The Experts below are selected from a list of 100965 Experts worldwide ranked by ideXlab platform
Paul Stuart Cally - One of the best experts on this subject based on the ideXlab platform.
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probing sunspots with two skip time distance helioseismology
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Thomas L. Duvall, Paul Stuart Cally, Kaori Nagashima, Damien Przybylski, Laurent GizonAbstract: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.
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probing sunspots with two skip time distance helioseismology
Astronomy and Astrophysics, 2018Co-Authors: Thomas L. Duvall, Paul Stuart Cally, Kaori Nagashima, Damien Przybylski, Laurent GizonAbstract:Context. Previous helioseismology of sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure. Aims. We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated. Methods. 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. Results. 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. Conclusions. With sufficient modeling effort, these should lead to better understanding of sunspot structure.
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numerical models of travel time inhomogeneities in sunspots
The Astrophysical Journal, 2009Co-Authors: Hamed Moradi, Shravan M Hanasoge, Paul Stuart CallyAbstract:We investigate the direct contribution of strong, sunspot-like magnetic fields to helioseismic wave travel-time shifts via two numerical forward models, a three-dimensional ideal MHD solver and MHD Ray Theory. The simulated data cubes are analyzed using the traditional time-distance center-to-annulus measurement technique. We also isolate and analyze the direct contribution from purely thermal perturbations to the observed travel-time shifts, confirming some existing ideas and bringing forth new ones: (i) that the observed travel-time shifts in the vicinity of sunspots are largely governed by MHD physics, (ii) the travel-time shifts are sensitively dependent on frequency and phase-speed filter parameters and the background power below the p 1 ridge, and finally, (iii) despite its seeming limitations, Ray Theory succeeds in capturing the essence of the travel-time variations as derived from the MHD simulations.
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numerical models of travel time inhomogeneities in sunspots
arXiv: Astrophysics, 2008Co-Authors: Hamed Moradi, Shravan M Hanasoge, Paul Stuart CallyAbstract:We investigate the direct contribution of strong, sunspot-like magnetic fields to helioseismic wave travel-time shifts via two numerical forward models, a 3D ideal MHD solver and MHD Ray Theory. The simulated data cubes are analyzed using the traditional time-distance center-to-annulus measurement technique. We also isolate and analyze the direct contribution from purely thermal perturbations to the observed travel-time shifts, confirming some existing ideas and bring forth new ones: (i) that the observed travel-time shifts in the vicinity of sunspots are largely governed by MHD physics, (ii) the travel-time shifts are sensitively dependent on frequency and phase-speed filter parameters and the background power below the $p_1$ ridge, and finally, (iii) despite its seeming limitations, Ray Theory succeeds in capturing the essence of the travel-time variations as derived from the MHD simulations.
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magnetic field inclination and atmospheric oscillations above solar active regions
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: H Schunker, Paul Stuart CallyAbstract:Recent observational evidence for magnetic field direction effects on helioseismic signals in sunspot penumbrae is suggestive of magnetohydrodynamic (MHD) mode conversion occurring at lower levels. This possibility is explored using wave mechanical and Ray Theory in a model of the Sun's surface layers permeated by uniform inclined magnetic field. It is found that fast-to-slow conversion near the equipartition depth at which the sound and Alfven speeds coincide can indeed greatly enhance the atmospheric acoustic signal at heights observed by Solar and Heliospheric Observatory/Michelson Doppler Imager and other helioseismic instruments, but that this effect depends crucially on the wave attack angle, i.e. the angle between the wavevector and the magnetic field at the conversion/transmission depth. A major consequence of this insight is that the magnetic field acts as a filter, preferentially allowing through acoustic signal from a narrow range of incident directions. This is potentially testable by observation.
Jeffrey R Blundell - One of the best experts on this subject based on the ideXlab platform.
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long extratropical planetary wave propagation in the presence of slowly varying mean flow and bottom topography part ii Ray propagation and comparison with observations
Journal of Physical Oceanography, 2003Co-Authors: Peter D Killworth, Jeffrey R BlundellAbstract:Ray Theory is used to predict phase and group velocities for long planetary waves under realistic, albeit slowly varying, oceanic conditions. The results are compared with local Theory using fields smoothed to the same amount (9° latitude/longitude) as well as those with much less smoothing (1°). The agreement is excellent, showing that local Theory forms a good proxy for Ray Theory results. The predicted speeds agree well with observations of planetary waves deduced from sea surface height data. The Theory uses purely baroclinic mean flow; the inclusion of barotropic flow has little effect except at high latitudes.
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the effect of bottom topography on the speed of long extratropical planetary waves
Journal of Physical Oceanography, 1999Co-Authors: Peter D Killworth, Jeffrey R BlundellAbstract:Abstract This paper examines how slowly varying topography induces changes in all aspects of long planetary wave propagation, including phase speed and surface signature, through steering effects. The approach introduces a method for the exact solution of the vertical topographic eigenvalue problem for arbitrary realistic stratification and Ray Theory in the horizontal. It is shown that, for observed stratifications, first internal mode topographic waves have phase speeds between about 0.4 and twice the local flat-bottom phase speed. Increases occur on the western and equatorward sides of hills. Focusing of Ray trajectories and caustics are common features of the solutions. Despite a bias between slowdown and speedup, on average there is little speedup except in high latitudes (where long-wave Theory is less applicable). Calculations are performed for five main ocean basins, assuming waves are generated at the eastern coastline, using smoothed topography. These calculations confirm the above findings: the...
Clive G. Parini - One of the best experts on this subject based on the ideXlab platform.
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uwb on body radio channel modelling using Ray Theory and sub band fdtd method
arXiv: Other Condensed Matter, 2006Co-Authors: Yan Zhao, Akram Alomainy, Yang Hao, Clive G. PariniAbstract:This paper presents the ultra-wideband (UWB) on-body radio channel modelling using a sub-band Finite-Difference Time-Domain (FDTD) method and a model combining the uniform geometrical Theory of diffraction (UTD) and Ray tracing (RT). In the sub-band FDTD model, the frequency band (3 - 9 GHz) is uniformly divided into 12 sub-bands in order to take into account the material frequency dispersion. Each sub-band is simulated separately and then a combination technique is used to recover all simulations at the receiver. In the UTD/RT model, the RT technique is used to find the surface diffracted Ray path while the UTD is applied for calculating the received signal. Respective modelling results from two-dimensional (2-D) and three-dimensional (3-D) sub-band FDTD and UTD/RT models indicate that antenna patterns have significant impacts on the on-body radio channel. The effect of different antenna types on on-body radio channels is also investigated through the UTD/RT approach.
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uwb on body radio channel modeling using Ray Theory and subband fdtd method
IEEE Transactions on Microwave Theory and Techniques, 2006Co-Authors: Yan Zhao, Akram Alomainy, Clive G. PariniAbstract:This paper presents the ultra-wideband on-body radio channel modeling using a subband finite-difference time-domain (FDTD) method and a model combining the uniform geometrical Theory of diffraction (UTD) and Ray tracing (RT). In the subband FDTD model, the frequency band (3-9 GHz) is uniformly divided into 12 subbands in order to take into account the material frequency dispersion. Each subband is simulated separately and then a combination technique is used to recover all simulations at the receiver. In the UTD/RT model, the RT technique is used to find the surface diffracted Ray path, while the UTD is applied for calculating the received signal. Respective modeling results from two- and three-dimensional subband FDTD and UTD/RT models indicate that antenna patterns have significant impacts on the on-body radio channel. The effect of different antenna types on on-body radio channels is also investigated through the UTD/RT approach.
Alejandro Luque - One of the best experts on this subject based on the ideXlab platform.
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on the accuracy of Ray Theory methods to determine the altitudes of intracloud electric discharges and ionospheric reflections application to narrow bipolar events
Journal of Geophysical Research, 2020Co-Authors: Feifan Liu, Zilong Qin, Baoyou Zhu, F J Perezinvernon, Alejandro LuqueAbstract:Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground-based very low frequency (VLF)/low-frequency (LF) observations. The simplified Ray-Theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high-frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth-ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full-wave Finite-Difference Time-Domain (FDTD) approach. The simulated results are compared with ground-based measurements at different distances in Southern China, and we assess the accuracy of the simplified Ray-Theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full-wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km.
Roel Snieder - One of the best experts on this subject based on the ideXlab platform.
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the effect of scattering in surface wave tomography
Geophysical Journal International, 2002Co-Authors: Jesper Spetzler, Jeannot Trampert, Roel SniederAbstract:Summary We present a new technique in surface wave tomography that takes the finite frequency of surface waves into account using first-order scattering Theory in a SNREI Earth. Physically, propagating surface waves with a finite frequency are diffracted by heterogeneity distributed on a sphere and then interfere at the receiver position. Paradoxically, surface waves have the largest sensitivity to velocity anomalies off the path of the geometrical Ray. The non-Ray geometrical effect is increasingly important for increasing period and distance. Therefore, it is expected that the violation of Ray Theory in surface wave tomography is most significant for the longest periods. We applied scattering Theory to phaseshift measurements of Love waves between periods of 40 and 150 s to obtain global phase velocity maps expanded in spherical harmonics to angular degree and order 40. These models obtained with scattering Theory were compared with those constructed with Ray Theory. We observed that Ray Theory and scattering Theory predict the same structure in the phase velocity maps to degree and order 25–30 for Love waves at 40 s and to degree and order 12–15 for Love waves at 150 s. For reasons of spectral leakage, a smoothness condition was included in the phaseshift inversions to construct the phase velocity maps, so we could not access the small length-scale structure in the obtained Earth models. We carried out a synthetic experiment for phase velocity measurements to investigate the limits of classical Ray Theory in surface wave tomography. In the synthetic experiment, we computed, using the source–receiver paths of our surface wave data set, the discrepancy between Ray theoretical and scattering theoretical phase velocity measurements for an input model with slowness heterogeneity for increasing angular degree. We found that classical Ray Theory in global surface wave tomography is only applicable for structures with angular degrees smaller than 25 (equivalent to 1600 km) and 15 (equivalent to 2700 km) for Love waves at 40 and 150 s, respectively. The synthetic experiment suggests that the Ray theoretical great circle approximation is appropriate to use in present-day global surface wave tomography. On the other hand, in order to obtain reliable models with a higher resolution we must take the non-Ray geometrical effect of surface waves into account.
