1,454 research outputs found
Centre-to-limb spectro-polarimetric diagnostics of simulated solar photospheric magneto-convection: signatures of photospheric Alfven waves
Using numerical simulations of the magnetised solar photosphere carried out
with the radiative magneto-hydrodynamic code, MURaM, and detailed
spectro-polarimetric diagnostics of the simulated photospheric 6302A FeI line,
spectro-polarimetric signatures of Alfven waves in magnetised intergranular
lanes of the simulated solar photosphere were analysed at different positions
at the solar disk. The torsional Alfven waves in the intergranular lanes are
horizontal plasma motions, which do not have a thermal perturbation
counterpart. We find signatures of Alfven waves as small-scale line profile
Doppler shifts and Stokes-V area asymmetry enhancements in the simulated
off-disk centre observations. These photospheric features disappear when the
simulated observations are degraded with a telescope PSF similar to the one of
Hinode. We analyse the possibilities for direct observations and confirmation
of Alfven wave presence in the solar photosphere.Comment: 6 pages, 3 figures, accepted to PAS
Directional Time-Distance Probing of Model Sunspot Atmospheres
A crucial feature not widely accounted for in local helioseismology is that
surface magnetic regions actually open a window from the interior into the
solar atmosphere, and that the seismic waves leak through this window, reflect
high in the atmosphere, and then re-enter the interior to rejoin the seismic
wave field normally confined there. In a series of recent numerical studies
using translation invariant atmospheres, we utilised a "directional
time-distance helioseismology" measurement scheme to study the implications of
the returning fast and Alfv\'en waves higher up in the solar atmosphere on the
seismology at the photosphere (Cally & Moradi 2013; Moradi & Cally 2014). In
this study, we extend our directional time-distance analysis to more realistic
sunspot-like atmospheres to better understand the direct effects of the
magnetic field on helioseismic travel-time measurements in sunspots. In line
with our previous findings, we uncover a distinct frequency-dependant
directional behaviour in the travel-time measurements, consistent with the
signatures of MHD mode conversion. We found this to be the case regardless of
the sunspot field strength or depth of its Wilson depression. We also isolated
and analysed the direct contribution from purely thermal perturbations to the
measured travel times, finding that waves propagating in the umbra are much
more sensitive to the underlying thermal effects of the sunspot.Comment: 9 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Probing sunspots with two-skip time-distance helioseismology
Previous helioseismology of sunspots has been sensitive to both the
structural and magnetic aspects of sunspot structure. We aim to develop a
technique that is insensitive to the magnetic component so the two aspects can
be more readily separated. We study waves reflected almost vertically from the
underside of a sunspot. Time-distance helioseismology was used to measure
travel times for the waves. Ray theory and a detailed sunspot model were used
to calculate travel times for comparison. It is shown that these large distance
waves are insensitive to the magnetic field in the sunspot. The largest travel
time differences for any solar phenomena are observed. With sufficient modeling
effort, these should lead to better understanding of sunspot structure
Multi-objective integer programming: An improved recursive algorithm
This paper introduces an improved recursive algorithm to generate the set of
all nondominated objective vectors for the Multi-Objective Integer Programming
(MOIP) problem. We significantly improve the earlier recursive algorithm of
\"Ozlen and Azizo\u{g}lu by using the set of already solved subproblems and
their solutions to avoid solving a large number of IPs. A numerical example is
presented to explain the workings of the algorithm, and we conduct a series of
computational experiments to show the savings that can be obtained. As our
experiments show, the improvement becomes more significant as the problems grow
larger in terms of the number of objectives.Comment: 11 pages, 6 tables; v2: added more details and a computational stud
Inversions of synthetic umbral flashes: effects of the scanning time on the inferred atmospheres
The use of instruments that record narrow band images at selected wavelengths
is a common approach in solar observations. They allow the scanning of a
spectral line by sampling the Stokes profiles with 2D images at each line
position, but require a compromise between spectral resolution and temporal
cadence. We evaluate the impact of the time-dependent acquisition of different
wavelengths on the inversion of spectropolarimetric profiles from chromospheric
lines during umbral flashes. Simulations of non-linear wave propagation in a
sunspot were performed with the code MANCHA. Synthetic Stokes parameters in the
Ca II 8542 A line in NLTE were computed for an umbral flash using the code
NICOLE. Artificial profiles with the same wavelength coverage and temporal
cadence from reported observations were constructed and inverted. The inferred
atmospheric stratifications were compared with the original models. The
inferred atmospheres provide a reasonable characterization of the thermodynamic
properties of the atmosphere during most of the phases of the umbral flash.
Only at the early stages of the flash, when the shock wave reaches the
formation height of the line, the Stokes profiles present apparent wavelength
shifts and other spurious deformations. These features are misinterpreted by
the inversion code, which can return unrealistic atmospheric models from a good
fit of the Stokes profiles. The misguided results include flashed atmospheres
with strong downflows, even though the simulation exhibits upflows during the
umbral flash, and large variations in the magnetic field strength. Our analyses
validate the inversion of Stokes profiles acquired by sequentially scanning
certain selected wavelengths of a line profile, even in the case of
rapidly-changing events such as umbral flashes. However, the inversions are
unreliable during a short period at the development phase of the flash.Comment: Accepted for publication in A&
Optimising a nonlinear utility function in multi-objective integer programming
In this paper we develop an algorithm to optimise a nonlinear utility
function of multiple objectives over the integer efficient set. Our approach is
based on identifying and updating bounds on the individual objectives as well
as the optimal utility value. This is done using already known solutions,
linear programming relaxations, utility function inversion, and integer
programming. We develop a general optimisation algorithm for use with k
objectives, and we illustrate our approach using a tri-objective integer
programming problem.Comment: 11 pages, 2 tables; v3: minor revisions, to appear in Journal of
Global Optimizatio
First principles calculation of structural and magnetic properties for Fe monolayers and bilayers on W(110)
Structure optimizations were performed for 1 and 2 monolayers (ML) of Fe on a
5 ML W(110) substrate employing the all-electron full-potential linearized
augmented plane-wave (FP-LAPW) method. The magnetic moments were also obtained
for the converged and optimized structures. We find significant contractions
( 10 %) for both the Fe-W and the neighboring Fe-Fe interlayer spacings
compared to the corresponding bulk W-W and Fe-Fe interlayer spacings. Compared
to the Fe bcc bulk moment of 2.2 , the magnetic moment for the surface
layer of Fe is enhanced (i) by 15% to 2.54 for 1 ML Fe/5 ML W(110), and
(ii) by 29% to 2.84 for 2 ML Fe/5 ML W(110). The inner Fe layer for 2
ML Fe/5 ML W(110) has a bulk-like moment of 2.3 . These results agree
well with previous experimental data
Low-Frequency Conducted Emissions Assessment for Electric Powertrain
The conducted emissions (CE) in powertrain applications are a major concern due to the high current peaks and transients from torque profiles and sudden speed changes. This paper investigates the time-domain noises analytically through a curve-fitting process, whose results can be used to reproduce the noise waveforms avoiding the need to store the entire waveform. Besides, common-mode (CM) and differential-mode (DM) voltages from speed changes of a PMSM (permanent magnet synchronous motor) powertrain are compared, in terms of harmonic components and noise amplitude, by using line impedance stabilization network (LISN) according to different standards. It was observed that DM emissions, at low frequencies, are better coupled to DO-160G LISN and increase progressively with speed rise, while CM noise levels are kept constant
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