2,162 research outputs found
Alfven Wave Reflection and Turbulent Heating in the Solar Wind from 1 Solar Radius to 1 AU: an Analytical Treatment
We study the propagation, reflection, and turbulent dissipation of Alfven
waves in coronal holes and the solar wind. We start with the Heinemann-Olbert
equations, which describe non-compressive magnetohydrodynamic fluctuations in
an inhomogeneous medium with a background flow parallel to the background
magnetic field. Following the approach of Dmitruk et al, we model the nonlinear
terms in these equations using a simple phenomenology for the cascade and
dissipation of wave energy, and assume that there is much more energy in waves
propagating away from the Sun than waves propagating towards the Sun. We then
solve the equations analytically for waves with periods of hours and longer to
obtain expressions for the wave amplitudes and turbulent heating rate as a
function of heliocentric distance. We also develop a second approximate model
that includes waves with periods of roughly one minute to one hour, which
undergo less reflection than the longer-period waves, and compare our models to
observations. Our models generalize the phenomenological model of Dmitruk et al
by accounting for the solar wind velocity, so that the turbulent heating rate
can be evaluated from the coronal base out past the Alfven critical point -
that is, throughout the region in which most of the heating and acceleration
occurs. The simple analytical expressions that we obtain can be used to
incorporate Alfven-wave reflection and turbulent heating into fluid models of
the solar wind.Comment: 9 pages, 9 figures, accepted for publication in Ap
Transmission of Mycobacterium tuberculosis in a Rural Community, Arkansas, 1945–2000
A cluster of tuberculosis cases in a rural community in Arkansas persisted from 1991 to 1999. The cluster had 13 members, 11 linked epidemiologically. Old records identified 24 additional patients for 40 linked case-patients during a 54-year period. Residents of this neighborhood represent a population at high risk who should be considered for tuberculin testing and treatment for latent tuberculosis infection
Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells
Cadmium telluride (CdTe) is the leading
commercialized thin-film photovoltaic technology. Copper is
commonly used in back contacts to obtain high efficiency, but has
also been implicated as a harmful factor for device stability. T hus
it is critical to understand its composition and distribution within
complete devices. In this work the composition and structure of
the back contact region was examined in high efficiency devices
(-16%) contacted using a ZnTe:Cu buffer layer followed by gold
metallization. T he microstructure was examined in the asdeposited
state and after rapid thermal processing (RTP) using
high resolution transmission electron microscopy and EDX
chemical mapping. After RTP the ZnTe exhibits a bilayer
structure with polycrystalline, twinned grains adjacent to Au and
an amorphous region adjacent to CdTe characterized by extensive
Cd-Zn interdiffusion. T he copper that is co-deposited uniformly
within ZnTe is found to segregate dramatically after RTP
activation, either collecting near the ZnTe/Au interface or forming
CUxTe clusters in CdTe at defects or grain boundaries near the
interface with ZnTe. Chlorine, present throughout CdTe and
concentrated at grain boundaries, does not penetrate significantly
into the back contact region during RTP activation
Generality of shear thickening in suspensions
Suspensions are of wide interest and form the basis for many smart fluids.
For most suspensions, the viscosity decreases with increasing shear rate, i.e.
they shear thin. Few are reported to do the opposite, i.e. shear thicken,
despite the longstanding expectation that shear thickening is a generic type of
suspension behavior. Here we resolve this apparent contradiction. We
demonstrate that shear thickening can be masked by a yield stress and can be
recovered when the yield stress is decreased below a threshold. We show the
generality of this argument and quantify the threshold in rheology experiments
where we control yield stresses arising from a variety of sources, such as
attractions from particle surface interactions, induced dipoles from applied
electric and magnetic fields, as well as confinement of hard particles at high
packing fractions. These findings open up possibilities for the design of smart
suspensions that combine shear thickening with electro- or magnetorheological
response.Comment: 11 pages, 9 figures, accepted for publication in Nature Material
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Police, public, and offender perceptions of body-worn video: a single jurisdictional multiple-perspective analysis
Objectives. Police, public, and offender survey responses from a single jurisdiction give a multiple-perspective insight into the use of body-worn video (BWV) cameras by police.
Methods. Police attitudinal data was collected from before (n = 190), during (n = 139), and at the conclusion (n = 221) of a BWV implementation trial. Public attitudes were collected at the conclusion of the BWV implementation trial via online survey (n = 995 respondents) and intercept survey (n = 428 respondents). Offender attitudes (n = 302) were collected in police custody over a 6-month period immediately preceding the BWV trial.
Results. The extent to which police felt BWV influenced their behavior tempered during the trial. All three perspectives were supportive of the use of BWV. The public who had encountered BWV-wearing officers and the offender sample indicated limited belief that BWV would reduce bad behavior. There was also clear contention about the policy and practice decisions around recording.
Conclusions. These findings have significance for BWV trials, commenting on the importance of (a) collecting police attitudes at multiple points, (b) separating the attitudes of public who did encounter police wearing BWV, and (c) data collection and policy for evaluation outcomes
Transit Timing Observations from Kepler: VII. Confirmation of 27 planets in 13 multiplanet systems via Transit Timing Variations and orbital stability
We confirm 27 planets in 13 planetary systems by showing the existence of
statistically significant anti-correlated transit timing variations (TTVs),
which demonstrates that the planet candidates are in the same system, and
long-term dynamical stability, which places limits on the masses of the
candidates---showing that they are planetary. %This overall method of planet
confirmation was first applied to \kepler systems 23 through 32. All of these
newly confirmed planetary systems have orbital periods that place them near
first-order mean motion resonances (MMRs), including 6 systems near the 2:1
MMR, 5 near 3:2, and one each near 4:3, 5:4, and 6:5. In addition, several
unconfirmed planet candidates exist in some systems (that cannot be confirmed
with this method at this time). A few of these candidates would also be near
first order MMRs with either the confirmed planets or with other candidates.
One system of particular interest, Kepler-56 (KOI-1241), is a pair of planets
orbiting a 12th magnitude, giant star with radius over three times that of the
Sun and effective temperature of 4900 K---among the largest stars known to host
a transiting exoplanetary system.Comment: 12 pages, 13 figures, 5 tables. Submitted to MNRA
Spitzer Quasar and ULIRG Evolution Study (QUEST). IV. Comparison of 1-Jy Ultraluminous Infrared Galaxies with Palomar-Green Quasars
We report the results from a comprehensive study of 74 ultraluminous infrared
galaxies (ULIRGs) and 34 Palomar-Green (PG) quasars within z ~ 0.3$ observed
with the Spitzer Infrared Spectrograph (IRS). The contribution of nuclear
activity to the bolometric luminosity in these systems is quantified using six
independent methods that span a range in wavelength and give consistent results
within ~ +/-10-15% on average. The average derived AGN contribution in ULIRGs
is ~35-40%, ranging from ~15-35% among "cool" (f_25/f_60 =< 0.2) optically
classified HII-like and LINER ULIRGs to ~50 and ~75% among warm Seyfert 2 and
Seyfert 1 ULIRGs, respectively. This number exceeds ~80% in PG QSOs. ULIRGs
fall in one of three distinct AGN classes: (1) objects with small extinctions
and large PAH equivalent widths are highly starburst-dominated; (2) systems
with large extinctions and modest PAH equivalent widths have larger AGN
contributions, but still tend to be starburst-dominated; and (3) ULIRGs with
both small extinctions and small PAH equivalent widths host AGN that are at
least as powerful as the starbursts. The AGN contributions in class 2 ULIRGs
are more uncertain than in the other objects, and we cannot formally rule out
the possibility that these objects represent a physically distinct type of
ULIRGs. A morphological trend is seen along the sequence (1)-(2)-(3), in
general agreement with the standard ULIRG - QSO evolution scenario and
suggestive of a broad peak in extinction during the intermediate stages of
merger evolution. However, the scatter in this sequence, implies that black
hole accretion, in addition to depending on the merger phase, also has a strong
chaotic/random component, as in local AGN. (abridged)Comment: 61 pages, 39 figures, 16 tables, accepted for publication in ApJS,
June 2009 issue. Unabbreviated version can be found at
http://www.astro.umd.edu/~veilleux/pubs/quest4.pd
NRES: the network of robotic Echelle spectrographs
Las Cumbres Observatory Global Network (LCOGT) is building the Network of Robotic Echelle Spectrographs (NRES), which will consist of six identical, optical (390 - 860 nm) high-precision spectrographs, each fiber-fed simultaneously by up to two 1-meter telescopes and a thorium argon calibration source. We plan to install one at up to 6 observatory sites in the Northern and Southern hemispheres, creating a single, globally-distributed, autonomous spectrograph facility using up to twelve 1-meter telescopes. Simulations suggest we will achieve long-term radial velocity precision of 3 m/s in less than an hour for stars brighter than V = 12. We have been funded with NSF MRI and ATI grants, and expect our first spectrograph to be deployed in fall 2016, with the full network operation of 5 or 6 units beginning in 2017. We will briefly overview the NRES design, goals, robotic operation, and status. In addition, we will discuss early results from our prototype spectrograph, the laboratory and on-sky performance of our first production unit, and the ongoing software development effort to bring this resource online
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