324 research outputs found
Magnetic field diagnostics and spatio-temporal variability of the solar transition region
Magnetic field diagnostics of the transition region from the chromosphere to
the corona faces us with the problem that one has to apply extreme UV
spectro-polarimetry. While for coronal diagnostic techniques already exist
through infrared coronagraphy above the limb and radio observations on the
disk, for the transition region one has to investigate extreme UV observations.
However, so far the success of such observations has been limited, but there
are various projects to get spectro-polarimetric data in the extreme UV in the
near future. Therefore it is timely to study the polarimetric signals we can
expect for such observations through realistic forward modeling.
We employ a 3D MHD forward model of the solar corona and synthesize the
Stokes I and Stokes V profiles of C IV 1548 A. A signal well above 0.001 in
Stokes V can be expected, even when integrating for several minutes in order to
reach the required signal-to-noise ratio, despite the fact that the intensity
in the model is rapidly changing (just as in observations). Often this
variability of the intensity is used as an argument against transition region
magnetic diagnostics which requires exposure times of minutes. However, the
magnetic field is evolving much slower than the intensity, and thus when
integrating in time the degree of (circular) polarization remains rather
constant. Our study shows the feasibility to measure the transition region
magnetic field, if a polarimetric accuracy on the order of 0.001 can be
reached, which we can expect from planned instrumentation.Comment: Accepted for publication in Solar Physics (4.Mar.2013), 19 pages, 9
figure
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
Disentangling flows in the solar transition region
The measured average velocities in solar and stellar spectral lines formed at
transition region temperatures have been difficult to interpret. However,
realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of
the solar atmosphere are able to reproduce the observed dominant line shifts
and may thus hold the key to resolve these issues. Our new 3D rMHD simulations
aim to shed light on how mass flows between the chromosphere and corona and on
how the coronal mass is maintained. Passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of changes
in plasma temperature and velocity not only locally, but also in a co-moving
frame. By following the trajectories of the corks, we can investigate mass and
energy flows and understand the composition of the observed velocities. Our
findings show that most of the transition region mass is cooling. The
preponderance of transition region redshifts in the model can be explained by
the higher percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be explained by
a combination of both stronger upflows than downflows and a higher percentage
of upflowing mass. The most common combination at lower and middle transition
region temperatures are corks that are cooling and traveling downward. For
these corks, a strong correlation between the pressure gradient along the
magnetic field line and the velocity along the magnetic field line has been
observed, indicating a formation mechanism that is related to downward
propagating pressure disturbances. Corks at upper transition region
temperatures are subject to a rather slow and highly variable but continuous
heating process.Comment: 13 pages, 10 figures, online movi
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
Electrically Driven Light Emission from Individual CdSe Nanowires
We report electroluminescence (EL) measurements carried out on three-terminal
devices incorporating individual n-type CdSe nanowires. Simultaneous optical
and electrical measurements reveal that EL occurs near the contact between the
nanowire and a positively biased electrode or drain. The surface potential
profile, obtained by using Kelvin probe microscopy, shows an abrupt potential
drop near the position of the EL spot, while the band profile obtained from
scanning photocurrent microscopy indicates the existence of an n-type Schottky
barrier at the interface. These observations indicate that light emission
occurs through a hole leakage or an inelastic scattering induced by the rapid
potential drop at the nanowire-electrode interface.Comment: 12 pages, 4 figure
Many-Body Effects on Tunneling of Electrons in Magnetic-Field-Induced Quasi One-Dimensional Electron Systems in Semiconductor Nanowhiskers
Effects of the electron-electron interaction on tunneling in a semiconductor
nanowhisker are studied in a magnetic quantum limit. We consider the system
with which bulk and edge states coexist. In bulk states, the temperature
dependence of the transmission probability is qualitatively similar to that of
a one-dimensional electron system. We investigate contributions of edge states
on transmission probability in bulk states. Those contributions can be
neglected within our approximation which takes into account only most divergent
terms at low temperatures.Comment: 9 pages, 6 figure
Chromospheric emission from nanoflare heating in RADYN simulations
Heating signatures from small-scale magnetic reconnection events in the solar
atmosphere have proven to be difficult to detect through observations.
Numerical models that reproduce flaring conditions are essential in the
understanding of how nanoflares may act as a heating mechanism of the corona.
We study the effects of non-thermal electrons in synthetic spectra from 1D
hydrodynamic RADYN simulations of nanoflare heated loops to investigate the
diagnostic potential of chromospheric emission from small-scale events. The Mg
II h and k, Ca II H and K, Ca II 854.2 nm, H-alpha and H-beta chromospheric
lines were synthesised from various RADYN models of coronal loops subject to
electron beams of nanoflare energies. The contribution function to the line
intensity was computed to better understand how the atmospheric response to the
non-thermal electrons affects the formation of spectral lines and the detailed
shape of their spectral profiles. The spectral line signatures arising from the
electron beams highly depend on the density of the loop and the lower cutoff
energy of the electrons. Low-energy (5 keV) electrons deposit their energy in
the corona and transition region, producing strong plasma flows that cause both
redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15
keV) electrons deposit their energy in the lower transition region and
chromosphere, resulting in increased emission from local heating. Our results
indicate that effects from small-scale events can be observed with ground-based
telescopes, expanding the list of possible diagnostics for the presence and
properties of nanoflares
Heat conduction in a 1D harmonic chain with three dimensional vibrations
We study vibrational energy transport in a quasi 1-D harmonic chain with both
longitudinal and transverse vibrations. We demonstrate via both numerical
simulation and theoretic analysis that for 1-D atomic chain connected by 3D
harmonic springs, the coefficient of heat conduction changes it continuously
with its lattice constant, indicating the qualitative difference from the
corresponding 1-D case where the coefficient is independent of the lattice
constant.Comment: 4 pages, 4 figure
Structure of nanoparticles embedded in micellar polycrystals
We investigate by scattering techniques the structure of water-based soft
composite materials comprising a crystal made of Pluronic block-copolymer
micelles arranged in a face-centered cubic lattice and a small amount (at most
2% by volume) of silica nanoparticles, of size comparable to that of the
micelles. The copolymer is thermosensitive: it is hydrophilic and fully
dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into
micelles at room temperature, where the block-copolymer is amphiphilic. We use
contrast matching small-angle neuron scattering experiments to probe
independently the structure of the nanoparticles and that of the polymer. We
find that the nanoparticles do not perturb the crystalline order. In addition,
a structure peak is measured for the silica nanoparticles dispersed in the
polycrystalline samples. This implies that the samples are spatially
heterogeneous and comprise, without macroscopic phase separation, silica-poor
and silica-rich regions. We show that the nanoparticle concentration in the
silica-rich regions is about tenfold the average concentration. These regions
are grain boundaries between crystallites, where nanoparticles concentrate, as
shown by static light scattering and by light microscopy imaging of the
samples. We show that the temperature rate at which the sample is prepared
strongly influence the segregation of the nanoparticles in the
grain-boundaries.Comment: accepted for publication in Langmui
Sign-reversal of drag in bilayer systems with in-plane periodic potential modulation
We develop a theory for describing frictional drag in bilayer systems with
in-plane periodic potential modulations, and use it to investigate the drag
between bilayer systems in which one of the layers is modulated in one
direction. At low temperatures, as the density of carriers in the modulated
layer is changed, we show that the transresistivity component in the direction
of modulation can change its sign. We also give a physical explanation for this
behavior.Comment: 4 pages, 4 figure
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