33 research outputs found
Accelerating Cosmic Microwave Background map-making procedure through preconditioning
Estimation of the sky signal from sequences of time ordered data is one of
the key steps in Cosmic Microwave Background (CMB) data analysis, commonly
referred to as the map-making problem. Some of the most popular and general
methods proposed for this problem involve solving generalised least squares
(GLS) equations with non-diagonal noise weights given by a block-diagonal
matrix with Toeplitz blocks. In this work we study new map-making solvers
potentially suitable for applications to the largest anticipated data sets.
They are based on iterative conjugate gradient (CG) approaches enhanced with
novel, parallel, two-level preconditioners. We apply the proposed solvers to
examples of simulated non-polarised and polarised CMB observations, and a set
of idealised scanning strategies with sky coverage ranging from nearly a full
sky down to small sky patches. We discuss in detail their implementation for
massively parallel computational platforms and their performance for a broad
range of parameters characterising the simulated data sets. We find that our
best new solver can outperform carefully-optimised standard solvers used today
by a factor of as much as 5 in terms of the convergence rate and a factor of up
to in terms of the time to solution, and to do so without significantly
increasing the memory consumption and the volume of inter-processor
communication. The performance of the new algorithms is also found to be more
stable and robust, and less dependent on specific characteristics of the
analysed data set. We therefore conclude that the proposed approaches are well
suited to address successfully challenges posed by new and forthcoming CMB data
sets.Comment: 19 pages // Final version submitted to A&
The solar chromosphere at millimetre and ultraviolet wavelengths. I. Radiation temperatures and a detailed comparison
Solar observations with the Atacama Large Millimeter/submillimeter Array
(ALMA) provide us with direct measurements of the brightness temperature in the
solar chromosphere. We study the temperature distributions obtained with ALMA
Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas
at, and in the vicinity of, a sunspot, comprising quasi-quiet and active
regions with different amounts of underlying magnetic fields. We compare these
temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths
(and with the line-core intensities of the optically-thin far-UV spectra),
co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer.
These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h
280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si
IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition
region. Splitting the ALMA sub-bands resulted in an slight increase of spatial
resolution in individual temperature maps, thus, resolving smaller-scale
structures compared to those produced with the standard averaging routines. We
find that the radiation temperatures have different, though somewhat
overlapping, distributions in different wavelengths and in the various magnetic
regions. Comparison of the ALMA temperatures with those of the UV diagnostics
should, however, be interpreted with great caution, the former is formed under
the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE.
The mean radiation temperature of the ALMA Band 6 is similar to that extracted
from the IRIS C II line in all areas with exception of the sunspot and pores
where the C II poses higher radiation temperatures. In all magnetic regions,
the Mg II lines associate with the lowest mean radiation temperatures in our
sample. These will provide constraints for future numerical models.Comment: Accepted for publication in the Astronomy & Astrophysics journa
Cosmic ray interactions in the solar atmosphere
High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce âalbedoâ emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P â Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P â B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations
Parallel Spherical Harmonic Transforms on heterogeneous architectures (GPUs/multi-core CPUs)
Spherical Harmonic Transforms (SHT) are at the heart of many scientific and
practical applications ranging from climate modelling to cosmological
observations. In many of these areas new, cutting-edge science goals have been
recently proposed requiring simulations and analyses of experimental or
observational data at very high resolutions and of unprecedented volumes. Both
these aspects pose formidable challenge for the currently existing
implementations of the transforms.
This paper describes parallel algorithms for computing SHT with two variants
of intra-node parallelism appropriate for novel supercomputer architectures,
multi-core processors and Graphic Processing Units (GPU). It also discusses
their performance, alone and embedded within a top-level, MPI-based
parallelisation layer ported from the S2HAT library, in terms of their
accuracy, overall efficiency and scalability. We show that our inverse SHT run
on GeForce 400 Series GPUs equipped with latest CUDA architecture ("Fermi")
outperforms the state of the art implementation for a multi-core processor
executed on a current Intel Core i7-2600K. Furthermore, we show that an
MPI/CUDA version of the inverse transform run on a cluster of 128 Nvidia Tesla
S1070 is as much as 3 times faster than the hybrid MPI/OpenMP version executed
on the same number of quad-core processors Intel Nahalem for problem sizes
motivated by our target applications. Performance of the direct transforms is
however found to be at the best comparable in these cases. We discuss in detail
the algorithmic solutions devised for major steps involved in the transforms
calculation, emphasising those with a major impact on their overall
performance, and elucidates the sources of the dichotomy between the direct and
the inverse operations
Algebraic Domain Decomposition Methods for Highly Heterogeneous Problems
International audienceWe consider the solving of linear systems arising from porous media flow simulations with high heterogeneities. Using a Newton algorithm to handle the non-linearity leads to the solving of a sequence of linear systems with different but similar matrices and right hand sides. The parallel solver is a Schwarz domain decomposition method. The unknowns are partitioned with a criterion based on the entries of the input matrix. This leads to substantial gains compared to a partition based only on the adjacency graph of the matrix. From the information generated during the solving of the first linear system, it is possible to build a coarse space for a two-level domain decomposition algorithm that leads to an acceleration of the convergence of the subsequent linear systems. We compare two coarse spaces: a classical approach and a new one adapted to parallel implementation
The Sun at millimeter wavelengths -- II. Small-scale dynamic events in ALMA Band 3
Solar observations with the Atacama Large Millimeter/sub-millimeter Array
(ALMA) facilitate studying the atmosphere of the Sun at chromospheric heights
at high spatial and temporal resolution at millimeter wavelengths. ALMA
intensity data at mm-wavelengths are used for a first detailed systematic
assessment of the occurrence and properties of small-scale dynamical features
in the quiet Sun. ALMA Band 3 data (~ mm / GHz) with spatial
resolution ~ - arcsec and a duration of ~ min are analysed
together with SDO/HMI magnetograms. The temporal evolution of the mm-maps is
studied to detect pronounced dynamical features which are connected to
dynamical events via a k-means clustering algorithm. The physical properties of
the resulting events are studied and it is explored if they show properties
consistent with propagating shock waves. For this purpose, observable shock
wave signatures at mm wavelengths are calculated from one- and
three-dimensional model atmospheres. There are 552 dynamical events detected
with an excess in brightness temperature () of at least
K. The events show a large variety in size up to ~ arcsec,
amplitude up to ~ K with typical values between ~
- K and lifetime at FWHM of between ~ -
s, with typical values between ~ - s. Furthermore, many of the
events show signature properties that suggest that they are likely produced by
propagating shock waves. There are a lot of small-scale dynamic structures
detected in the Band 3 data, even though the spatial resolution sets
limitations of the size of events that can be detected. The amount of dynamic
signatures in the ALMA mm data is very low in areas with photospheric
footpoints with stronger magnetic fields, which is consistent with the
expectation for propagating shock waves.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 15
figure
First Spectral Analysis of a Solar Plasma Eruption Using ALMA
The aim of this study is to demonstrate how the logarithmic millimeter
continuum gradient observed using the Atacama Large Millimeter/submillimeter
Array (ALMA) may be used to estimate optical thickness in the solar atmosphere.
We discuss how using multi-wavelength millimeter measurements can refine plasma
analysis through knowledge of the absorption mechanisms. Here we use sub-band
observations from the publicly available science verification (SV) data, whilst
our methodology will also be applicable to regular ALMA data. The spectral
resolving capacity of ALMA SV data is tested using the enhancement coincident
with an X-ray Bright Point (XBP) and from a plasmoid ejection event near active
region NOAA12470 observed in Band 3 (84-116 GHz) on 17/12/2015. We compute the
interferometric brightness temperature light-curve for both features at each of
the four constituent sub-bands to find the logarithmic millimetre spectrum. We
compared the observed logarithmic spectral gradient with the derived
relationship with optical thickness for an isothermal plasma to estimate the
structure's optical thicknesses. We conclude, within 90% confidence, that the
stationary enhancement has an optical thickness between , and that the moving enhancement has , thus
both lie near to the transition between optically thin and thick plasma at 100
GHz. From these estimates, isothermal plasmas with typical Band 3 background
brightness temperatures would be expected to have electron temperatures of
K for the stationary enhancement and between K for the moving enhancement, thus demonstrating the benefit of sub-band
ALMA spectral analysis.Comment: To appear in Ap
Characterisation of shock wave signatures at millimetre wavelengths from Bifrost simulations
Observations at millimetre wavelengths provide a valuable tool to study the
small scale dynamics in the solar chromosphere. We evaluate the physical
conditions of the atmosphere in the presence of a propagating shock wave and
link that to the observable signatures in mm-wavelength radiation, providing
valuable insights into the underlying physics of mm-wavelength observations. A
realistic numerical simulation from the 3D radiative Magnetohydrodynamic (MHD)
code Bifrost is used to interpret changes in the atmosphere caused by shock
wave propagation. High-cadence (1 s) time series of brightness temperature
(T) maps are calculated with the Advanced Radiative Transfer (ART)
code at the wavelengths mm and mm, which represents opposite
sides of spectral band~ of the Atacama Large Millimeter/submillimeter Array
(ALMA). An example of shock wave propagation is presented. The brightness
temperatures show a strong shock wave signature with large variation in
formation height between to Mm. The results demonstrate that
millimetre brightness temperatures efficiently track upwardly propagating shock
waves in the middle chromosphere. In addition, we show that the gradient of the
brightness temperature between wavelengths within ALMA band can potentially
be utilised as a diagnostics tool in understanding the small-scale dynamics at
the sampled layers.Comment: 16 pages, 6 figures. Accepted for publication in Philosophical
Transactions A of the Royal Societ