Showing papers on "Amplitude published in 1999"

Multichannel analysis of surface waves

Abstract: The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear-wave velocity profile is dependent on frequency as well as distance from the source. Multichannel recording permits effective identification and isolation of noise according to distinctive traceto-trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real-time maximization of the S/N ratio during acquisition and subsequent processing steps. Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component. Breaks in coherent surface-wave arrivals, observable on the decomposed record, can be compensated for during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording. A multichannel shot gather decomposed into a sweptfrequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear-wave velocity (vs) profile with a downholevs profile.

Interpretational applications of spectral decomposition in reservoir characterization

Abstract: Spectral decomposition provides a novel means of utilizing seismic data and the discrete Fourier transform (DFT) for imaging and mapping temporal bed thickness and geologic discontinuities over large 3-D seismic surveys. By transforming the seismic data into the frequency domain via the DFT, the amplitude spectra delineate temporal bed thickness variability while the phase spectra indicate lateral geologic discontinuities. This technology has delineated stratigraphic settings (such as channel sands and structural settings involving complex fault systems) in 3-D surveys.

Attractive and repulsive tip-sample interaction regimes in tapping-mode atomic force microscopy

Abstract: Attractive and repulsive tip-sample interaction regimes of a force microscope operated with an amplitude modulation feedback were investigated as a function of tip-sample separation, free amplitude, and sample properties. In the attractive regime, a net attractive force dominates the amplitude reduction while in the repulsive regime the amplitude reduction is dominated by a net repulsive force. The transition between both regimes may be smooth or steplike, depending on free amplitude and sample properties. A steplike discontinuity is always a consequence of the existence of two oscillation states for the same conditions. Stiff materials and small free amplitudes give rise to steplike transitions while the use of large free amplitudes produce smooth transitions. Simulations performed on compliant samples showed cases where the cantilever dynamics is fully controlled by a net attractive force. Phase-shift measurements provide a practical method to determine the operating regime. Finally, we discuss the influence of those regimes in data acquisition and image interpretation.

An Adiabatic Quantum Electron Pump

Abstract: A quantum pumping mechanism that produces dc current or voltage in response to a cyclic deformation of the confining potential in an open quantum dot is reported. The voltage produced at zero current bias is sinusoidal in the phase difference between the two ac voltages deforming the potential and shows random fluctuations in amplitude and direction with small changes in external parameters such as magnetic field. The amplitude of the pumping response increases linearly with the frequency of the deformation. Dependencies of pumping on the strength of the deformations, temperature, and breaking of time-reversal symmetry were also investigated.

Drag reduction in fish-like locomotion

Abstract: We present experimental force and power measurements demonstrating that the power required to propel an actively swimming, streamlined, fish-like body is significantly smaller than the power needed to tow the body straight and rigid at the same speed U. The data have been obtained through accurate force and motion measurements on a laboratory fish-like robotic mechanism, 1.2 m long, covered with a flexible skin and equipped with a tail fin, at Reynolds numbers up to 106, with turbulence stimulation. The lateral motion of the body is in the form of a travelling wave with wavelength λ and varying amplitude along the length, smoothly increasing from the front to the tail end. A parametric investigation shows sensitivity of drag reduction to the non-dimensional frequency (Strouhal number), amplitude of body oscillation and wavelength λ, and angle of attack and phase angle of the tail fin. A necessary condition for drag reduction is that the phase speed of the body wave be greater than the forward speed U. Power estimates using an inviscid numerical scheme compare favourably with the experimental data. The method employs a boundary-integral method for arbitrary flexible body geometry and motions, while the wake shed from the fish-like form is modelled by an evolving desingularized dipole sheet.

Encoding amplitude information onto phase-only filters

Abstract: We report a new, to our knowledge, technique for encoding amplitude information onto a phase-only filter with a single liquid-crystal spatial light modulator. In our approach we spatially modulate the phase that is encoded onto the filter and, consequently, spatially modify the diffraction efficiency of the filter. Light that is not diffracted into the first order is sent into the zero order, effectively allowing for amplitude modulation of either the first-order or the zero-order diffracted light. This technique has several applications in both optical pattern recognition and image processing, including amplitude modulation and inverse filters. Experimental results are included for the new technique.

A kinematic model of a ducted flame

Abstract: A premixed ducted flame, burning in the wake of a bluff-body flame-holder, is considered. For such a flame, interaction between acoustic waves and unsteady combustion can lead to self-excited oscillations. The concept of a time-invariant turbulent flame speed is used to develop a kinematic model of the response of the flame to flow disturbances. Variations in the oncoming flow velocity at the flame-holder drive perturbations in the flame initiation surface and hence in the instantaneous rate of heat release. For linear fluctuations, the transfer function between heat release and velocity can be determined analytically from the model and is in good agreement with experiment across a wide frequency range. For nonlinear fluctuations, the model reproduces the flame surface distortions seen in schlieren films.Coupling this kinematic flame model with an analysis of the acoustic waves generated in the duct by the unsteady combustion enables the time evolution of disturbances to be calculated. Self-excited oscillations occur above a critical fuel–air ratio. The frequency and amplitude of the resulting limit cycles are in satisfactory agreement with experiment. Flow reversal is predicted to occur during part of the limit-cycle oscillation and the flame then moves upstream of the flame-holder, just as in experimental visualizations. The main nonlinearity is identified in the rate of heat release, which essentially ‘saturates’ once the amplitude of the velocity fluctuation exceeds its mean. We show that, for this type of nonlinearity, describing function analysis can be used to give a good estimate of the limit-cycle frequency and amplitude from a quasi-nonlinear theory.

Theory of optical scintillation

Abstract: A heuristic model of irradiance fluctuations for a propagating optical wave in a weakly inhomogeneous medium is developed under the assumption that small-scale irradiance fluctuations are modulated by large-scale irradiance fluctuations of the wave. The upper bound for small turbulent cells is defined by the smallest cell size between the Fresnel zone and the transverse spatial coherence radius of the optical wave. A lower bound for large turbulent cells is defined by the largest cell size between the Fresnel zone and the scattering disk. In moderate-to-strong irradiance fluctuations, cell sizes between those defined by the spatial coherence radius and the scattering disk are eliminated through spatial-frequency filtering as a consequence of the propagation process. The resulting scintillation index from this theory has the form σI2=σx2+σy2+σx2σy2, where σx2 denotes large-scale scintillation and σy2 denotes small-scale scintillation. By means of a modification of the Rytov method that incorporates an amplitude spatial-frequency filter function under strong-fluctuation conditions, tractable expressions are developed for the scintillation index of a plane wave and a spherical wave that are valid under moderate-to-strong irradiance fluctuations. In many cases the models also compare well with conventional results in weak-fluctuation regimes. Inner-scale effects are taken into account by use of a modified atmospheric spectrum that exhibits a bump at large spatial frequencies. Quantitative values predicted by these models agree well with experimental and simulation data previously published. In addition to the scintillation index, expressions are also developed for the irradiance covariance function of a plane wave and a spherical wave, both of which have the form BI(ρ)=Bx(ρ)+By(ρ)+Bx(ρ)By(ρ), where Bx(ρ) is the covariance function associated with large-scale fluctuations and By(ρ) is the covariance function associated with small-scale fluctuations. In strong turbulence the derived covariance shows the characteristic two-scale behavior, in which the correlation length is determined by the spatial coherence radius of the field and the width of the long residual correlation tail is determined by the scattering disk.

General circulation model simulations of the Mars Pathfinder atmospheric structure investigation/meteorology data

Abstract: The NASA Ames Mars General Circulation Model is used to interpret selected results from the Mars Pathfinder atmospheric structure instrument/meteorology (ASI/MET) experiment. The present version of the model has an improved soil thermal model, a new boundary layer scheme, and a correction for non-local thermodynamic equilibrium effects at solar wavelengths. We find good agreement with the ASI/MET entry data if the dust observed at the Pathfinder site is assumed to be distributed throughout the lowest five to six scale heights. This implies that the dust is globally distributed as well. In the lower atmosphere the inversion between 10 and 16 km in Pathfinder's entry profile is likely due to thermal emission from a water ice cloud in that region. In the upper atmosphere (above 50 km), dynamical processes, tides in particular, appear to have a cooling effect and may play an important role in driving temperatures toward the CO2 condensation temperature near 80 km. Near-surface air temperatures and wind directions are well simulated by the model by assuming a low surface albedo (0.16) and moderately high soil thermal inertia (336 SI). However, modeled tidal surface pressure amplitudes are about a factor of 2 smaller than observed. This may indicate that the model is not properly simulating interference effects between eastward and westward modes.

A synthesis of solar cycle prediction techniques

Abstract: A number of techniques currently in use for predicting solar activity on a solar cycle timescale are tested with historical data. Some techniques, e.g., regression and curve fitting, work well as solar activity approaches maximum and provide a month-by-month description of future activity, while others, e.g., geomagnetic precursors, work well near solar minimum but only provide an estimate of the amplitude of the cycle. A synthesis of different techniques is shown to provide a more accurate and useful forecast of solar cycle activity levels. A combination of two uncorrelated geomagnetic precursor techniques provides a more accurate prediction for the amplitude of a solar activity cycle at a time well before activity minimum. This combined precursor method gives a smoothed sunspot number maximum of 154 ± 21 at the 95% level of confidence for the next cycle maximum. A mathematical function dependent on the time of cycle initiation and the cycle amplitude is used to describe the level of solar activity month by month for the next cycle. As the time of cycle maximum approaches a better estimate of the cycle activity is obtained by including the fit between previous activity levels and this function. This Combined Solar Cycle Activity Forecast gives, as of January 1999, a smoothed sunspot maximum of 146 ± 20 at the 95% level of confidence for the next cycle maximum.

Richtmyer–Meshkov instability growth: experiment, simulation and theory

Abstract: Richtmyer–Meshkov instability is investigated for negative Atwood number and two-dimensional sinusoidal perturbations by comparing experiments, numerical simulations and analytic theories. The experiments were conducted on the NOVA laser with strong radiatively driven shocks with Mach numbers greater than 10. Three different hydrodynamics codes (RAGE, PROMETHEUS and FronTier) reproduce the amplitude evolution and the gross features in the experiment while the fine-scale features differ in the different numerical techniques. Linearized theories correctly calculate the growth rates at small amplitude and early time, but fail at large amplitude and late time. A nonlinear theory using asymptotic matching between the linear theory and a potential flow model shows much better agreement with the late-time and large-amplitude growth rates found in the experiments and simulations. We vary the incident shock strength and initial perturbation amplitude to study the behaviour of the simulations and theory and to study the effects of compression and nonlinearity.

A distinct class of isolated intracloud lightning discharges and their associated radio emissions

Abstract: Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short-duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 μs. The HF emissions associated with NPBPs were broadband noise-like radiation bursts with a mean duration of 2.8 μs and amplitudes 10 times larger than emissions from typical intracloud and cloud-to-ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.

FAST Observations of Inertial Alfven Waves in the Dayside Aurora

Abstract: Bursts of large amplitude impulsive electric and magnetic field oscillations are a common feature observed from FAST when crossing the dayside auroral oval at altitudes from 1500–2500 km. The oscillations have transverse amplitudes of up to 1 V/m and 100nT and exhibit a parallel electric field component with amplitudes which may be as large as 100mV/m. Calculation of E1/B1 over 100 events yields an average value four times the local Alfven speed. The wave period is usually less than 0.25s with ‘perpendicular wavelengths’ which average to 7.1 electron skin depths (c/ωpe∼80m). Poynting flux calculations indicate predominately downward fluxes with magnitude up 10−2 Wm−2 usually accompanied by a smaller upwards component. Invariably these waves are accompanied by field-aligned fluxes of down going and sometimes counterstreaming suprathermal electrons. Comparison with theoretical studies indicate that these observations are consistent with the characteristics of a shear Alfven wave with k⟂∼ωpe/c propagating in the inertial dispersive regime and interfering with a reflected component. However the observed large parallel electric field component, if real, has yet to be explained

Gamma Doradus Stars: Defining a New Class of Pulsating Variables

Abstract: In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf, Gamma Doradus. These stars typically have between 1 and 5 periods ranging from 0.4 to 3 days with photometric amplitudes up to 0.1 in Johnson V. The mechanism for these observed variations is high-order, low-degree, non-radial, gravity-mode pulsation. Gamma Doradus stars exhibit variability on a time scale that is an order of magnitude slower than Delta Scuti stars. They may offer additional insight into stellar physics when they are better understood (e.g., they may represent the cool portion of an "iron opacity instability strip" currently formed by the Beta Cephei stars, the SPB stars, and the subdwarf B stars; they may also offer insight into the presence of g-modes in solar-like stars).

Visualization and measurement of internal waves by ‘synthetic schlieren’. Part 1. Vertically oscillating cylinder

Abstract: We present measurements of the density and velocity fields produced when an oscillating circular cylinder excites internal gravity waves in a stratified fluid. These measurements are obtained using a novel, non-intrusive optical technique suitable for determining the density fluctuation field in temporally evolving flows which are nominally two-dimensional. Although using the same basic principles as conventional methods, the technique uses digital image processing in lieu of large and expensive parabolic mirrors, thus allowing more flexibility and providing high sensitivity: perturbations of the order of 1% of the ambient density gradient may be detected. From the density gradient field and its time derivative it is possible to construct the perturbation fields of density and horizontal and vertical velocity. Thus, in principle, momentum and energy fluxes can be determined.In this paper we examine the structure and amplitude of internal gravity waves generated by a cylinder oscillating vertically at different frequencies and amplitudes, paying particular attention to the role of viscosity in determining the evolution of the waves. In qualitative agreement with theory, it is found that wave motions characterized by a bimodal displacement distribution close to the source are attenuated by viscosity and eventually undergo a transition to a unimodal displacement distribution further from the source. Close quantitative agreement is found when comparing our results with the theoretical ones of Hurley & Keady (1997). This demonstrates that the new experimental technique is capable of making accurate measurements and also lends support to analytic theories. However, theory predicts that the wave beams are narrower than observed, and the amplitude is significantly under-predicted for low-frequency waves. The discrepancy occurs in part because the theory neglects the presence of the viscous boundary layers surrounding the cylinder, and because it does not take into account the effects of wave attenuation resulting from nonlinear wave–wave interactions between the upward and downward propagating waves near the source.

Attenuation and excitation of three-component ground motion in southern California

Abstract: Ground motion attenuation with distance and the variation of excitation with magnitude are parameterized using three-component, 0.25 to 5.0-Hz earthquake ground motions recorded in the distance range of 15-500 km for southern California to define a consistent model that describes both peak ground motion and Fourier spectra observations. The data set consists of 820 three-component TERRAscope recordings from 140 earthquakes, recorded at 17 stations, with moment magnitudes between 3.1 and 6.7. Regression analysis uses a simple model to relate the logarithm of measured ground motion to excitation, site, and propagation effects. The peak motions are Fourier velocity spectra and peak velocities in selected narrow bandpass-filtered frequency ranges. Regression results for Fourier amplitude spectra and peak velocities are used to define a piecewise continuous geometrical spreading function, frequency dependent Q ( f ), and a distance dependent duration that can be used with random vibration theory (RVT) or stochastic simulations to predict other characteristics of the ground motion. The duration results indicate that both the variation of the duration data with distance and its scattering decrease with increasing frequency. The ratio of horizontal to vertical component site terms is about √2 for all frequencies. However, this ratio is near unity for rock sites and is larger for soil sites. Simple modeling indicates that the Fourier velocity spectra are best fit by bilinear geometrical spreading of r −1 for r r −1/2 for r > 40 km. The frequency-dependent quality factor is Q ( f ) = 180 f 0.45 for each of the three components and also for the combined three-component data sets. The T 5%-75% duration window provides good agreement between observed and RVT predicted peak values.

Properties of large-amplitude internal waves

Abstract: Properties of solitary waves propagating in a two-layer fluid are investigated comparing experiments and theory. In the experiments the velocity eld induced by the waves, the propagation speed and the wave shape are quite accurately measured using particle tracking velocimetry (PTV) and image analysis. The experiments are calibrated with a layer of fresh water above a layer of brine. The depth of the brine is 4.13 times the depth of the fresh water. Theoretical results are given for this depth ratio, and, in addition, in a few examples for larger ratios, up to 100:1. The wave amplitudes in the experiments range from a small value up to almost maximal amplitude. The thickness of the pycnocline is in the range of approximately 0.13{0.26 times the depth of the thinner layer. Solitary waves are generated by releasing a volume of fresh water trapped behind a gate. By careful adjustment of the length and depth of the initial volume we always generate a single solitary wave, even for very large volumes. The experiments are very repeatable and the recording technique is very accurate. The error in the measured velocities non-dimensionalized by the linear long wave speed is less than about 7{8% in all cases. The experiments are compared with a fully nonlinear interface model and weakly nonlinear Korteweg{de Vries (KdV) theory. The fully nonlinear model compares excellently with the experiments for all quantities measured. This is true for the whole amplitude range, even for a pycnocline which is not very sharp. The KdV theory is relevant for small wave amplitude but exhibit a systematic deviation from the experiments and the fully nonlinear theory for wave amplitudes exceeding about 0.4 times the depth of the thinner layer. In the experiments with the largest waves, rolls develop behind the maximal displacement of the wave due to the Kelvin{Helmholtz instability. The recordings enable evaluation of the local Richardson number due to the flow in the pycnocline. We nd that stability or instability of the flow occurs in approximate agreement with the theorem of Miles and Howard.

Pulse Width Evolution in GRBs: Evidence for Internal Shocks

Abstract: Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we find that the width of the large amplitude pulses in GRB time histories remain remarkably constant throughout the classic GRB phase. This is also true for small amplitude pulses. However, small and large pulses do not have the same pulse width within a single time history. We find a quantitative relationship between pulse amplitude and pulse width: the smaller amplitude peaks tend to be wider, with amplitude following a power law with an index of about -2.8. Internal shocks simulated by randomly selecting the Lorentz factor and energy per shell are consistent with a power law relationship. This is strong quantitative evidence that GRBs are, indeed, caused by internal shocks. The dependency of the width-vs.-intensity relationship on the maximum Lorentz factor provides a way to estimate that elusive parameter. Our observed power law index indicates that \Gamma_{max} is less or equal than 10^3. We also interpret the narrowness of the pulse width distribution as indicating that the emission, that occurs when one shell over takes another, is produced over a small range of distances from the central site.

Relations between interaction force and frequency shift in large-amplitude dynamic force microscopy

Abstract: Large-amplitude dynamic force microscopy based on measuring shifts of the resonance frequency of the force sensor has proved to be a powerful imaging tool. General expressions relating arbitrary interaction forces to resonance frequency shifts are derived using variational methods and Fourier expansion of the tip motion. For interactions with a range much shorter than the vibration amplitude, the frequency shift can be expressed in terms of a convolution product involving the interaction force and a weakly divergent kernel. The convolution can be inverted, thus enabling one to recover unequivocally interaction potentials and forces from measured frequency shift data.

Some Observational Consequences of GRB Shock Models

Abstract: In the internal shock scenario for GRBs we expect some fraction of the energy of the burst to be carried by slow moving shells that were ejected at late times. These slow shells collide with faster moving outer shells when the outer shells have slowed down as a result of sweeping up material from the ISM. This gives rise to a forward shock that moves into the outer shell producing a bump in the afterglow light curve of amplitude roughly proportional to the ratio of the energy in the inner and the outer shells. In addition, a reverse shock propagates in the inner shell and produces emission at a characteristic frequency that is typically much smaller than the peak of the emission from the outer shell by a factor of $\sim 7 \gamma_{0c}^2 (E_2/E_1)^{1.1}$, and the observed flux at this frequency from the reverse shock is larger compared to the flux from the outer shell by a factor of $\sim 8 (\gamma_{0c} E_2/E_1)^{5/3} $; where $\gamma_{0c}$ is the bulk Lorentz factor of the outer shell at the time of collision, and $E_1 & E_2$ are the total energy in the outer and the inner shells respectively. The Lorentz factor is related to the observer time as $\sim 5 (t/day)^{3/8}$. The shell collision could produce initial temporal variability in the early afterglow signal. The lack of significant deviation from a power-law decline of the optical afterglow from half a dozen bursts suggests that $E_2/E_1$ is small. Future multi-wavelength observations should be able to either detect bumps in the light curve corresponding to both the forward and the reverse shocks or further constrain the late time release of energy in ejecta with small Lorentz factor, which is expected generically in the internal shock models for the gamma-ray bursts.

Measurement of the Coherence of a Bose-Einstein Condensate

Abstract: We present experimental and theoretical studies of the coherence properties of a Bose-Einstein condensate (BEC) using an interference technique. Two optical standing wave pulses of duration 100 ns and separation $\ensuremath{\Delta}t$ are applied to a condensate. Each standing wave phase grating makes small copies of the condensate displaced in momentum space. The quantum mechanical amplitudes of each copy interfere, depending on $\ensuremath{\Delta}t$ and on spatial phase variations across the condensate. We find that the behavior of a trapped BEC is consistent with a uniform spatial phase. A released BEC, however, exhibits large phase variation across the condensate.

Probability density of the surface electromyogram and its relation to amplitude detectors

Abstract: When the surface electromyogram (EMG) generated from constant-force, constant-angle, nonfatiguing contractions is modeled as a random process, its density is typically assumed to be Gaussian. This assumption leads to root-mean-square (RMS) processing as the maximum likelihood estimator of the EMG amplitude (where EMG amplitude is defined as the standard deviation of the random process). Contrary to this theoretical formulation, experimental work has found the signal-to-noise-ratio [(SNR), defined as the mean of the amplitude estimate divided by its standard deviation] using mean-absolute-value (MAV) processing to be superior to RMS. This paper reviews RMS processing with the Gaussian model and then derives the expected (inferior) SNR performance of MAV processing with the Gaussian model. Next, a new model for the surface EMG signal, using a Laplacian density, is presented. It is shown that the MAV processor is the maximum likelihood estimator of the EMG amplitude for the Laplacian model. SNR performance based on a Laplacian model is predicted to be inferior to that of the Gaussian model by approximately 32%. Thus, minor variations in the probability distribution of the EMG may result in large decrements in SNR performance. Lastly, experimental data from constant-force, constant-angle, nonfatiguing contractions were examined. The experimentally observed densities fell in between the theoretic Gaussian and Laplacian densities. On average, the Gaussian density best fit the experimental data, although results varied with subject. For amplitude estimation, MAV processing had a slightly higher SNR than RMS processing.

Magnetic cycles of HR 1099 and LQ Hydrae

Abstract: We present in this paper a 6-yr time series of magnetic (and brightness) surface images of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri) and of the young K0 dwarf LQ Hydrae, reconstructed (with the help of a dedicated maximum entropy image reconstruction software) from Zeeman--Doppler imaging observations collected at the Anglo-Australian Telescope. All the stellar magnetic images that we reconstruct host at least one high-contrast feature in which the field is predominantly azimuthal, thus confirming that such surface structures (already detected in previous similar studies) are indeed real. We take this as strong evidence that dynamo processes of late-type rapid rotators operate throughout the whole stellar convective envelope, rather than being confined to an interface layer between the convective and radiative zones as in the Sun. The latitudinal polarity pattern of azimuthal and radial fields that we observe at the surface of both stars suggests that these magnetic regions respectively reveal the toroidal and poloidal components of the large-scale dynamo field. The spatial structure of these two magnetic field components becomes increasingly more complex (with higher axisymmetric spherical harmonic degrees) for larger rotation rates and deeper convective zones. The strength of the toroidal and poloidal components is typically a few hundred G, more than two orders of magnitude stronger than in the Sun. Long-term evolution of the toroidal and poloidal components of the large-scale field is clearly detected in our time series. We also report the detection of small fluctuations in the orbital period of HR 1099, with a peak-to-peak amplitude of about 36±1 s (i.e. 0.015 per cent) and a period of about 18±2 yr (assuming sinusoidal variations around the nominal value). The most plausible way of explaining such fluctuations is that the quadrupole moment of the K1 subgiant is varying with time, and that this modulation is driven by the magnetic activity cycle of the primary star itself (through a periodic exchange between kinetic and magnetic energy within the convective zone). It provides in particular an independent confirmation that a dynamo operates within the whole convective zone, and suggests that the average azimuthal field in the convective envelope is of the order of 6 kG.

Measuring the equation of state of the intergalactic medium

Abstract: Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyα forest follows a well-defined temperature–density relation, which is well described by a power law . We demonstrate that such an equation of state results in a power-law cut-off in the distribution of linewidths (b-parameters) as a function of column density (N) for the low column density (N≲1014.5 cm−2) absorption lines. This explains the existence of the lower envelope that is clearly seen in scatter plots of the b(N) distribution in observed QSO spectra. Even a strict power-law equation of state will not result in an absolute cut-off because of line blending and contamination by unidentified metal lines. We develop an algorithm to determine the cut-off, which is insensitive to these narrow lines. We show that the parameters of the cut-off in the b(N) distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cut-off in the b(N) distribution to measure the equation of state of the IGM. We show that systematics that change the b(N) distribution, such as cosmology (for a fixed equation of state), peculiar velocities, the intensity of the ionizing background radiation and variations in the signal-to-noise ratio, do not affect the measured cut-off. We argue that physical processes that have not been incorporated in the simulations, e.g. feedback from star formation, are unlikely to affect the results. Using Monte Carlo simulations of Keck spectra at z=3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within 10 per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the Universe.

Phase-space electron holes along magnetic field lines

Abstract: Recent observations from satellites crossing active magnetic field lines have revealed solitary potential structures that move at speeds substantially greater than the ion thermal velocity. The structures appear as positive potential pulses rapidly drifting along the magnetic field. We interpret them as BGK electron holes supported by a population of trapped and passing electrons. Using Laplace transform techniques, we analyse the behavior of one phase-space electron hole. The resulting potential shapes and electron distribution functions are self-consistent and compatible with the field and particle data associated with the observed pulses. In particular, the spatial width increases with increasing amplitude. The stability of the analytic solution is tested by means of a two-dimensional particle-in-cell simulation code with open boundaries. We also use our code to briefly investigate the influence of the ions. The nonlinear structure appears to be remarkably resilient.