Velocity Signatures Research Articles

Using the Sun to study the impact of stellar activity on exoplanet detectability

AbstractStellar activity induced by spots and plages, even at a low level of activity, affects the radial velocity signatures. It is therefore crucial to determine how it impacts our ability to detect small planetary signals such as those produced by Earth‐mass planets in the habitable zone of stars. We focus on the solar case: thanks to the wealth of accurate data available, the Sun gives us a unique opportunity to test the impact of stellar activity on such detections. We use the Sun as a template and investigate in details the impact of spots and plages as well as the role played by the attenuation of convection due to the presence of magnetic activity. We present the detection limits obtained in various conditions. We find that the plage contribution due to the convective blueshift attenuation dominates the total signal, with a peak‐to‐peak amplitude over the solar cycle of about 8 m s–1, and show that this contribution would prevent the detection of the Earth around the Sun, even with forthcoming high precision radial velocity instruments, unless ways to correct the signal are found. We therefore propose practical ways to correct for the activity on various timescales from days to years, and use again the Sun to estimate the ultimate detection limits achievable once the signal is corrected from the Sun's activity. Estimations will be briefly presented. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Range-Correcting Azimuthal Shear in Doppler Radar Data

Abstract The current tornado detection algorithm (TDA) used by the National Weather Service produces a large number of false detections, primarily because it calculates azimuthal shear in a manner that is adversely impacted by noisy velocity data and range-degraded velocity signatures. Coincident with the advent of new radar-derived products and ongoing research involving new weather radar systems, the National Severe Storms Laboratory is developing an improved TDA. A primary component of this algorithm is the local, linear least squares derivatives (LLSD) azimuthal shear field. The LLSD method incorporates rotational derivatives of the velocity field and is affected less strongly by noisy velocity data in comparison with traditional “peak to peak” azimuthal shear calculations. LLSD shear is generally less range dependent than peak-to-peak shear, although some range dependency is unavoidable. The relationship between range and the LLSD shear values of simulated circulations was examined to develop a range correction for LLSD shear. A linear regression and artificial neural networks (ANNs) were investigated as range-correction models. Both methods were used to produce fits for the simulated shear data, although the ANN excelled as it could capture the nonlinear nature of the data. The range-correction methods were applied to real radar data from tornadic and nontornadic events to measure the capacity of the corrected shear to discriminate between tornadic and nontornadic circulations. The findings presented herein suggest that both methods increased shear values during tornadic periods by nearly an order of magnitude, facilitating differentiation between tornadic and nontornadic scans in tornadic events.

Vertical Variations of Boundary Layer Potential Buoyancy in Tornadic and Nontornadic Near-Storm Environments

Abstract Despite great strides in understanding the tornadic near-storm environment (NSE), at times it remains difficult to determine why some storms produce significant tornadoes, while others produce none, given similar pretornadic radar reflectivity and velocity signatures. Previous studies have shown that this is likely related to the potential buoyancy (θep) of the rear-flank downdraft (RFD) air. Unfortunately, to date there are few ways to operationally anticipate possible RFD thermodynamic character. Based upon previous research indicating that capping inversions may restrict much of the low-level RFD air to come from within the boundary layer, this study considers the relation of Δθep (vertical change in θep within the boundary layer below the cap) to tornadogenesis potential. This is because when a cap exists above a boundary layer and the descent of lower-θep air from aloft to the surface is potentially limited, then minimal Δθep may indicate more RFD air that has greater potential buoyancy. The Rapid Update Cycle (RUC) soundings used in this study and several observed soundings taken in the vicinity of violent tornadoes suggest that boundary layer Δθep shows promise as an additional means of discriminating between tornadic and nontornadic NSEs.

Circumstellar habitable zones of binary-star systems in the solar neighbourhood

Binary and multiple systems constitute more than half of the total stellar population in the Solar neighborhood (Kiseleva-Eggleton and Eggleton 2001). Their frequent occurrence as well as the fact that more than 70 (Schneider et al. 2011) planets have already been discovered in such configurations - most noteably the telluric companion of alpha Centauri B (Dumusque et al. 2012) - make them interesting targets in the search for habitable worlds. Recent studies (Eggl et al. 2012b, Forgan 2012) have shown, that despite the variations in gravitational and radiative environment, there are indeed circumstellar regions where planets can stay within habitable insolation limits on secular dynamical timescales. In this article we provide habitable zones for 19 near S-Type binary systems from the Hipparchos and WDS catalogues with semimajor axes between 1 and 100 AU. Hereby, we accounted for the combined dynamical and radiative influence of the second star on the Earth-like planet. Out of the 19 systems presented, 17 offer dynamically stable habitable zones around at least one component. The 17 potentially habitable systems contain 5 F, 3 G, 7 K and 16 M class stars. As their proximity to the Solar System (d < 31 pc) makes the selected binary stars exquisite targets for observational campaigns, we offer estimates on radial velocity, astrometric and transit signatures produced by habitable Earth-like planets in eccentric circumstellar orbits.

A global shear velocity model of the upper mantle from fundamental and higher Rayleigh mode measurements

We present DR2012, a global SV‐wave tomographic model of the upper mantle. We use an extension of the automated waveform inversion approach of Debayle (1999) which improves our mapping of the transition zone with extraction of fundamental and higher‐mode information. The new approach is fully automated and has been successfully used to match approximately 375,000 Rayleigh waveforms. For each seismogram, we obtain a path average shear velocity and quality factor model, and a set of fundamental and higher‐mode dispersion and attenuation curves. We incorporate the resulting set of path average shear velocity models into a tomographic inversion. In the uppermost 200 km of the mantle, SV wave heterogeneities correlate with surface tectonics. The high velocity signature of cratons is slightly shallower (≈200 km) than in other seismic models. Thicker continental roots are not required by our data, but can be produced by imposinga priori a smoother model in the vertical direction. Regions deeper than 200 km show no velocity contrasts larger than ±1% at large scale, except for high velocity slabs within the transition zone. Comparisons with other seismic models show that current surface wave datasets allow to build consistent models up to degrees 40 in the upper 200 km of the mantle. The agreement is poorer in the transition zone and confined to low harmonic degrees (≤10).

Velocity Imprecision in Finite-Beamwidth Shipboard Doppler Sonar: A First-Generation Correction Algorithm

Abstract The finite angular width of Doppler sonar beams introduces errors into the measurement of ocean velocity from moving ships. These errors are exacerbated as ship speed increases and as the acoustic scatter field becomes more inhomogeneous in space. Zooplanktonic scatters often reside in distinct quasi-isopycnal scattering layers. When measured from a moving ship, such layers appear to have a distinct velocity signature, even if the ocean is quiescent. Here, this effect is explored in a simple analytic model and a least squares algorithm is presented for remediating its signature. The algorithm corrects the measured velocity at any given depth, with the correction being a weighted sum of the depth gradient of log acoustic intensity at surrounding depths. The correction weights are a property of the specific sonar beam pattern. Once determined, they can be considered constant. The algorithm, when applied to a 50-kHz sonar on the Research Vessel (R/V) Roger Revelle, is found to remove roughly 90% of the error in vertical wavenumber spectra of shear. More sophisticated algorithms can be developed as experience with the present approach is gained.

Challenges in marine mammal habitat modelling: evidence of multiple foraging habitats from the identification of feeding events in blue whales

Effective conservation of animal species depends on accurate identification of their critical habitat. Marine mammals, however, often transit through heterogeneous habitats and perform various activities within short periods of time. The predictive power of habitat modelling techniques can thus suffer from variability in behaviour and the use of multiple habitat types. We used data loggers and ecological-niche factor analysis (ENFA) modelling techniques to determine blue whale Balaenoptera musculus associations with underwater topography on a feeding ground in the St. Lawrence River estuary, Canada. We compared a naive model that had no knowledge of behaviour with a model that used the locations of feeding events inferred from specific velocity signatures. Blue whales travelled over several habitat types with different characteristics, which confounded modelling efforts when pooled together. The model based on the feeding set had considerably higher predictive power but could not highlight all suitable habitats at the same time. Using cluster analysis, we identified 4 habitat types used for feeding, each corresponding to distinct underwater topographies. Feeding depth and behaviour varied across these habitats, which were used preferentially at different times of the tidal cycle and appeared linked to known prey aggregation mechanisms. Our results suggest that failure to identify feeding activity or to take into account the existence of multiple foraging habitats at a fine scale could result in spurious modelling results.

Transient fuel spray impingement at atmospheric and elevated ambient conditions

This study analyses detailed temporally resolved full flow field data for pre- and post-impingement fuel sprays under atmospheric and elevated ambient conditions. A comprehensive suite of diagnostic techniques is utilised comprising of phase-resolved Phase Doppler Interferometry—employing a very fine grid and velocity signature to differentiate between pre- and post-impingement droplets—high-speed imaging (utilising Mie-scattering), high-magnification shadowgraphy and an adapted instantaneous mass-rate tube. Raised ambient conditions are achieved in a high-temperature, high-pressure constant volume rig affording large optical access. Temporal data for the free and impinging sprays is compared and analysed within three phases—early, mid and late injection—defined by the mass-rate tube data. All experimental techniques employed present consistent temporal and spatial trends at atmospheric and elevated ambient conditions, and global trends are consistent with the phenomenological flow structure originally proposed by Ozdemir and Whitelaw. Detailed analysis close to the piston surface reveals approximately a threefold increase in the D50 mean spray diameter for the post-impingement droplet size distribution at the wall spray tip. It is suggested that this is due to droplet coalescence, with some supporting evidence from high-speed imaging. Comparison of transient mass deposited on the surface with model predictions show reasonable agreement with the ‘Wet’ model assumption.

The Tornadic Vortex Signature: An Update

Abstract A tornadic vortex signature (TVS) is a degraded Doppler velocity signature of a tornado that occurs when the core region of a tornado is smaller than the half-power beamwidth of the sampling Doppler radar. Soon after the TVS was discovered in the mid-1970s, simulations were conducted to verify that the signature did indeed represent a tornado. The simulations, which used a uniform reflectivity distribution across a Rankine vortex model, indicated that the extreme positive and negative Doppler velocity values of the signature should be separated by about one half-power beamwidth regardless of tornado size or strength. For a Weather Surveillance Radar-1988 Doppler (WSR-88D) with an effective half-power beamwidth of approximately 1.4° and data collected at 1.0° azimuthal intervals, the two extreme Doppler velocity values should be separated by 1.0°. However, with the recent advent of 0.5° azimuthal sampling (“superresolution”) by WSR-88Ds at lower elevation angles, some of the extreme Doppler velocity values unexpectedly were found to be separated by 0.5° instead of 1.0° azimuthal intervals. To understand this dilemma, the choice of vortex model and reflectivity profile is investigated. It is found that the choice of vortex model does not have a significant effect on the simulation results. However, using a reflectivity profile with a minimum at the vortex center does make a difference. The revised simulations indicate that it is possible for the distance between the peak Doppler velocity values of a TVS to be separated by 0.5° with superresolution data collection.

The radial velocity signature of tides raised in stars hosting exoplanets

Close-in, massive exoplanets raise significant tides in their stellar hosts. We compute the radial velocity (RV) signal due to this fluid motion in the equilibrium tide approximation. The predicted radial velocities in the observed sample of exoplanets exceed 1 m/s for 17 systems, with the largest predicted signal being ~30 m/s for WASP-18 b. Tidally-induced RV's are thus detectable with present methods. Both tidal fluid flow and the epicyclic motion of a slightly eccentric orbit produce an RV signal at twice the orbital frequency. If care is not taken, the tidally induced RV may, in some cases, be confused with a finite orbital eccentricity. Indeed, WASP-18 b is reported to have an eccentric orbit with small e=0.009 and pericenter longitude \omega=-\pi/2. Whereas such a close alignment of the orbit and line of sight to the observer requires fine tuning, this phase in the RV signal is naturally explained by the tidal velocity signature of an e=0 orbit. Additionally, the equilibrium tide estimate for the amplitude is in rough agreement with the data. Thus the reported eccentricity for WASP-18 b is instead likely a signature of the tidally-induced RV in the stellar host. Measurement of both the orbital and tidal velocity for non-transiting planets may allow planet mass and inclination to be separately determined solely from radial velocity data. We suggest that high precision fitting of RV data should include the tidal velocity signal in those cases where it may affect the determination of orbital parameters.

VERY LARGE ARRAY OBSERVATIONS OF AMMONIA IN INFRARED-DARK CLOUDS. II. INTERNAL KINEMATICS

Infrared-dark clouds (IRDCs) are believed to be the birthplaces of rich clusters and thus contain the earliest phases of high-mass star formation. We use the Green Bank Telescope (GBT) and Very Large Array (VLA) maps of ammonia (NH3) in six IRDCs to measure their column density and temperature structure (Paper 1), and here, we investigate the kinematic structure and energy content. We find that IRDCs overall display organized velocity fields, with only localized disruptions due to embedded star formation. The local effects seen in NH3 emission are not high velocity outflows but rather moderate (few km/s) increases in the line width that exhibit maxima near or coincident with the mid-infrared emission tracing protostars. These line width enhancements could be the result of infall or (hidden in NH3 emission) outflow. Not only is the kinetic energy content insufficient to support the IRDCs against collapse, but also the spatial energy distribution is inconsistent with a scenario of turbulent cloud support. We conclude that the velocity signatures of the IRDCs in our sample are due to active collapse and fragmentation, in some cases augmented by local feedback from stars.

KINEMATICS AND CHEMISTRY OF STARS ALONG THE SAGITTARIUS TRAILING TIDAL TAIL AND CONSTRAINTS ON THE MILKY WAY MASS DISTRIBUTION

We present three-dimensional kinematics of Sagittarius (Sgr) trailing tidal debris in six fields located 70-130 degrees along the stream from the Sgr dwarf galaxy core. The data are from our proper-motion (PM) survey of Kapteyn's Selected Areas, in which we have measured accurate PMs to faint magnitudes in 40x40 arcmin fields evenly spaced across the sky. The radial velocity (RV) signature of Sgr has been identified among our follow-up spectroscopic data in four of the six fields and combined with mean PMs of spectroscopically-confirmed members to derive space motions of Sgr debris based on 15-64 confirmed stream members per field. These kinematics are compared to predictions of the Law & Majewski (2010) model of Sgr disruption; we find reasonable agreement with model predictions in RVs and PMs along Galactic latitude. However, an upward adjustment of the Local Standard of Rest velocity Theta_LSR from its standard 220 km/s to at least $232\pm14$ km/s (and possibly as high as $264\pm23$ km/s) is necessary to bring 3-D model debris kinematics and our measurements into agreement. Satisfactory model fits that simultaneously reproduce known position, distance, and radial velocity trends of the Sgr tidal streams, while significantly increasing Theta_LSR}, could only be achieved by increasing the Galactic bulge and disk mass while leaving the dark matter halo fixed to the best-fit values from Law & Majewski (2010). We derive low-resolution spectroscopic abundances along this stretch of the Sgr stream and find a constant [Fe/H] = -1.15 (with ~0.5 dex scatter in each field -- typical for dwarf galaxy populations) among the four fields with reliable measurements. A constant metallicity suggests that debris along the ~60-degree span of this study was all stripped from Sgr on the same orbital passage.

Simulated Tornadic Vortex Signatures of Tornado-Like Vortices Having One- and Two-Celled Structures

AbstractA tornadic vortex signature (TVS) is a degraded Doppler velocity signature that occurs when the tangential velocity core region of a tornado is smaller than the effective beamwidth of a sampling Doppler radar. Early Doppler radar simulations, which used a uniform reflectivity distribution across an idealized Rankine vortex, showed that the extreme Doppler velocity peaks of a TVS profile are separated by approximately one beamwidth. The simulations also indicated that neither the size nor the strength of the tornado is recoverable from a TVS. The current study was undertaken to investigate how the TVS might change if vortices having more realistic tangential velocity profiles were considered. The one-celled (axial updraft only) Burgers–Rott vortex model and the two-celled (annular updraft with axial downdraft) Sullivan vortex model were selected. Results of the simulations show that the TVS peaks still are separated by approximately one beamwidth—signifying that the TVS not only is unaffected by the size or strength of a tornado but also is unaffected by whether the tornado structure consists of one or two cells.

A predictive inner–outer model for streamwise turbulence statistics in wall-bounded flows

A model is proposed with which the statistics of the fluctuating streamwise velocity in the inner region of wall-bounded turbulent flows are predicted from a measured large-scale velocity signature from an outer position in the logarithmic region of the flow. Results, including spectra and all moments up to sixth order, are shown and compared to experimental data for zero-pressure-gradient flows over a large range of Reynolds numbers. The model uses universal time-series and constants that were empirically determined from zero-pressure-gradient boundary layer data. In order to test the applicability of these for other flows, the model is also applied to channel, pipe and adverse-pressure-gradient flows. The results support the concept of a universal inner region that is modified through a modulation and superposition of the large-scale outer motions, which are specific to the geometry or imposed streamwise pressure gradient acting on the flow.

Surface Velocity Profiles in a Vessel's Turbulent Wake Observed by a Dual-Beam Along-Track Interferometric SAR

The dual-beam interferometer is an airborne instrument that combines two vertically polarized C-band along-track interferometric synthetic aperture radars (AT-InSARs) observing the surface below at different squints. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s. An imagery of a small vessel with a rather pronounced wake pattern captured during one of such flights is the subject of this letter. Specifically, the interferometric phase in the turbulent wake exhibits a conspicuous banding structure that is still visible at distances more than 1 km behind the craft. The phase signatures from the fore and aft looks are combined to retrieve both longitudinal and lateral velocity components along cuts traversing the wake 400 and 750 m behind the boat. The results identify appreciable variations in the longitudinal velocity across the turbulent wake which are apparently consistent with the combined effect of the hull drag and the propeller backwash. A persistent pattern for the lateral component is also observed but is harder to interpret without the detailed knowledge of the vessel. The examples demonstrate the utility of AT-InSAR and, particularly, of a dual-beam AT-InSAR, for studies of centerline ship wakes. Readily available velocity signatures of a turbulent wake obtained with such systems can help with vessel classification tasks.

Detecting the signatures of Uranus and Neptune

With more than 15 years since the first radial velocity discovery of a planet orbiting a Sun-like star, the time baseline for radial velocity surveys is now extending out beyond the orbit of Jupiter analogs. The sensitivity to exoplanet orbital periods beyond that of Saturn orbital radii however is still beyond our reach such that very few clues regarding the prevalence of ice giants orbiting solar analogs are available to us. Here we simulate the radial velocity, transit, and photometric phase amplitude signatures of the Solar System giant planets, in particular Uranus and Neptune, and assess their detectability. We scale these results for application to monitoring low-mass stars and compare the relative detection prospects with other potential methods, such as astrometry and imaging. These results quantitatively show how many of the existing techniques are suitable for the detection of ice giants beyond the snow line for late-type stars and the challenges that lie ahead for the detection true Uranus/Neptune analogs around solar-type stars.

On the velocity and dissipation signature of vortex tubes in isotropic turbulence

The properties of vortex tubes are extracted and analyzed from a DNS database at various R e λ , with the objective to characterize the associated distributions of induced velocity and kinetic energy dissipation. The induced velocity exhibits an inverse power-law scaling in the far field, different from Burgers’ r − 1 scaling, supporting the interpretation that tubes are the remnants of vortex sheets after roll-up, and suggesting a possible link with the Kolmogorov k − 5 / 3 spectral scaling. The energy dissipation signature is characterized by a local maximum near the edge of the vortex core, and an absolute peak at its center, which can be tentatively explained appealing to the occurrence of a bi-axial configuration of the strain-rate tensor.

Planetary detection limits taking into account stellar noise

The radial velocity signature of stellar noise is small, around the meter-per-second, but already too much for the detection of Earth mass planets in habitable zones. In this paper, we address the important role played by observational strategies in averaging out the radial velocity signature of stellar noise. We also derive the planetary mass detection limits expected in presence of stellar noise. We start with HARPS asteroseismology measurements for 4 stars (beta Hyi, alpha Cen A, mu Ara and tau Ceti) available in the ESO archive plus very precise measurements of alpha Cen B. This sample covers different spectral types, from G2 to K1 and different evolutionary stage, from subgiant to dwarf stars. Since the span of our data ranges between 5 to 8 days, we will have access to oscillation modes and granulation phenomena, without important contribution of activity noise which is present at larger time scales. For those 5 stars, we generate synthetic radial velocity measurements after fitting corresponding models of stellar noise in Fourier space. These measurements allows us to study the radial velocity variation due to stellar noise for different observational strategies as well as the corresponding planetary mass detection limits. Applying 3 measurements per night of 10 minutes exposure each, 2 hours apart, seems to average out most efficiently the stellar noise considered. For quiet K1V stars as alpha Cen B, such a strategy allows us to detect planets of ~3 times the mass of Earth with an orbital period of 200 days, corresponding to the habitable zone of the star. Since activity is not yet included in our simulation, these detection limits correspond to a case, which exist, where the host star has few magnetic features. In this case stellar noise is dominated by oscillation modes and granulation phenomena.

Thalamic synchrony and the adaptive gating of information flow to cortex

Although it has long been posited that sensory adaptation serves to enhance information flow in sensory pathways, the neural basis remains elusive. Simultaneous single–unit recordings in the thalamus and cortex in anesthetized rats reveal that adaptation differentially influences thalamus and cortex in a manner that fundamentally changes the nature of information conveyed about vibrissae motion. Utilizing an ideal observer of cortical activity, performance in detecting vibrissa deflections degrades with adaptation, while performance in discriminating between vibrissa deflections of different velocities is enhanced, a trend not observed in thalamus. Analysis of simultaneously recorded thalamic neurons does reveal, however, an analogous adaptive change in thalamic synchrony that mirrors the cortical response. An integrate–and–fire model using experimentally measured thalamic input reproduces the observed transformations. The results here suggest a shift in coding strategy with adaptation that directly controls information relayed to cortex, which could have implications for encoding velocity signatures of textures.

Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure

We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to 20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5–2 km thick, and deviatoric stress was increased until failure of the specimens. Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack densities: ρ i the crack density of the pre-existing thermal cracks and ρ v the crack density of the stress-induced cracks. In addition a link was established between elastic properties (elastic wave velocities V p and V s ) and permeability using a statistical permeability model. Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density ρ i from ~ 1 to 0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases of more than two orders of magnitude. Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack density, from the location of the acoustic emission sources, and from microstructural observations. This competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on permeability change than pre-existing thermal cracks.