85 research outputs found
Metastable states in classical and quantum systems
The classical vibrational metastable states in a one-dimensional two-mass system are investigated experimentally and theoretically via the transmission of a coherent wave packet propagating through the system. The Fourier transform of the vibrational signal recorded in between the masses reveals resonant excitations by the coherent wave packet. The time-resolved spectrum indicates that the lifetime of a metastable state of higher frequency is longer than that of lower frequency, which is in contrast with the quantum mechanical double-barrier system. This study, which is easily accessible to physics majors, also demonstrates quantum resonant tunneling in a very simple classical way
Mass Flow Control in a Magnesium Hall-effect Thruster
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83626/1/AIAA-2010-6861-269.pd
Vibrational properties of a loaded string
In this paper we discuss our study of a stringâmass chain and its anology to quantum mechanical systems. Theoretical predictions are made based upon the numerical solution to the wave equation. These predictions are tested experimentally using both normal mode analysis and pulse analysis. The frequency band structures for periodic and disordered string mass chains are studied as well as their corresponding eigenfunctions. The theoretical and experimental results are in accord. This experiment, suitable for advanced physics majors, demonstrates many important features of quantum mechanics: eigenvalues, superposition principle, band structure, gap modes, and Anderson localization
Phononic band structure in a mass chain
The vibrational properties of a finite oneâdimensional stringâmass chain are studied experimentally and theoretically. In the experiment both normal mode analysis and pulse analysis are used to obtain the eigenfrequencies of the stringâmass chain. The theoretical predictions are made based upon the numerical solution to the wave equation. The phononic band structure for a periodically massed string as well as Anderson localized gap modes for a disordered system are found. The theoretical and experimental results match satisfactorily well
Seasonal dependence of northern highâlatitude upper thermospheric winds: A quiet time climatological study based on groundâbased and spaceâbased measurements
This paper investigates the largeâscale seasonal dependence of geomagnetically quiet time, northern highâlatitude FÂ region thermospheric winds by combining extensive observations from eight groundâbased (optical remote sensing) and three spaceâbased (optical remote sensing and in situ) instruments. To provide a comprehensive picture of the wind morphology, data are assimilated into a seasonal empirical vector wind model as a function of season, latitude, and local time in magnetic coordinates. The model accurately represents the behavior of the constituent data sets. There is good general agreement among the various data sets, but there are some major offsets between GOCE and the other data sets, especially on the duskside. The assimilated wind patterns exhibit a strong and large duskside anticyclonic circulation cell, sharp latitudinal gradients in the duskside auroral zone, strong antisunward winds in the polar cap, and a weaker tendency toward a dawnside cyclonic circulation cell. The highâlatitude wind system shows a progressive intensification of wind patterns from winter to equinox to summer. The latitudinal extent of the duskside circulation cell does not depend strongly on season. Zonal winds show a mainly diurnal variation (two extrema) around polar and middle latitudes and semidiurnal variation (four extrema) at auroral latitudes; meridional winds are primarily diurnal at all high latitudes. The strength of zonal winds channeling through the auroral zone on the duskside is strongest in the summer season. The vorticity of the wind pattern increases from winter to summer, whereas divergence is maximum in equinox. In all three seasons, divergence is weaker than vorticity.Key PointsFirst ever investigation of the largeâscale seasonal dependence of northern highâlatitude upper thermospheric winds in magnetic coordinatesResults show progressive intensification of wind circulation from winter to equinox to summerThe vorticity increases from winter to summer. In all the seasons, the strongest divergences occur primarily in and above auroral latitudesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136373/1/jgra53329.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136373/2/jgra53329_am.pd
HLâTWiM Empirical Model of HighâLatitude Upper Thermospheric Winds
We present an empirical model of thermospheric winds (Highâlatitude Thermospheric Wind Model [HLâTWiM]) that specifies F region highâlatitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HLâTWiM represents the largeâscale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical highâlatitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 groundâbased stations (FabryâPerot Interferometers and Scanning Doppler Imaging FabryâPerot Interferometers) located at various northern and southern high latitudes and two spaceâbased instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HLâTWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HLâTWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HLâTWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Modelâdata intraâannual comparisons in this study show semiannual oscillationâlike behavior of GOCE winds, rarely observed before in wind data.Key PointsWe developed a comprehensive empirical model of highâlatitude F region thermospheric winds (HLâTWiM)Universal Time variations in highâlatitude winds are stronger in the Southern than Northern HemisphereHLâTWiM provides a necessary benchmark for validating new highâlatitude wind observations and tuning first principal modelsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/1/jgra55363_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/2/jgra55363-sup-0001-Figure_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/3/jgra55363.pd
Precision pulse shape simulation for proton detection at the Nab experiment
The Nab experiment at Oak Ridge National Laboratory, USA, aims to measure the
beta-antineutrino angular correlation following neutron decay to an
anticipated precision of approximately 0.1\%. The proton momentum is
reconstructed through proton time-of-flight measurements, and potential
systematic biases in the timing reconstruction due to detector effects must be
controlled at the nanosecond level. We present a thorough and detailed
semiconductor and quasiparticle transport simulation effort to provide precise
pulse shapes, and report on relevant systematic effects and potential
measurement schemes
Comparative Genomics of the Conjugation Region of F-like Plasmids: Five Shades of F
The F plasmid is the foremost representative of a large group of conjugative plasmids, prevalent in Escherichia coli, and widely distributed among the Enterobacteriaceae. These plasmids are of clinical relevance, given their frequent association with virulence determinants, colicins, and antibiotic resistance genes. Originally defined by their sensitivity to certain male-specific phages, IncF plasmids share a conserved conjugative system and regulatory circuits. In order to determine whether the genetic architecture and regulation circuits are preserved among these plasmids, we analyzed the natural diversity of F-like plasmids. Using the relaxase as a phylogenetic marker, we identified 256 plasmids belonging to the IncF/ MOBF12group, present as complete DNA sequences in the NCBI database. By comparative genomics, we identified five major groups of F-like plasmids. Each shows a particular operon structure and alternate regulatory systems. Results show that the IncF/MOBF12 conjugation gene cluster conforms a diverse and ancient group, which evolved alternative regulatory schemes in its adaptation to different environments and bacterial hosts
Horizontally acquired glycosyltransferase operons drive salmonellae lipopolysaccharide diversity.
The immunodominant lipopolysaccharide is a key antigenic factor for Gram-negative pathogens such as salmonellae where it plays key roles in host adaptation, virulence, immune evasion, and persistence. Variation in the lipopolysaccharide is also the major differentiating factor that is used to classify Salmonella into over 2600 serovars as part of the Kaufmann-White scheme. While lipopolysaccharide diversity is generally associated with sequence variation in the lipopolysaccharide biosynthesis operon, extraneous genetic factors such as those encoded by the glucosyltransferase (gtr) operons provide further structural heterogeneity by adding additional sugars onto the O-antigen component of the lipopolysaccharide. Here we identify and examine the O-antigen modifying glucosyltransferase genes from the genomes of Salmonella enterica and Salmonella bongori serovars. We show that Salmonella generally carries between 1 and 4 gtr operons that we have classified into 10 families on the basis of gtrC sequence with apparent O-antigen modification detected for five of these families. The gtr operons localize to bacteriophage-associated genomic regions and exhibit a dynamic evolutionary history driven by recombination and gene shuffling events leading to new gene combinations. Furthermore, evidence of Dam- and OxyR-dependent phase variation of gtr gene expression was identified within eight gtr families. Thus, as O-antigen modification generates significant intra- and inter-strain phenotypic diversity, gtr-mediated modification is fundamental in assessing Salmonella strain variability. This will inform appropriate vaccine and diagnostic approaches, in addition to contributing to our understanding of host-pathogen interactions
Relationship between alcohol-attributable disease and socioeconomic status, and the role of alcohol consumption in this relationship: a systematic review and meta-analysis
Background: Studies show that alcohol consumption appears to have a disproportionate impact on people of low socioeconomic status. Further exploration of the relationship between alcohol consumption, socioeconomic status and the development of chronic alcohol-attributable diseases is therefore important to inform the development of
effective public health programmes.
Methods: We used systematic review methodology to identify published studies of the association between
socioeconomic factors and mortality and morbidity for alcohol-attributable conditions. To attempt to quantify
differences in the impact of alcohol consumption for each condition, stratified by SES, we (i) investigated the relationship between SES and risk of mortality or morbidity for each alcohol-attributable condition, and (ii) where, feasible explored alcohol consumption as a mediating or interacting variable in this relationship.
Results: We identified differing relationships between a range of alcohol-attributable conditions and socioeconomic
indicators. Pooled analyses showed that low, relative to high socioeconomic status, was associated with an increased
risk of head and neck cancer and stroke, and in individual studies, with hypertension and liver disease. Conversely, risk of female breast cancer tended to be associated with higher socioeconomic status. These findings were attenuated
but held when adjusted for a number of known risk factors and other potential confounding factors. A key finding was
the lack of studies that have explored the interaction between alcohol-attributable disease, socioeconomic status and alcohol use.
Conclusions: Despite some limitations to our review, we have described relationships between socioeconomic
status and a range of alcohol-attributable conditions, and explored the mediating and interacting effects of alcohol
consumption where feasible. However, further research is needed to better characterise the relationship between
socioeconomic status alcohol consumption and alcohol-attributable disease risk so as to gain a greater understanding of the mechanisms and pathways that influence the differential risk in harm between people of low and high
socioeconomic status
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