2,907 research outputs found
The dynamical structure of the MEO region: long-term stability, chaos, and transport
It has long been suspected that the Global Navigation Satellite Systems exist
in a background of complex resonances and chaotic motion; yet, the precise
dynamical character of these phenomena remains elusive. Recent studies have
shown that the occurrence and nature of the resonances driving these dynamics
depend chiefly on the frequencies of nodal and apsidal precession and the rate
of regression of the Moon's nodes. Woven throughout the inclination and
eccentricity phase space is an exceedingly complicated web-like structure of
lunisolar secular resonances, which become particularly dense near the
inclinations of the navigation satellite orbits. A clear picture of the
physical significance of these resonances is of considerable practical interest
for the design of disposal strategies for the four constellations. Here we
present analytical and semi-analytical models that accurately reflect the true
nature of the resonant interactions, and trace the topological organization of
the manifolds on which the chaotic motions take place. We present an atlas of
FLI stability maps, showing the extent of the chaotic regions of the phase
space, computed through a hierarchy of more realistic, and more complicated,
models, and compare the chaotic zones in these charts with the analytical
estimation of the width of the chaotic layers from the heuristic Chirikov
resonance-overlap criterion. As the semi-major axis of the satellite is
receding, we observe a transition from stable Nekhoroshev-like structures at
three Earth radii, where regular orbits dominate, to a Chirikov regime where
resonances overlap at five Earth radii. From a numerical estimation of the
Lyapunov times, we find that many of the inclined, nearly circular orbits of
the navigation satellites are strongly chaotic and that their dynamics are
unpredictable on decadal timescales.Comment: Submitted to Celestial Mechanics and Dynamical Astronomy. Comments
are greatly appreciated. 28 pages, 15 figure
Attention is allocated closely ahead of the target during smooth pursuit eye movements: Evidence from EEG frequency tagging
It is under debate whether attention during smooth pursuit is centered right on the pursuit target or allocated preferentially ahead of it. Attentional deployment was previously probed using a secondary task, which might have altered attention allocation and led to inconsistent findings. We measured frequency-tagged steady-state visual evoked potentials (SSVEP) to measure attention allocation in the absence of any secondary probing task. The observers pursued a moving dot while stimuli flickering at different frequencies were presented at various locations ahead or behind the pursuit target. We observed a significant increase in EEG power at the flicker frequency of the stimulus in front of the pursuit target, compared to the frequency of the stimulus behind. When testing many different locations, we found that the enhancement was detectable up to about 1.5° ahead during pursuit, but vanished by 3.5°. In a control condition using attentional cueing during fixation, we did observe an enhanced EEG response to stimuli at this eccentricity, indicating that the focus of attention during pursuit is narrower than allowed for by the resolution of the attentional system. In a third experiment, we ruled out the possibility that the SSVEP enhancement was a byproduct of the catch-up saccades occurring during pursuit. Overall, we showed that attention is on average allocated ahead of the pursuit target during smooth pursuit. EEG frequency tagging seems to be a powerful technique that allows for the investigation of attention/perception implicitly when an overt task would be confounding
Advantage in Reading Lexical Bundles is Reduced in Non-Native Speakers
Formulaic sequences such as idioms, collocations, and lexical bundles, which may be processed as holistic units, make up a large proportion of natural language. For language learners, however, formulaic patterns are a major barrier to achieving native like competence. The present study investigated the processing of lexical bundles by native speakers and less advanced non-native English speakers using corpus analysis for the identification of lexical bundles and eye-tracking to measure the reading times. The participants read sentences containing 4-grams and control phrases which were matched for sub-string frequency. The results for native speakers demonstrate a processing advantage for formulaic sequences over the matched control units. We do not find any processing advantage for non-native speakers which suggests that native like processing of lexical bundles comes only late in the acquisition process
Migration of Jupiter-family comets and resonant asteroids to near-Earth space
We estimated the rate of comet and asteroid collisions with the terrestrial
planets by calculating the orbits of 13000 Jupiter-crossing objects (JCOs) and
1300 resonant asteroids and computing the probabilities of collisions based on
random-phase approximations and the orbital elements sampled with a 500 yr
step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results
for orbital evolution, but sometimes give different collision probabilities
with the Sun. A small fraction of former JCOs reached orbits with aphelia
inside Jupiter's orbit, and some reached Apollo orbits with semi-major axes
less than 2 AU, Aten orbits, and inner-Earth orbits (with aphelia less than
0.983 AU) and remained there for millions of years. Though less than 0.1% of
the total, these objects were responsible for most of the collision probability
of former JCOs with Earth and Venus. Some Jupiter-family comets can reach
inclinations i>90 deg. We conclude that a significant fraction of near-Earth
objects could be extinct comets that came from the trans-Neptunian region.Comment: Proc. of the international conference "New trends in astrodynamics
and applications" (20-22 January 2003, University of Maryland, College Park
Volumetric Absorptive Microsampling (VAMS) for Targeted LC-MS/MS Determination of Tryptophan-Related Biomarkers
L-Tryptophan (TRP) metabolites and related biomarkers play crucial roles in physiological functions, and their imbalances are implicated in central nervous system pathologies and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease, Parkinson's disease, schizophrenia and depression. The measurement of TRP metabolites and related biomarkers possesses great potential to elucidate the disease mechanisms, aid preclinical drug development, highlight potential therapeutic targets and evaluate the outcomes of therapeutic interventions. An effective, straightforward, sensitive and selective liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the simultaneous determination of 24 TRP-related compounds in miniaturised murine whole blood samples. Sampling and sample pretreatment miniaturisation were achieved thanks to the development of a volumetric dried blood microsampling approach. Volumetric absorptive microsampling (VAMS) allows the accurate sampling of microvolumes of blood with advantages including, but not limited to, minimal sampling invasiveness, logistical improvements, method sustainability in terms of solvents and energy consumption, and improvement of animal studies in the framework of the 3Rs (Replacement, Reduction and Refinement) principles on animal welfare. The VAMS-LC-MS/MS method exhibited good selectivity, and correlation coefficient values for the calibration curves of each analyte were >0.9987. The limits of quantitation ranged from 0.1 to 25 ng/mL. The intra- and inter-day precisions in terms of RSD were <9.6%. All analytes were stable in whole blood VAMS samples stored at room temperature for at least 30 days with analyte losses < 14%. The developed method was successfully applied to the analysis of biological samples from mice, leading to the unambiguous determination of all the considered target analytes. This method can therefore be applied to analyse TRP metabolites and related biomarkers levels to monitor disease states, perform mechanistic studies and investigate the outcomes of therapeutic interventions
Tidally-Induced Apsidal Precession in Double White Dwarfs: a new mass measurement tool with LISA
Galactic interacting double white dwarfs (DWD) are guaranteed gravitational
wave (GW) sources for the GW detector LISA, with more than 10^4 binaries
expected to be detected over the mission's lifetime. Part of this population is
expected to be eccentric, and here we investigate the potential for
constraining the white dwarf (WD) properties through apsidal precession in
these binaries. We analyze the tidal, rotational, and general relativistic
contributions to apsidal precession by using detailed He WD models, where the
evolution of the star's interior is followed throughout the cooling phase. In
agreement with previous studies of zero-temperature WDs, we find that apsidal
precession in eccentric DWDs can lead to a detectable shift in the emitted GW
signal when binaries with cool (old) components are considered. This shift
increases significantly for hot (young) WDs. We find that apsidal motion in hot
(cool) DWDs is dominated by tides at orbital frequencies above ~10^{-4}Hz
(10^{- 3}$Hz). The analysis of apsidal precession in these sources while
ignoring the tidal component would lead to an extreme bias in the mass
determination, and could lead us to misidentify WDs as neutron stars or black
holes. We use the detailed WD models to show that for older, cold WDs, there is
a unique relationship that ties the radius and apsidal precession constant to
the WD masses, therefore allowing tides to be used as a tool to constrain the
source masses.Comment: 23 pages, 7 figures, revised to match accepted ApJ versio
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