9,330 research outputs found
Using the local gyrokinetic code, GS2, to investigate global ITG modes in tokamaks. (I) s- model with profile and flow shear effects
This paper combines results from a local gyrokinetic code with analytical
theory to reconstruct the global eigenmode structure of the linearly unstable
ion-temperature-gradient (ITG) mode with adiabatic electrons. The simulations
presented here employ the s- tokamak equilibrium model. Local
gyrokinetic calculations, using GS2 have been performed over a range of radial
surfaces, x, and for ballooning phase angle, p, in the range -, to map out the complex local mode frequency, . Assuming a quadratic radial profile for the
drive, namely , (holding constant all other equilibrium
profiles such as safety factor, magnetic shear etc.), has a
stationary point. The reconstructed global mode then sits on the outboard mid
plane of the tokamak plasma, and is known as a conventional or isolated mode,
with global growth rate, ~ Max[], where
is the local growth rate. Taking the radial variation in
other equilibrium profiles (e.g safety factor q(x)) into account, removes the
stationary point in and results in a mode that peaks
slightly away from the outboard mid-plane with a reduced global growth rate.
Finally, the influence of flow shear has also been investigated through a
Doppler shift, , where n
is the toroidal mode number and incorporates the effect of
flow shear. The equilibrium profile variation introduces an asymmetry to the
growth rate spectrum with respect to the sign of ,
consistent with recent global gyrokinetic calculations.Comment: 10 pages, 8 figures and 1 tabl
Lower thermosphere densities of N2, O and Ar under high latitude winter conditions
Measurements of the neutral thermosphere were conducted in northern Scandinavia during the Energy Budget Campaign. These measurements included determinations of N2, O, and Ar densities using rocket-borne experiments. The results obtained in the experiments are presented, taking into account also details regarding the employed experimental methods, and an evaluation of the significance of the data. It is found that there are striking differences in thermospheric distributions of the neutral constituents under different geomagnetic conditions. Under quiet geomagnetic conditions there was reasonable agreement with the United States Standard Atmosphere. The concentrations of N2 and Ar were about 70 percent of the predicted values, while the O concentration was about 2.5 times greater
Deep Halpha imagery of the Eridanus shells
A deep \ha image of interlocking filamentary arcs of nebulosity has been
obtained with a wide-field ( 30\degree diameter) narrow-band filter
camera combined with a CCD as a detector. The resultant mosaic of images,
extending to a galactic latitude of 65, has been corrected for field
distortions and had galactic coordinates superimposed on it to permit accurate
correlations with the most recent H{\sc i} (21 cm), X-ray (0.75 kev) and FIR
(IRAS 100 m) maps.
Furthermore, an upper limit of 0.13 arcsec/yr to the expansion proper motion
of the primary 25\degree long nebulous arc has been obtained by comparing a
recent \ha image obtained with the San Pedro Martir telescope of its
filamentary edge with that on a POSS E plate obtained in 1951.
It is concluded that these filamentary arcs are the superimposed images of
separate shells (driven by supernova explosions and/or stellar winds) rather
than the edges of a single `superbubble' stretching from Barnard's Arc (and the
Orion Nebula) to these high galactic latitudes. The proper motion measurement
argues against the primary \ha emitting arc being associated with the giant
radio loop (Loop 2) except in extraordinary circumstances.Comment: 9 pages, 5 figures, accepted for MNRAS publicatio
A search for 183-GHz emission from water in late-type stars
A search was made for 183 GHz line emission from water vapor in the direction of twelve Mira and two semiregular variables. Upper limits to the emission are in the range of 2000 to 5000 Jy. It is estimated that thermal emission from the inner regions of late type stellar envelopes will be on the order of ten Jy. Maser emission, according to one model, would be an order of magnitude stronger. From the limited set sampled, the possibility of very strong maser emission at 183 GHz cannot yet be ruled out
Dispersity-Driven Melting Transition in Two Dimensional Solids
We perform extensive simulations of Lennard-Jones particles to study
the effect of particle size dispersity on the thermodynamic stability of
two-dimensional solids. We find a novel phase diagram in the dispersity-density
parameter space. We observe that for large values of the density there is a
threshold value of the size dispersity above which the solid melts to a liquid
along a line of first order phase transitions. For smaller values of density,
our results are consistent with the presence of an intermediate hexatic phase.
Further, these findings support the possibility of a multicritical point in the
dispersity-density parameter space.Comment: In revtex format, 4 pages, 6 postscript figures. Submitted to PR
The Rest-Frame Optical Properties of z~3 Galaxies
We present the results of a near-infrared imaging survey of z~3 Lyman Break
Galaxies (LBGs). The survey covers a total of 30 arcmin^2 and includes 118
photometrically selected LBGs with K_s band measurements, 63 of which also have
J band measurements, and 81 of which have spectroscopic redshifts. Using the
distribution of optical {\cal R} magnitudes from previous work and {\cal R}-K_s
colors for this sub-sample, we compute the rest-frame optical luminosity
function of LBGs. At the brightest magnitudes, where it is fairly well
constrained, this luminosity function strikingly exceeds locally determined
optical luminosity functions. The V-band luminosity density of only the
observed bright end of the z~3 LBG luminosity function already approaches that
of all stars in the local universe. For the 81 galaxies with measured
redshifts, we investigate the range of LBG stellar populations implied by the
photometry which generally spans the range 900--5500 AA in the rest-frame.
While there are only weak constraints on the parameters for most of the
individual galaxies, there are strong trends in the sample as a whole. A
unified scenario which accounts for the observed trends in bright LBGs is one
in which a relatively short period of very rapid star-formation (hundreds of
M_sun/yr) lasts for roughly 50--100 Myr, after which both the extinction and
star-formation rate are considerably reduced and stars are formed at a more
quiescent, but still rapid, rate for at least a few hundred Myr. In our sample,
a considerable fraction (~20%) of the LBGs have best-fit star-formation ages ~>
1 Gyr, implied stellar masses of ~> 10^10 M_sun, and are still forming stars at
\~30 M_sun/yr.Comment: 61 pages including 19 figures. Accepted for publication in Ap
Hierarchical Bayesian CMB Component Separation with the No-U-Turn Sampler
Key to any cosmic microwave background (CMB) analysis is the separation of
the CMB from foreground contaminants. In this paper we present a novel
implementation of Bayesian CMB component separation. We sample from the full
posterior distribution using the No-U-Turn Sampler (NUTS), a gradient based
sampling algorithm. Alongside this, we introduce new foreground modelling
approaches. We use the mean-shift algorithm to define regions on the sky,
clustering according to naively estimated foreground spectral parameters. Over
these regions we adopt a complete pooling model, where we assume constant
spectral parameters, and a hierarchical model, where we model individual
spectral parameters as being drawn from underlying hyper-distributions. We
validate the algorithm against simulations of the LiteBIRD and C-BASS
experiments, with an input tensor-to-scalar ratio of .
Considering multipoles , we are able to recover estimates
for . With LiteBIRD only observations, and using the complete pooling model,
we recover . For C-BASS and LiteBIRD observations
we find using the complete pooling model, and
using the hierarchical model. By adopting the
hierarchical model we are able to eliminate biases in our cosmological
parameter estimation, and obtain lower uncertainties due to the smaller
Galactic emission mask that can be adopted for power spectrum estimation.
Measured by the rate of effective sample generation, NUTS offers performance
improvements of over using Metropolis-Hastings to fit the complete
pooling model. The efficiency of NUTS allows us to fit the more sophisticated
hierarchical foreground model, that would likely be intractable with
non-gradient based sampling algorithms.Comment: 19 pages, 9 figure
Radio Astronomy
Contains reports on one research project.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Aeronautics and Space Administration (Grant NGR 22-009-421)National Science Foundation Grant GP-2076
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