHD turbulence heating.
We extract the quantities needed for these two parametrizations from the
reference model and investigate the spatial distribution of the heat input in
all three cases, globally and along individual field lines. To study
differences in the resulting coronal emission we employ 1D loop models with a
prescribed heat input based on the heating rate we extracted along a bundle of
field lines. On average, all heating implementations show a roughly drop of the
heating rate with height. This also holds for individual field lines. While all
mechanism show a concentration of the energy input towards the low parts of the
atmosphere, for individual field lines the concentration towards the footpoints
is much stronger for the DC mechanisms than for the Alfven wave AC case. In
contrast, the AC model gives a stronger concentration of the emission towards
the footpoints. This is because the more homogeneous distribution of the energy
input leads to higher coronal temperatures and a more extended transition
region. The significant difference in the concentration of the heat input
towards the foot points for the AC and DC mechanisms, and the pointed
difference in the spatial distribution of the coronal emission for these cases
shows that the two mechanisms should be discriminable by observations. Before
drawing final conclusions, these parametrizations should be implemented in new
3D models in a more self-consistent way.Comment: accepted for publication in A&A, 10 pages, 9 figure
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Investigation of mass flows in the transition region and corona in a three-dimensional numerical model approach
The origin of solar transition region redshifts is not completely understood.
Current research is addressing this issue by investigating three-dimensional
magneto-hydrodynamic models that extend from the photosphere to the corona. By
studying the average properties of emission line profiles synthesized from the
simulation runs and comparing them to observations with present-day
instrumentation, we investigate the origin of mass flows in the solar
transition region and corona. Doppler shifts were determined from the emission
line profiles of various extreme-ultraviolet emission lines formed in the range
of K. Plasma velocities and mass flows were investigated for
their contribution to the observed Doppler shifts in the model. In particular,
the temporal evolution of plasma flows along the magnetic field lines was
analyzed. Comparing observed vs. modeled Doppler shifts shows a good
correlation in the temperature range /[K])=4.5-5.7, which is the basis
of our search for the origin of the line shifts. The vertical velocity obtained
when weighting the velocity by the density squared is shown to be almost
identical to the corresponding Doppler shift. Therefore, a direct comparison
between Doppler shifts and the model parameters is allowed. A simple
interpretation of Doppler shifts in terms of mass flux leads to overestimating
the mass flux. Upflows in the model appear in the form of cool pockets of gas
that heat up slowly as they rise. Their low temperature means that these
pockets are not observed as blueshifts in the transition region and coronal
lines. For a set of magnetic field lines, two different flow phases could be
identified. The coronal part of the field line is intermittently connected to
subjacent layers of either strong or weak heating, leading either to mass flows
into the loop or to the draining of the loop.Comment: 7 pages, 7 figure
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Influence of large-strain deformation on the microstructure, texture, and mechanical response of tantalum bar
Numerous studies have established the influence of impurities, crystallographic texture, temperature, and strain rate separately or collectively on the constitutive response of annealed tantalum, in particular plate Ta-stock. However, fewer detailed studies have examined the evolution of crystallographic texture and the mechanical response of tantalum bar or rod material following prestraining to large strains {epsilon} > I. In this paper the influence of large plastic prestraining on the microstructure evolution, texture evolution, and mechanical response of high-purity tantalum bar material is presented. Tantalum cylinders annealed at 1200 {degrees}C were quasi-statically upset forged, with intermediate lubrication, to true strains of 0.4, 0.95, and 1.85. Microstructural and textural banding within the starting Ta-bar was characterized in detail. It was found that different oriented bands evolved differently during large-strain forging leading to significant scatter in the mechanical response. Aspects of defect storage, work-hardening response, and texture evolution in Ta-bar as a function of forging strain are discussed
Ejection of cool plasma into the hot corona
We investigate the processes that lead to the formation, ejection and fall of
a confined plasma ejection that was observed in a numerical experiment of the
solar corona. By quantifying physical parameters such as mass, velocity, and
orientation of the plasma ejection relative to the magnetic field, we provide a
description of the nature of this particular phenomenon. The time-dependent
three-dimensional magnetohydrodynamic (3D MHD) equations are solved in a box
extending from the chromosphere to the lower corona. The plasma is heated by
currents that are induced through field line braiding as a consequence of
photospheric motions. Spectra of optically thin emission lines in the extreme
ultraviolet range are synthesized, and magnetic field lines are traced over
time. Following strong heating just above the chromosphere, the pressure
rapidly increases, leading to a hydrodynamic explosion above the upper
chromosphere in the low transition region. The explosion drives the plasma,
which needs to follow the magnetic field lines. The ejection is then moving
more or less ballistically along the loop-like field lines and eventually drops
down onto the surface of the Sun. The speed of the ejection is in the range of
the sound speed, well below the Alfven velocity. The plasma ejection is
basically a hydrodynamic phenomenon, whereas the rise of the heating rate is of
magnetic nature. The granular motions in the photosphere lead (by chance) to a
strong braiding of the magnetic field lines at the location of the explosion
that in turn is causing strong currents which are dissipated. Future studies
need to determine if this process is a ubiquitous phenomenon on the Sun on
small scales. Data from the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory (AIA/SDO) might provide the relevant information.Comment: 12 pages, 10 figure
Structure of solar coronal loops: from miniature to large-scale
We will use new data from the High-resolution Coronal Imager (Hi-C) with
unprecedented spatial resolution of the solar corona to investigate the
structure of coronal loops down to 0.2 arcsec. During a rocket flight Hi-C
provided images of the solar corona in a wavelength band around 193 A that is
dominated by emission from Fe XII showing plasma at temperatures around 1.5 MK.
We analyze part of the Hi-C field-of-view to study the smallest coronal loops
observed so far and search for the a possible sub-structuring of larger loops.
We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal
loops. These have length of the coronal segment above the chromosphere of only
about 1 Mm and a thickness of less than 200 km. They could be interpreted as
the coronal signature of small flux tubes breaking through the photosphere with
a footpoint distance corresponding to the diameter of a cell of granulation. We
find loops that are longer than 50 Mm to have a diameter of about 2 arcsec or
1.5 Mm, consistent with previous observations. However, Hi-C really resolves
these loops with some 20 pixels across the loop. Even at this greatly improved
spatial resolution the large loops seem to have no visible sub-structure.
Instead they show a smooth variation in cross-section. The fact that the large
coronal loops do not show a sub-structure at the spatial scale of 0.1 arcsec
per pixel implies that either the densities and temperatures are smoothly
varying across these loops or poses an upper limit on the diameter of strands
the loops might be composed of. We estimate that strands that compose the 2
arcsec thick loop would have to be thinner than 15 km. The miniature loops we
find for the first time pose a challenge to be properly understood in terms of
modeling.Comment: Accepted for publication in A&A (Jun 19, 2013), 11 pages, 10 figure
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