Variation Of Mode Frequencies Research Articles

Gravity Modes on Rapidly Rotating Accreting White Dwarfs and Their Variation after Dwarf Novae

Accreting white dwarfs (WDs) in cataclysmic variables (CVs) show short-period (tens of minutes) brightness variations that are consistent with nonradial oscillations similar to gravity (g) modes observed in isolated WDs. The dwarf nova GW Librae was the first CV in which nonradial oscillations were observed and continues to be the best-studied accreting WD displaying these pulsations. Unlike isolated WDs, accreting WDs rotate rapidly, with spin periods comparable to or shorter than typical low-order oscillation periods. Accreting WDs also have a different relationship between their interior and surface temperatures. The surface temperature of an accreting WD varies on a months-to-year timescale between dwarf nova accretion events, allowing study of how this temperature change affects g-mode behavior. Here we show results from adiabatic seismological calculations for accreting WDs, focusing on low-order (ℓ = 1) modes. We demonstrate how g-modes vary in response to temperature changes in the subsurface layers due to a dwarf nova accretion event. These calculations include rotation nonperturbatively, required by the high spin rate. We discuss the thermal history of these accreting WDs and compare the seismological properties with and without rotation. Comparison of g-mode frequencies to observed objects may allow inference of features of the structure of the WD such as mass, surface abundance, accretion history, and more. The variation of mode frequencies during cooling after an outburst provides a novel method of identifying modes.

Signatures of Magnetic Activity: On the Relation between Stellar Properties and p-mode Frequency Variations

Abstract In the Sun, the properties of acoustic modes are sensitive to changes in the magnetic activity. In particular, mode frequencies are observed to increase with increasing activity level. Thanks to CoRoT and Kepler, such variations have been found in other solar-type stars and encode information on the activity-related changes in their interiors. Thus, the unprecedented long-term Kepler photometric observations provide a unique opportunity to study stellar activity through asteroseismology. The goal of this work is to investigate the dependencies of the observed mode frequency variations on the stellar parameters and whether those are consistent with an activity-related origin. We select the solar-type oscillators with highest signal-to-noise ratio, in total, 75 targets. Using the temporal frequency variations determined in Santos et al., we study the relation between those variations and the fundamental stellar properties. We also compare the observed frequency shifts with chromospheric and photometric activity indexes, which are only available for a subset of the sample. We find that frequency shifts increase with increasing chromospheric activity, which is consistent with an activity-related origin of the observed frequency shifts. Frequency shifts are also found to increase with effective temperature, which is in agreement with the theoretical predictions for the activity-related frequency shifts by Metcalfe et al. Frequency shifts are largest for fast rotating and young stars, which is consistent with those being more active than slower rotators and older stars. Finally, we find evidence for frequency shifts increasing with stellar metallicity.

Time-domain study of spin-wave dynamics in two-dimensional arrays of bi-component magnetic structures

Spin-waves in single Ni80Fe20 (Py) and Co bi-component units embedded in two-dimensional arrays thereof are investigated by all-optical time-resolved magneto-optical Kerr effect microscope. Two bands of modes observed for the bias field applied along the long-axis of the bi-component unit convert into four distinct bands, and the mode frequencies change significantly as the field is rotated to the short-axis. Micromagnetic simulations reproduce the experimental results, and comparison with single Py and Co elements and single bi-component unit reveals significant variations of mode frequencies and mode profiles, indicating opportunities for magnonic band engineering in this type of bi-component arrays.

Vibrations of post-buckled rods: The singular inextensible limit

The small-amplitude in-plane vibrations of an elastic rod clamped at both extremities are studied. The rod is modeled as an extensible, shearable, planar Kirchhoff elastic rod under large displacements and rotations, and the vibration frequencies are computed both analytically and numerically as a function of the loading. Of particular interest is the variation of mode frequencies as the load is increased through the buckling threshold. While for some modes there are no qualitative changes in the mode frequencies, other frequencies experience rapid variations after the buckling threshold, the thinner the rod, the more abrupt the variations. Eventually, a mismatch for half of the frequencies at buckling arises between the zero thickness limit of the extensible model and the inextensible model.

J161032 自動車走行制御向け3軸複合センサ : ひずみにロバストな角速度センサの構造設計について([J16103]マイクロナノメカト口ニクス(3))

A microelectromechanical systems (MEMS) combined sensor measuring two-axis accelerations and an angular rate (rotation) has been developed for an electronic stability control system of automobiles. With the recent trend to mount the combined sensors in the engine compartment, the operation temperature range increased drastically, with the request of immunity to environmental disturbances such as vibration. In this paper, we report the combined sensor which has a gyroscopic part and two acceleration parts in single die. Especially, we focused on the structure design of the deformation-robust gyroscope which has three sets of symmetrically arranged folded beams as a main topic. The developed gyroscope has less than 0.6% temperature dependant variation of mode frequencies as well as less than 0.04% variation of the difference between driving and sensing mode frequencies.

High degree modes & instrumental effects

Full-disk observations taken with the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) spacecraft, or the upgraded Global Oscillations Network Group (GONG) instruments, have enough spatial resolution to resolve modes up to ι = 1000 if not ι = 1500. The inclusion of such high-degree modes (i.e., ι ≤ 1000) improves dramatically inferences near the surface. Unfortunately, observational and instrumental effects cause the characterization of high degree modes to be quite complicated.Indeed, the characteristics of the solar acoustic spectrum are such that, for a given order, mode lifetimes get shorter and spatial leaks get closer in frequency as the degree of a mode increases. A direct consequence of this property is that individual modes are resolved only at low and intermediate degrees. At high degrees the individual modes blend into ridges and the power distribution of the ridge defines the ridge central frequency, masking the underlying mode frequency. An accurate model of the amplitude of the peaks that contribute to the ridge power distribution is needed to recover the underlying mode frequency from fitting the ridge.We present a detailed discussion of the modeling of the ridge power distribution, and the contribution of the various observational and instrumental effects on the spatial leakage, in the context of the MDI instrument. We have constructed a physically motivated model (rather than an ad hoc correction scheme) that results in a methodology that can produce unbiased estimates of high-degree modes. This requires that the instrumental characteristics are well understood, a task that has turned out to pose a major challenge.We also present our latest results, where most of the known instrumental and observational effects that affect specifically high-degree modes were removed. These new results allow us to focus our attention on changes with solar activity.Finally, we present variations of mode frequencies resulting from solar activity over most of solar cycle 23. We present the correlation of medium and high degree modes with different solar indices. Our results confirm that the frequency shift scaled by the relative mode inertia is a function of frequency alone and follows a simple power law.

Microwave spectroscopy with vector network analyzer for interlayer exchange-coupledsymmetrical and asymmetrical NiFe/Ru/NiFe

Vector network analyzer ferromagnetic resonance spectroscopy (VNA-FMR) isused here to study the different excited modes of sputtered asymmetricalNiFe(13.6 nm)/Ru(dRu)/NiFe(27.2 nm) exchange-coupled films with variable Ru thicknesses. Theobtained results have been compared to those of the symmetricalNiFe(30 nm)/Ru(dRu)/NiFe(30 nm). In both cases, the measurements show the existence of an optic and an acousticprecessional mode. The optic mode was only observed over limited field ranges, especiallyin the symmetrical trilayers. To overcome such a limitation, we developed a new techniquesimilar to the longitudinal FMR, where the bias and the rf field are parallel to eachother and perpendicular to the pinning field. Interestingly, and in contrast to thesymmetrical trilayers, we observed a mode anti-crossing in the dispersion relation ofthe asymmetrical layers that we attributed to the thickness difference betweenthe two NiFe layers. Our experimental results on the effect of the biquadraticcoupling on the mode frequency variations are in good agreement with the theory.

Numerical study of an optically pumped multimode D2O laser

A numerical simulation of an optically pumped, multimode D2O laser was carried out based on Lamb’s semiclassical laser theory. The numerical code includes the dispersion effect of the laser medium (D2O gas in the cavity) in order to simulate the strength of mode coupling accurately. As a result, multiple-longitudinal-mode oscillations around the Raman resonance frequency were obtained that are roughly in agreement with experimental observations. Energy spectra of the D2O laser emission obtained by the simulation were compared with those of the experimental results for various D2O gas pressures. By including the dispersion effect, temporal variations of mode frequencies were calculated. In addition, resonant pumping was examined to obtain spectrally narrow outputs by tuning a frequency of a pump CO2 laser pulse to the line center of the absorption band of D2O molecules.

Mode frequency and damping changes due to chemical treatments of the violin bridge

Hammer‐impact modal analysis using a microphone as a no‐load vibration transducer was performed on violin bridge blanks chemically treated two different ways to extract free–free vibrational mode frequencies, dampings and shapes for comparison with untreated bridges. Data were taken for both in‐plane (IP) and out‐of‐plane (OP) vibrations over a 0–20 kHz range. Average effects of these chemical treatments on mode frequency and damping were determined from IP and OP single‐point excitation on six Formalization (FA), six Resorcin/formaldehyde (RE) bridges, and eight untreated (UN) maple violin bridge blanks. All the bridges exhibited three well‐defined IP modes and six OP modes. Few significant changes were observed between the frequency and damping parameters of the IP and OP mode shapes, although the RE treated bridge exhibited a possibly unique doublet at ∼9 kHz. Since no mode frequency variations ≳1.5% were found, even though the RE treatment increased bridge masses 9.1%, it was concluded that the combined mass/stiffness changes of the wood were related to the acoustic effects observed with these bridges [H. Yano and K. Minato, 1222–1227 (1992)].

Temperature Variation of Elastic Properties of .ALPHA.-Quartz up to the .ALPHA.-.BETA. Transition.

The temperature variations of normal mode frequencies of a specimen of α-quartz single-crystal have been measured nearly up to the α-β transition (T0 573°C), and all of the independent elastic constants were determined simultaneously by the rectangular parallelepiped resonance method. The results demonstrate that bulk moduli decrease largely toward T0, while shear moduli show only a slight decrease. The trigonal constant C14 decreases just below T0, but seems to remain a finite value even at T0. It is noted that the isotropic aggregate of α-quartz is a curious and scarce material in the sense that its bulk modulus is smaller than the shear modulus, and Poisson's ratio is very small or even negative at temperatures higher than 450°C. In the measured temperature variations of mode frequencies, a characteristic feature was observed when two branches of Ag modes came close, suggesting mode coupling. Except for this coupling between Ag modes themselves, no definite evidence was observed which was indicative of coupling between modes of different symmetry. So it can be said the present sample is uniform in its property enough to suppress the coupling between modes of different symmetry.

Thermodynamic and anharmonic properties of forsterite, α‐Mg2SiO4: Computer modelling versus high‐pressure and high‐temperature measurements

Self‐consistent minimum free‐energy calculations of the structure and lattice dynamics of forsterite at high pressure (up to 30 GPa) and high temperature (up to 1850 K) have been performed using an approach based on the Born model of solids. In the free‐energy minimization procedure, lattice dynamics and thermodynamic properties are calculated self‐consistently within the quasi‐harmonic approximation (QHA). The results of the free‐energy minimizations are compared with recent spectroscopic studies of forsterite at high pressure and high temperature. The predicted variations of mode frequencies with compression are consistent with those obtained from high‐pressure spectroscopy. On the other hand, the variations with temperature are underestimated. Because the variations of lattice dynamics with pressure are related to anharmonic properties such as the Gruneisen parameter and thermal expansion, it is concluded that most of the extrinsic (i.e., volume dependent) anharmonicity can be accounted for within the QHA. On the other hand, the variations of the lattice dynamics with temperature also include intrinsic (i.e., volume free) anharmonic effects, which are not accounted for in the QHA. In forsterite, these effects become significant for thermodynamic properties (heat capacities and thermal expansion) above 1200 K.

Controller Design of Flexible Spacecraft Attitude Control

A controller design method for flexible spacecraft attitude control is presented. The design method uses the frequency domain approach. The system is first described by partial differential equation with internal damping model. Then the frequency response is analized, and the three basic characteristics of flexible system, namely, average function, lower bound and upper bound are defined. A compensator design method is proposed on the basis of these characteristics. The result shows that the direct velocity feedback combined with weak lag networks seems to be the optimum in the sense that the bandwidth required for the actuator is the narrowest. This approach is very robust and immune to the parameter changes such as mode frequency variations. This is because the design mehtod is based on only the general information of flexible structure and not on the specific ones. Some experiment is performed to investigate the validity of the method.