1,271 research outputs found
The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data
Solar granules are bright patterns surrounded by dark channels called
intergranular lanes in the solar photosphere and are a manifestation of
overshooting convection. Observational studies generally find stronger upflows
in granules and weaker downflows in intergranular lanes. This trend is,
however, inconsistent with the results of numerical simulations in which
downflows are stronger than upflows through the joint action of gravitational
acceleration/deceleration and pressure gradients. One cause of this discrepancy
is the image degradation caused by optical distortion and light diffraction and
scattering that takes place in an imaging instrument. We apply a deconvolution
technique to Hinode/SP data in an attempt to recover the original solar scene.
Our results show a significant enhancement in both, the convective upflows and
downflows, but particularly for the latter. After deconvolution, the up- and
downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average
geometrical height of roughly 50 km, respectively. We found that the velocity
distributions after deconvolution match those derived from numerical
simulations. After deconvolution the net LOS velocity averaged over the whole
FOV lies close to zero as expected in a rough sense from mass balance.Comment: 32 pages, 13 figures, accepted for publication in Ap
Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7
We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole
resonance (NQR), and transmission electron microscopy (TEM) studies on the
quasi-two-dimensional spin-gap system (CuCl)LaNb2O7, a possible candidate for
the J1-J2 model on a square lattice. A sharp single NQR line is observed at the
Cu and Cl sites, indicating that both Cu and Cl atoms occupy a unique site.
However, the electric field gradient tensors at the Cu, Cl, and La sites do not
have axial symmetry. This is incompatible with the reported crystal structure.
Thus the J1-J2 model has to be modified. We propose alternative two-dimensional
dimer models based on the NMR, NQR, and TEM results. The value of the hyperfine
coupling constant at the Cu sites indicates that the spin density is mainly on
the d(3z2-r2) orbital (z parallel c). At 1.5 K, Cu- and Nb-NMR signals
disappear above the critical field Bc1 = 10.3 T determined from the onset of
the magnetization, indicating a field-induced magnetic phase transition at Bc1.Comment: 9 pages, 16 figure
Approximation Algorithms for Connected Maximum Cut and Related Problems
An instance of the Connected Maximum Cut problem consists of an undirected
graph G = (V, E) and the goal is to find a subset of vertices S V
that maximizes the number of edges in the cut \delta(S) such that the induced
graph G[S] is connected. We present the first non-trivial \Omega(1/log n)
approximation algorithm for the connected maximum cut problem in general graphs
using novel techniques. We then extend our algorithm to an edge weighted case
and obtain a poly-logarithmic approximation algorithm. Interestingly, in stark
contrast to the classical max-cut problem, we show that the connected maximum
cut problem remains NP-hard even on unweighted, planar graphs. On the positive
side, we obtain a polynomial time approximation scheme for the connected
maximum cut problem on planar graphs and more generally on graphs with bounded
genus.Comment: 17 pages, Conference version to appear in ESA 201
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region
Future solar missions and ground-based telescopes aim to understand the
magnetism of the solar chromosphere. We performed a supporting study in
Quintero Noda et al. (2016) focused on the infrared Ca II 8542 A line and we
concluded that is one of the best candidates because it is sensitive to a large
range of atmospheric heights, from the photosphere to the middle chromosphere.
However, we believe that it is worth to try improving the results produced by
this line observing additional spectral lines. In that regard, we examined the
neighbour solar spectrum looking for spectral lines that could increase the
sensitivity to the atmospheric parameters. Interestingly, we discovered several
photospheric lines that greatly improve the photospheric sensitivity to the
magnetic field vector. Moreover, they are located close to a second
chromospheric line that also belongs to the Ca II infrared triplet, i.e. the Ca
II 8498 A line, and enhances the sensitivity to the atmospheric parameters at
chromospheric layers. We conclude that the lines in the vicinity of the Ca II
8542 A line not only increase its sensitivity to the atmospheric parameters at
all layers, but also they constitute an excellent spectral window for
chromospheric polarimetry.Comment: 11 pages, 8 figures, 1 tabl
Solar polarimetry through the K I lines at 770 nm
We characterize the K I D1 & D2 lines in order to determine whether they
could complement the 850 nm window, containing the Ca II infrared triplet lines
and several Zeeman sensitive photospheric lines, that was studied previously.
We investigate the effect of partial redistribution on the intensity profiles,
their sensitivity to changes in different atmospheric parameters, and the
spatial distribution of Zeeman polarization signals employing a realistic
magnetohydrodynamic simulation. The results show that these lines form in the
upper photosphere at around 500 km and that they are sensitive to the line of
sight velocity and magnetic field strength at heights where neither the
photospheric lines nor the Ca II infrared lines are. However, at the same time,
we found that their sensitivity to the temperature essentially comes from the
photosphere. Then, we conclude that the K I lines provide a complement to the
lines in the 850 nm window for the determination of atmospheric parameters in
the upper photosphere, especially for the line of sight velocity and the
magnetic field.Comment: 10 pages, 9 figures, main journal publicatio
Cold Diffusion on the Replay Buffer: Learning to Plan from Known Good States
Learning from demonstrations (LfD) has successfully trained robots to exhibit
remarkable generalization capabilities. However, many powerful imitation
techniques do not prioritize the feasibility of the robot behaviors they
generate. In this work, we explore the feasibility of plans produced by LfD. As
in prior work, we employ a temporal diffusion model with fixed start and goal
states to facilitate imitation through in-painting. Unlike previous studies, we
apply cold diffusion to ensure the optimization process is directed through the
agent's replay buffer of previously visited states. This routing approach
increases the likelihood that the final trajectories will predominantly occupy
the feasible region of the robot's state space. We test this method in
simulated robotic environments with obstacles and observe a significant
improvement in the agent's ability to avoid these obstacles during planning
Study of the polarization produced by the Zeeman effect in the solar Mg I b lines
The next generation of solar observatories aim to understand the magnetism of
the solar chromosphere. Therefore, it is crucial to understand the polarimetric
signatures of chromospheric spectral lines. For this purpose, we here examine
the suitability of the three Fraunhofer Mg I b1, b2, and b4 lines at 5183.6,
5172.7, and 5167.3 A, respectively. We start by describing a simplified atomic
model of only 6 levels and 3 line transitions for computing the atomic
populations of the 3p-4s (multiplet number 2) levels involved in the Mg I b
line transitions assuming non-local thermodynamic conditions and considering
only the Zeeman effect using the field-free approximation. We test this
simplified atom against more complex ones finding that, although there are
differences in the computed profiles, they are small compared with the
advantages provided by the simple atom in terms of speed and robustness. After
comparing the three Mg I lines, we conclude that the most capable one is the b2
line as b1 forms at similar heights and always show weaker polarization signals
while b4 is severely blended with photospheric lines. We also compare Mg I b2
with the K I D1 and Ca II 8542 A lines finding that the former is sensitive to
the atmospheric parameters at heights that are in between those covered by the
latter two lines. This makes Mg I b2 an excellent candidate for future
multi-line observations that aim to seamlessly infer the thermal and magnetic
properties of different features in the lower solar atmosphere.Comment: 14 pages, 11 figures, and 5 table
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region II: A magnetic flux tube scenario
In this publication we continue the work started in Quintero Noda et al.
(2017) examining this time a numerical simulation of a magnetic flux tube
concentration. Our goal is to study if the physical phenomena that take place
in it, in particular, the magnetic pumping, leaves a specific imprint on the
examined spectral lines. We find that the profiles from the interior of the
flux tube are periodically dopplershifted following an oscillation pattern that
is also reflected in the amplitude of the circular polarization signals. In
addition, we analyse the properties of the Stokes profiles at the edges of the
flux tube discovering the presence of linear polarization signals for the Ca II
lines, although they are weak with an amplitude around 0.5% of the continuum
intensity. Finally, we compute the response functions to perturbations in the
longitudinal field and we estimate the field strength using the weak field
approximation. Our results indicate that the height of formation of the
spectral lines changes during the magnetic pumping process which makes the
interpretation of the inferred magnetic field strength and its evolution more
difficult. These results complement those from previous works demonstrating the
capabilities and limitations of the 850 nm spectrum for chromospheric Zeeman
polarimetry in a very dynamic and complex atmosphere.Comment: 12 pages, 12 figures, 0 tables, MNRAS main journal publicatio
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region III: Chromospheric jets driven by twisted magnetic fields
We investigate the diagnostic potential of the spectral lines at 850 nm for
understanding the magnetism of the lower atmosphere. For that purpose, we use a
newly developed 3D simulation of a chromospheric jet to check the sensitivity
of the spectral lines to this phenomenon as well as our ability to infer the
atmospheric information through spectropolarimetric inversions of noisy
synthetic data. We start comparing the benefits of inverting the entire
spectrum at 850 nm versus only the Ca II 8542 A spectral line. We found a
better match of the input atmosphere for the former case, mainly at lower
heights. However, the results at higher layers were not accurate. After several
tests, we determined that we need to weight more the chromospheric lines than
the photospheric ones in the computation of the goodness of the fit. The new
inversion configuration allows us to obtain better fits and consequently more
accurate physical parameters. Therefore, to extract the most from multi-line
inversions, a proper set of weights needs to be estimated. Besides that, we
conclude again that the lines at 850 nm, or a similar arrangement with Ca II
8542 A plus Zeeman sensitive photospheric lines, poses the best observing
configuration for examining the thermal and magnetic properties of the lower
solar atmosphere.Comment: 14 pages, 11 figure
- …