216 research outputs found
Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method
The role of magnetic fields in the chromospheric heating problem remains
greatly unconstrained. Most theoretical predictions from numerical models rely
on a magnetic configuration, field strength and connectivity whose details have
not been well established with observational studies. High-resolution studies
of chromospheric magnetic fields in plage are very scarce or non-existent in
general. Our aim is to study the stratification of the magnetic field vector in
plage regions. We use high-spatial resolution full-Stokes observations acquired
with CRISP instrument at the Swedish 1-m Solar Telescope in the Mg I
5173, Na I 5896 and Ca II 8542 lines. We have
developed a spatially-regularized weak-field approximation (WFA) method based
on the idea of spatial regularization. This method allows for a fast
computation of magnetic field maps for an extended field of view. The fidelity
of this new technique has been assessed using a snapshot from a realistic 3D
magnetohydrodynamics simulation. We have derived the depth-stratification of
the line-of-sight component of the magnetic field from the photosphere to the
chromosphere in a plage region. The magnetic fields are concentrated in the
intergranular lanes in the photosphere and expand horizontally toward the
chromosphere, filling all the space and forming a canopy. Our results suggest
that the lower boundary of this canopy must be located around 400-600 km from
the photosphere. The mean canopy total magnetic field strength in the lower
chromosphere ( km) is 658 G. At km we estimate
G. We propose a modification to the WFA that
improves its applicability to data with worse signal-to-noise ratio. These
methods provide a quick and reliable way of studying multi-layer magnetic field
observations without the many difficulties inherent to other inversion methods.Comment: Accepted for publication on 2020-08-2
Atomic structure and vibrational properties of icosahedral BC boron carbide
The atomic structure of icosahedral BC boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter
Spatio-temporal analysis of chromospheric heating in a plage region
Our knowledge of the heating mechanisms that are at work in the chromosphere
of plage regions remains highly unconstrained from observational studies. The
purpose of our study is to estimate the chromospheric heating terms from a
plage dataset, characterize their spatio-temporal distribution and set
constraints on the heating processes that are at work. We make use of NLTE
inversions to infer a model of the photosphere and chromosphere of a plage
dataset acquired with the Swedish 1-m Solar Telescope. We use this model
atmosphere to calculate the chromospheric radiative losses from H i, Ca ii and
Mg ii atoms. We approximate the chromospheric heating terms by the net
radiative losses predicted by the inverted model. In order to make the analysis
of time-series over a large field-of-view computationally tractable, we make
use of a neural network. In the lower chromosphere, the contribution from the
Ca ii lines is dominant and located in the surroundings of the photospheric
footpoints. In the upper chromosphere, the H i contribution is dominant.
Radiative losses in the upper chromosphere form an homogeneous patch that
covers the plage region. The net radiative losses can be split in a periodic
component with an average amplitude of ampQ = 7.6 kW m^{-2} and a static (or
very slowly evolving) component with a mean value of -26.1 kW m^{-2}. Our
interpretation is that in the lower chromosphere, the radiative losses are
tracing the sharp lower edge of the hot magnetic canopy, where the electric
current is expected to be large. In the upper chromosphere, both the magnetic
field and the distribution of net radiative losses are room-filling, whereas
the amplitude of the periodic component is largest. Our results suggest that
acoustic wave heating may be responsible for one third of the energy deposition
in the upper chromosphere, whereas other heating mechanisms must be responsible
for the rest
Stratification of physical parameters in a C-class solar flare using multi-line observations
We present high-resolution and multi-line observations of a C2-class solar
flare, occurred in NOAA AR 12740 on May 6, 2019. The rise, peak and decay
phases of the flare were recorded continuously and quasi-simultaneously in the
Ca II K line with the CHROMIS instrument, the Ca II 8542 and Fe I 6173 \AA
lines with the CRISP instrument at the SST. The observations in the
chromospheric Ca II lines exhibit intense brightening near the flare
footpoints. At these locations, a non-LTE inversion code was employed to infer
the temperature, magnetic field, line-of-sight (LOS) velocity and
microturbulent velocity stratification in the flaring atmosphere. During the
flare peak time, the LOS velocity shows both upflows and downflows around the
flare footpoints in the upper chromosphere and lower chromosphere,
respectively. Moreover, the temporal analysis of the LOS magnetic field at the
flarepoints exhibits a maximum change of ~600 G. After the flare, the LOS
magnetic field decreases to the non-flaring value, exhibiting no permanent or
step-wise change. The analysis of response functions to the temperature, LOS
magnetic field and velocity shows that the Ca II lines exhibit enhanced
sensitivity to the deeper layers (i.e., log_t ~ -3) of the flaring atmosphere,
whereas for the non-flaring atmosphere they are mainly sensitive around log_t ~
-4. We suggest that a fraction of the apparent increase in the LOS magnetic
field at the flare footpoints may be due to the increase in the sensitivity of
the Ca II 8542 \AA line in the deeper layers, where the field strength is
relatively stronger. The rest can be due to magnetic field reconfiguration
during the flare. Our observations illustrate that even a less intense C-class
flare can heat the deeper layers of the solar chromosphere, mainly at the flare
footpoints, without affecting the photosphere.Comment: Short abstract, 12 figures, 1 table, submitted to A&
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XRAYL: a program for producing idealized powder diffraction line profiles from overlapped powder patterns
The X-ray diffraction patterns of samples of polycrystalline materials are used to identify and characterize phases. Very often the total (or composite) profile consists of a series of overlapping profiles. In many applications it is necessary to separate the component profiles from the total profile. (In this document the terms {ital profile, line}, and {ital peak} are used interchangeably to represent these features of X-ray or neutron diffraction patterns.) A computer program, XRAYL, first developed in the 1980s and subsequently enlarged and improved, allows the fitting of analytical functions to powder diffraction lines. The fitting process produces parameters of chosen profile functions, diffraction line by diffraction line. The resulting function parameters may then be used to generate ``idealized`` powder diffraction lines as counts at steps in 2{Omega}. The generated lines are effectively free of statistical noise and contributions from overlapping lines. Each separated line extends to background on both sides of the generated profile. XRAYL may, therefore, be used in X-ray powder diffraction profile analysis as a preprocessor program that is, separating peaks and feeding the ``resolved`` data to subsequent analysis programs. This self- contained document includes: (1) a description of the fitting functions coded into XRAYL, (2) an outline of the least-squares algorithm used in fitting the profile function, (3) the file formats and contents utilized by the computer code, (4) the user options and their presentation requirements for execution of the program, (5) an example of input and output for a test case, and (6) source code listings on a diskette
1-Hydroxy-1,1,3,3,3-pentaphenyldisiloxane, [Si2O(OH)(Ph)5], at 150 K
In the crystal structure of the title compound, C30H26O2Si2, one Si(Ph)3 residue is bound to another Si(OH)(Ph)2 residue via a nonlinear Si—O—Si bridge. The asymmetric unit is composed of four [Si2O(OH)(Ph)5] molecules. Each pair of adjacent molecules interacts via strong and highly directional O—H⋯O hydrogen bonds connecting neighbouring Si—OH units, and via inter-unit O—H⋯π contacts connecting the second hydroxyl groups with adjacent phenyl groups
Non-LTE inversions of a confined X2.2 flare: I. Vector magnetic field in the photosphere and chromosphere : I. The vector magnetic field in the photosphere and chromosphere
Obtaining the magnetic field vector accurately in the solar atmosphere is essential for studying changes in field topology during flares and to reliably model space weather. We tackle this problem by applying various inversion methods to a confined X2.2 flare in NOAA AR 12673 on September 6, 2017, comparing the photospheric and chromospheric magnetic field vector with those from two numerical models of this event. We obtain the photospheric field from Milne-Eddington (ME) and (non-)local thermal equilibrium (non-LTE) inversions of Hinode SOT/SP Fe I 6301.5Å and 6302.5Å. The chromospheric field is obtained from a spatially-regularised weak field approximation (WFA) and non-LTE inversions of Ca II 8542Å observed with CRISP at the Swedish 1-m Solar Telescope. The LTE- and non-LTE-inferred photospheric field components are strongly correlated throughout the atmosphere, with stronger field and higher temperatures in the non-LTE inversions. For the chromospheric field, the non-LTE inversions correlate well with the spatially-regularised WFA. We find strong-field patches of over 4.5 kG in the photosphere, co-located with similar concentrations exceeding 3 kG in the chromosphere. The obtained field strengths are up to 2-3 times higher than in the numerical models, with more concentrated and structured photosphere-to-chromosphere shear close to the polarity inversion line. The LTE and non-LTE Fe I inversions yield essentially the same photospheric field, while ME inversions fail to reproduce the field vector orientation where Fe I is in emission. Our inversions confirm the locations of flux rope footpoints that are predicted by numerical models. However, pre-processing and lower spatial resolution lead to weaker and smoother field in the models than what the data indicate. This emphasises the need for higher spatial resolution in the models to better constrain pre-eruptive flux ropes.Peer reviewe
A neutron scattering study of two-magnon states in the quantum magnet copper nitrate
We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using
inelastic neutron scattering we have measured the one and two magnon excitation
spectra in a large single crystal. The data are in excellent agreement with a
perturbative expansion of the alternating Heisenberg Hamiltonian from the
strongly dimerized limit. The expansion predicts a two-magnon bound state for q
~ (2n+1)pi*d which is consistent with the neutron scattering data.Comment: 11 pages of revtex style with 6 figures include
Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds
Using a Rayleigh-Schr\"odinger perturbation expansion of multi-band Hubbard
models, we present analytic expressions for the super-exchange coupling
constants between magnetic transition metal ions of arbitrary separation in
Mott-Hubbard insulators. The only restrictions are i) all ligand ions are
closed shell anions and ii) all contributing interaction paths are of equal
length. For short paths, our results essentially confirm the
Goodenough-Kanamori-Anderson rules, yet in general there does not exist any
simple rule to predict the sign of the magnetic coupling constants. The most
favorable situation for ferromagnetic coupling is found for ions with less than
half filled d shells, the (relative) tendency to ferromagnetic coupling
increases with increasing path length. As an application, the magnetic
interactions of the Cr compounds RbCrCl, CrCl, CrBr and CrI
are investigated, all of which except CrCl are ferromagnets.Comment: 13 pages, 6 eps figures, submitted to Phys Rev
Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa
The partial density of vibrational states has been measured for Fe in
compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering.
Substantial changes have been observed in the overall shape of the density of
states close to the magnetic transiton around 20 GPa from the paramagnetic (low
pressure) to the antiferromagnetic (high pressure) state. Our data indicate a
substantial softening of the aggregate sound velocities far below the
transition, starting between 5 and 10 GPa. This is consistent with recent
radial x-ray diffraction measurements of the elastic constants in FeO. The
results indicate that strong magnetoelastic coupling in FeO is the driving
force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure
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