1,929 research outputs found
Filamentation Instability of Interacting Current Sheets in Striped Relativistic Winds: The Origin of Low Sigma?
I outline a mechanism, akin to Weibel instabilities of interpenetrating
beams, in which the neighboring current sheets in a striped wind from an
oblique rotator interact through a two stream-like mechanism (a Weibel
instability in flatland), to create an anomalous resistivity that heats the
sheets and causes the magnetic field to diffusively annihilate in the wind
upstream of the termination shock. The heating has consequences for observable
unpulsed emission from pulsars.Comment: 7 pages, 9 figures. To be published in the proceedings of ``40 Years
of Pulsars'
Improved alignment of nucleosome DNA sequences using a mixture model
DNA sequences that are present in nucleosomes have a preferential ∼10 bp periodicity of certain dinucleotide signals (1,2), but the overall sequence similarity of the nucleosomal DNA is weak, and traditional multiple sequence alignment tools fail to yield meaningful alignments. We develop a mixture model that characterizes the known dinucleotide periodicity probabilistically to improve the alignment of nucleosomal DNAs. We assume that a periodic dinucleotide signal of any type emits according to a probability distribution around a series of ‘hot spots’ that are equally spaced along nucleosomal DNA with 10 bp period, but with a 1 bp phase shift across the middle of the nucleosome. We model the three statistically most significant dinucleotide signals, AA/TT, GC and TA, simultaneously, while allowing phase shifts between the signals. The alignment is obtained by maximizing the likelihood of both Watson and Crick strands simultaneously. The resulting alignment of 177 chicken nucleosomal DNA sequences revealed that all 10 distinct dinucleotides are periodic, however, with only two distinct phases and varying intensity. By Fourier analysis, we show that our new alignment has enhanced periodicity and sequence identity compared with center alignment. The significance of the nucleosomal DNA sequence alignment is evaluated by comparing it with that obtained using the same model on non-nucleosomal sequences
Concurrent encoding of frequency and amplitude modulation in human auditory cortex: MEG evidence
September 26, 2007; doi:10.1152/jn.00342.2007. Complex natural sounds (e.g., animal vocalizations or speech) can be characterized by specific spectrotemporal patterns the components of which change in both frequency (FM) and amplitude (AM). The neural coding of AM and FM has been widely studied in humans and animals but typically with either pure AM or pure FM stimuli. The neural mechanisms employed to perceptually unify AM and FM acoustic features remain unclear. Using stimuli with simultaneous sinusoidal AM (at rate f AM � 37 Hz) and FM (with varying rates ƒ FM), magnetoencephalography (MEG) is used to investigate the elicited auditory steady-state response (aSSR) at relevant frequencies (ƒ AM, ƒ FM, ƒ AM � f FM). Previous work demonstrated that for sounds with slower FM dynamics (f FM � 5 Hz), the phase of the aSSR at ƒ AM tracked the FM; in other words, AM and FM features were co-tracked and co-represented by “phase modulation ” encoding. This stud
Controlling a Chaotic System
Using both experimental and theoretical results, this Letter describes how low-energy, feedback control signals can be successfully utilized to suppress (laminarize) chaotic flow in a thermal convection loop
Design earthquake ground motion prediction for Perth metropolitan area with microtremor measurements for site characterization
Perth is the largest city in Western Australia and home to three-quarters of the state\u27s residents. In recent decades, there have been a lot of earthquake activities just east of Perth in an area known as the South-West Seismic Zone. Previous numerical results of site response analyses based on limited available geology information for PMA indicated that Perth Basin might amplify the bedrock motion by more than 10 times at some frequencies and at some sites. Hence, more detailed studies on site characterization and amplification are necessary. The microtremor method using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments and has been adopted in many previous studies. In this study, the response spectrum of rock site corresponding to the 475-year return period for PMA is defined according to the probabilistic seismic hazard analysis (PSHA) based on the latest ground motion attenuation model of Southwest Western Australia. Site characterization in PMA is performed using two microtremor measurements, namely SPAC technique and H/V method. The clonal selection algorithm (CSA) is introduced to perform direct inversion of SPAC curves to determine the soil profiles of representative PMA sites investigated in this study. Using the simulated bedrock motion as input, the responses of the soil sites are estimated using numerical method based on the shear-wave velocity vs. depth profiles determined from the SPAC technique. The response spectrum of the earthquake ground motion on surface of each site is derived from the numerical results of the site response analysis, and compared with the respective design spectrum defined in the Australian Earthquake Loading Code. The comparison shows that the code spectra are conservative in the short period range, but may slightly underestimate the response spectrum at some long period range. <br /
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The influence of eddy parameterizations on the transport of the Antarctic Circumpolar Current in coupled climate models
The transport of the Antarctic Circumpolar Current (ACC) varies strongly across the coupled GCMs (general
circulation models) used for the IPCC AR4. This note shows that a large fraction of this across-model
variance can be explained by relating it to the parameterization of eddy-induced transports. In the majority
of models this parameterization is based on the study by Gent and McWilliams (1990). The main
parameter is the quasi-Stokes diffusivity kappa (often referred to less accurately as ’’thickness diffusion’’).
The ACC transport and the meridional density gradient both correlate strongly with kappa across those models
where kappa is a prescribed constant. In contrast, there is no correlation with the isopycnal diffusivity jiso
across the models. The sensitivity of the ACC transport to kappa is larger than to the zonal wind stress maximum.
Experiments with the fast GCM FAMOUS show that changing kappa directly affects the ACC transport
by changing the density structure throughout the water column. Our results suggest that this limits the
role of the wind stress magnitude in setting the ACC transport in FAMOUS. The sensitivities of the ACC
and the meridional density gradient are very similar across the AR4 GCMs (for those models where kappa
is a prescribed constant) and among the FAMOUS experiments. The strong sensitivity of the ACC transport
to kappa needs careful assessment in climate models
An X-ray View of Radio Millisecond Pulsars
In recent years, X-ray observations with Chandra and XMM-Newton have
significantly increased our understanding of rotation-powered (radio)
millisecond pulsars (MSPs). Deep Chandra studies of several globular clusters
have detected X-ray counterparts to a host of MSPs, including 19 in 47 Tuc
alone. These surveys have revealed that most MSPs exhibit thermal emission from
their heated magnetic polar caps. Realistic models of this thermal X-ray
emission have provided important insight into the basic physics of pulsars and
neutron stars. In addition, intrabinary shock X-ray radiation observed in
``black-widow'' and peculiar globular cluster ``exchanged'' binary MSPs give
interesting insight into MSP winds and relativistic shock. Thus, the X-ray band
contains valuable information regarding the basic properties of MSPs that are
not accesible by radio timing observations.Comment: 5 pages, 5 figures, To appear in the proceedings of "40 Years of
Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
Fully Complex Magnetoencephalography
Complex numbers appear naturally in biology whenever a system can be analyzed
in the frequency domain, such as physiological data from magnetoencephalography
(MEG). For example, the MEG steady state response to a modulated auditory
stimulus generates a complex magnetic field for each MEG channel, equal to the
Fourier transform at the stimulus modulation frequency. The complex nature of
these data sets, often not taken advantage of, is fully exploited here with new
methods. Whole-head, complex magnetic data can be used to estimate complex
neural current sources, and standard methods of source estimation naturally
generalize for complex sources. We show that a general complex neural vector
source is described by its location, magnitude, and direction, but also by a
phase and by an additional perpendicular component. We give natural
interpretations of all the parameters for the complex equivalent-current dipole
by linking them to the underlying neurophysiology. We demonstrate complex
magnetic fields, and their equivalent fully complex current sources, with both
simulations and experimental data.Comment: 23 pages, 1 table, 5 figures; to appear in Journal of Neuroscience
Method
Digital-analog quantum learning on Rydberg atom arrays
We propose hybrid digital-analog learning algorithms on Rydberg atom arrays,
combining the potentially practical utility and near-term realizability of
quantum learning with the rapidly scaling architectures of neutral atoms. Our
construction requires only single-qubit operations in the digital setting and
global driving according to the Rydberg Hamiltonian in the analog setting. We
perform a comprehensive numerical study of our algorithm on both classical and
quantum data, given respectively by handwritten digit classification and
unsupervised quantum phase boundary learning. We show in the two representative
problems that digital-analog learning is not only feasible in the near term,
but also requires shorter circuit depths and is more robust to realistic error
models as compared to digital learning schemes. Our results suggest that
digital-analog learning opens a promising path towards improved variational
quantum learning experiments in the near term.Comment: 22 pages, 20 figure
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