1,451 research outputs found
Representing anisotropic subduction zones with isotropic velocity models: A characterization of the problem and some steps on a possible path forward
Despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy, most P-wave tomography efforts still rely on the assumption of isotropy. In this study, we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach. We conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback. The simulation results provide a self-consistent distribution of isotropic (thermal) anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions. We find that anisotropy-induced artifacts are abundant and significant for teleseismic, local and mixed event distributions. The occurrence of artifacts is not reduced, and indeed can be exacerbated, by increasing richness in ray-path azimuths and incidence angles. The artifacts that we observe are, in all cases, important enough to significantly impact the interpretation of the images. We test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results. Using approximate anisotropy, fields can result in reduced RMS misfit to the travel time delays and reduced abundance and severity of imaging artifacts. We propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model
Stability of the nonlinear dynamics of an optically injected VCSEL
Automated protocols have been developed to characterize time series data in terms of stability. These techniques are applied to the output power time series of an optically injected vertical cavity surface emitting laser (VCSEL) subject to varying injection strength and optical frequency detuning between master and slave lasers. Dynamic maps, generated from high resolution, computer controlled experiments, identify regions of dynamic instability in the parameter space. © 2012 Optical Society of America
Advances in seismic imaging of magma and crystal mush
Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (<10 km3) and medium-sized (<100 km3) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs
Thyroid hormone regulates distinct paths to maturation in pigment cell lineages
Thyroid hormone (TH) regulates diverse developmental events and can drive disparate cellular outcomes. In zebrafish, TH has opposite effects on neural crest derived pigment cells of the adult stripe pattern, limiting melanophore population expansion, yet increasing yellow/orange xanthophore numbers. To learn how TH elicits seemingly opposite responses in cells having a common embryological origin, we analyzed individual transcriptomes from thousands of neural crest-derived cells, reconstructed developmental trajectories, identified pigment cell-lineage specific responses to TH, and assessed roles for TH receptors. We show that TH promotes maturation of both cell types but in distinct ways. In melanophores, TH drives terminal differentiation, limiting final cell numbers. In xanthophores, TH promotes accumulation of orange carotenoids, making the cells visible. TH receptors act primarily to repress these programs when TH is limiting. Our findings show how a single endocrine factor integrates very different cellular activities during the generation of adult form
Observational constraints on early dark energy
We review and update constraints on the Early Dark Energy (EDE) model from
cosmological data sets, in particular Planck PR3 and PR4 cosmic microwave
background (CMB) data and large-scale structure (LSS) data sets including
galaxy clustering and weak lensing data from the Dark Energy Survey, Subaru
Hyper Suprime-Cam, and KiDS+VIKING-450, as well as BOSS/eBOSS galaxy clustering
and Lyman- forest data. We detail the fit to CMB data, and perform the
first analyses of EDE using the CAMSPEC and Hillipop likelihoods for Planck CMB
data, rather than Plik, both of which yield a tighter upper bound on the
allowed EDE fraction than that found with Plik. We then supplement CMB data
with large-scale structure data in a series of new analyses. All these analyses
are concordant in their Bayesian preference for CDM over EDE, as
indicated by marginalized posterior distributions. We perform a series of tests
of the impact of priors in these results, and compare with frequentist analyses
based on the profile likelihood, finding qualitative agreement with the
Bayesian results. All these tests suggest prior volume effects are not a
determining factor in analyses of EDE. This work provides both a review of
existing constraints and several new analyses.Comment: 59 pages, 23 figures, 11 tables, Invited review for International
Journal of Modern Physics
Timing analysis for 20 millisecond pulsars in the Parkes Pulsar Timing Array
We present timing models for 20 millisecond pulsars in the Parkes Pulsar
Timing Array. The precision of the parameter measurements in these models has
been improved over earlier results by using longer data sets and modelling the
non-stationary noise. We describe a new noise modelling procedure and
demonstrate its effectiveness using simulated data. Our methodology includes
the addition of annual dispersion measure (DM) variations to the timing models
of some pulsars. We present the first significant parallax measurements for
PSRs J1024-0719, J1045-4509, J1600-3053, J1603-7202, and J1730-2304, as well as
the first significant measurements of some post-Keplerian orbital parameters in
six binary pulsars, caused by kinematic effects. Improved Shapiro delay
measurements have resulted in much improved pulsar mass measurements,
particularly for PSRs J0437-4715 and J1909-3744 with
and respectively. The improved orbital
period-derivative measurement for PSR J0437-4715 results in a derived distance
measurement at the 0.16% level of precision, pc, one of the
most fractionally precise distance measurements of any star to date.Comment: 21 pages, 5 figures, 7 tables. Accepted for publication in MNRA
Underdiagnosis of mild cognitive impairment: A consequence of ignoring practice effects
INTRODUCTION: Longitudinal testing is necessary to accurately measure cognitive change. However, repeated testing is susceptible to practice effects, which may obscure true cognitive decline and delay detection of mild cognitive impairment (MCI).
METHODS: We retested 995 late-middle-aged men in a ∼6-year follow-up of the Vietnam Era Twin Study of Aging. In addition, 170 age-matched replacements were tested for the first time at study wave 2. Group differences were used to calculate practice effects after controlling for attrition effects. MCI diagnoses were generated from practice-adjusted scores.
RESULTS: There were significant practice effects on most cognitive domains. Conversion to MCI doubled after correcting for practice effects, from 4.5% to 9%. Importantly, practice effects were present although there were declines in uncorrected scores.
DISCUSSION: Accounting for practice effects is critical to early detection of MCI. Declines, when lower than expected, can still indicate practice effects. Replacement participants are needed for accurately assessing disease progression.Published versio
Constraining Early Dark Energy with Large-Scale Structure
An axion-like field comprising of the energy density of the
universe near matter-radiation equality is a candidate to resolve the Hubble
tension; this is the "early dark energy" (EDE) model. However, as shown in Hill
et al. (2020), the model fails to simultaneously resolve the Hubble tension and
maintain a good fit to both cosmic microwave background (CMB) and large-scale
structure (LSS) data. Here, we use redshift-space galaxy clustering data to
sharpen constraints on the EDE model. We perform the first EDE analysis using
the full-shape power spectrum likelihood from the Baryon Oscillation
Spectroscopic Survey (BOSS), based on the effective field theory (EFT) of LSS.
The inclusion of this likelihood in the EDE analysis yields a tighter
error bar on compared to primary CMB data alone, yielding km/s/Mpc ( CL). In addition, we constrain the
maximum fractional energy density contribution of the EDE to ( CL). We explicitly demonstrate that the EFT BOSS likelihood
yields much stronger constraints on EDE than the standard BOSS likelihood.
Including further information from photometric LSS surveys,the constraints
narrow by an additional , yielding km/s/Mpc
( CL) and ( CL). These bounds are obtained
without including local-universe data, which is in strong tension with
the CMB and LSS, even in the EDE model. We also refute claims that MCMC
analyses of EDE that omit SH0ES from the combined dataset yield misleading
posteriors. Finally, we demonstrate that upcoming Euclid/DESI-like
spectroscopic galaxy surveys can greatly improve the EDE constraints. We
conclude that current data preclude the EDE model as a resolution of the Hubble
tension, and that future LSS surveys can close the remaining parameter space of
this model.Comment: 26 pages, 12 figures. v2: Version to appear in Phys Rev D. Appendices
B and C demonstrate that EDE provides a worse fit than LCDM to Planck+BOSS
data, and that constraints are not driven by prior volume effect
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