820 research outputs found
Threeādimensional, multifluid, high spatial resolution MHD model studies of the solar wind interaction with Mars
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95318/1/jgra21138.pd
Extinction and recurrence of multi-group SEIR epidemic
In this paper, we consider a class of multi-group SEIR epidemic models with stochastic perturbations. By the method of stochastic Lyapunov functions, we study their asymptotic behavior in terms of the intensity of the stochastic perturbations and the reproductive number R0R0. When the perturbations are sufficiently large, the exposed and infective components decay exponentially to zero whilst the susceptible components converge weakly to a class of explicit stationary distributions regardless of the magnitude of R0R0. An interesting result is that, if the perturbations are sufficiently small and R0ā¤1R0ā¤1, then the exposed, infective and susceptible components have similar behaviors, respectively, as in the case of large perturbations. When the perturbations are small and R0>1R0>1, we construct a new class of stochastic Lyapunov functions to show the ergodic property and the positive recurrence, and our results reveal some cycling phenomena of recurrent diseases. Computer simulations are carried out to illustrate our analytical results
Solar wind interaction with Mars upper atmosphere: Results from the oneāway coupling between the multifluid MHD model and the MTGCM model
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106987/1/grl51608.pd
The effects of primordial non-Gaussianity on giant-arc statistics
For over a decade, it has been debated whether the concordance LCDM model is
consistent with the observed abundance of giant arcs in clusters. While
previous theoretical studies have focused on properties of the lens and source
populations, as well as cosmological effects such as dark energy, the impact of
initial conditions on the giant-arc abundance is relatively unexplored. Here,
we quantify the impact of non-Gaussian initial conditions with the local
bispectrum shape on the predicted frequency of giant arcs. Using a
path-integral formulation of the excursion set formalism, we extend a
semi-analytic model for calculating halo concentrations to the case of
primordial non-Gaussianity, which may be useful for applications outside of
this work. We find that massive halos tend to collapse earlier in models with
positive f_NL, relative to the Gaussian case, leading to enhanced concentration
parameters. The converse is true for f_NL < 0. In addition to these effects,
which change the lensing cross sections, non-Gaussianity also modifies the
abundance of supercritical clusters available for lensing. These combined
effects work together to either enhance (f_NL > 0) or suppress (f_NL < 0) the
probability of giant-arc formation. Using the best value and 95% confidence
levels currently available from the Wilkinson Microwave Anisotropy Probe, we
find that the giant-arc optical depth for sources at z_s~2 is enhanced by ~20%
and ~45% for f_NL = 32 and 74 respectively. In contrast, we calculate a
suppression of ~5% for f_NL = -10. These differences translate to similar
relative changes in the predicted all-sky number of giant arcs.Comment: 16 pages, 8 figures, accepted by MNRA
The Large Scale Bias of Dark Matter Halos: Numerical Calibration and Model Tests
We measure the clustering of dark matter halos in a large set of
collisionless cosmological simulations of the flat LCDM cosmology. Halos are
identified using the spherical overdensity algorithm, which finds the mass
around isolated peaks in the density field such that the mean density is Delta
times the background. We calibrate fitting functions for the large scale bias
that are adaptable to any value of Delta we examine. We find a ~6% scatter
about our best fit bias relation. Our fitting functions couple to the halo mass
functions of Tinker et. al. (2008) such that bias of all dark matter is
normalized to unity. We demonstrate that the bias of massive, rare halos is
higher than that predicted in the modified ellipsoidal collapse model of Sheth,
Mo, & Tormen (2001), and approaches the predictions of the spherical collapse
model for the rarest halos. Halo bias results based on friends-of-friends halos
identified with linking length 0.2 are systematically lower than for halos with
the canonical Delta=200 overdensity by ~10%. In contrast to our previous
results on the mass function, we find that the universal bias function evolves
very weakly with redshift, if at all. We use our numerical results, both for
the mass function and the bias relation, to test the peak-background split
model for halo bias. We find that the peak-background split achieves a
reasonable agreement with the numerical results, but ~20% residuals remain,
both at high and low masses.Comment: 11 pages, submitted to ApJ, revised to include referee's coment
Solar wind interaction with the Martian upper atmosphere: Crustal field orientation, solar cycle, and seasonal variations
A comprehensive study of the solar wind interaction with the Martian upper atmosphere is presented. Three global models: the 3āD Mars multifluid Block Adaptive Tree Solarāwind Roe Upwind Scheme MHD code (MFāMHD), the 3āD Mars Global Ionosphere Thermosphere Model (MāGITM), and the Mars exosphere Monte Carlo model Adaptive Mesh Particle Simulator (MāAMPS) were used in this study. These models are oneāway coupled; i.e., the MFāMHD model uses the 3āD neutral inputs from MāGITM and the 3āD hot oxygen corona distribution from MāAMPS. By adopting this oneāway coupling approach, the Martian upper atmosphere ion escape rates are investigated in detail with the combined variations of crustal field orientation, solar cycle, and Martian seasonal conditions. The calculated ion escape rates are compared with Mars Express observational data and show reasonable agreement. The variations in solar cycles and seasons can affect the ion loss by a factor of ā¼3.3 and ā¼1.3, respectively. The crustal magnetic field has a shielding effect to protect Mars from solar wind interaction, and this effect is the strongest for perihelion conditions, with the crustal field facing the Sun. Furthermore, the fraction of cold escaping heavy ionospheric molecular ions [(2+ and/or 2+)/Total] are inversely proportional to the fraction of the escaping (ionospheric and corona) atomic ion [O+/Total], whereas 2+ and 2+ ion escape fractions show a positive linear correlation since both ion species are ionospheric ions that follow the same escaping path.Key PointsStudy crustal field, solar cycle, and seasons on Mars' upper atmosphere ion escapeTo understand the longāterm evolution of Mars atmosphere over its historyTo support MAVEN spacecraft mission data analysis (2014ā2016)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115901/1/jgra52040.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115901/2/jgra52040_am.pd
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
Halo statistics in non-Gaussian cosmologies: the collapsed fraction, conditional mass function, and halo bias from the path-integral excursion set method
Characterizing the level of primordial non-Gaussianity (PNG) in the initial
conditions for structure formation is one of the most promising ways to test
inflation and differentiate among different scenarios. The scale-dependent
imprint of PNG on the large-scale clustering of galaxies and quasars has
already been used to place significant constraints on the level of PNG in our
observed Universe. Such measurements depend upon an accurate and robust theory
of how PNG affects the bias of galactic halos relative to the underlying matter
density field. We improve upon previous work by employing a more general
analytical method - the path-integral extension of the excursion set formalism
- which is able to account for the non-Markovianity caused by PNG in the
random-walk model used to identify halos in the initial density field. This
non-Markovianity encodes information about environmental effects on halo
formation which have so far not been taken into account in analytical bias
calculations. We compute both scale-dependent and -independent corrections to
the halo bias, along the way presenting an expression for the conditional
collapsed fraction for the first time, and a new expression for the conditional
halo mass function. To leading order in our perturbative calculation, we
recover the halo bias results of Desjacques et. al. (2011), including the new
scale-dependent correction reported there. However, we show that the
non-Markovian dynamics from PNG can lead to marked differences in halo bias
when next-to-leading order terms are included. We quantify these differences
here. [abridged]Comment: Accepted for publication in MNRAS. Includes minor revisions
recommended by referee, slightly revised notation for clarity, and corrected
typo
The strongest gravitational lenses: II. Is the large Einstein radius of MACS J0717.5+3745 in conflict with LCDM?
Can the standard cosmological model be questioned on the basis of a single
observed extreme galaxy cluster? Usually, the word extreme refers directly to
cluster mass, which is not a direct observable and thus subject to substantial
uncertainty. Hence, it is desirable to extend studies of extreme clusters to
direct observables, such as the Einstein radius (ER). We aim to evaluate the
occurrence probability of the large observed ER of MACS J0717.5 within the
standard LCDM cosmology. In particular, we want to model the distribution
function of the single largest ER in a given cosmological volume and to study
which underlying assumptions and effects have the strongest impact on the
results. We obtain this distribution by a Monte Carlo approach, based on the
semi-analytic modelling of the halo population on the past lightcone. After
sampling the distribution, we fit the results with the general extreme value
(GEV) distribution which we use for the subsequent analysis. We find that the
distribution of the maximum ER is particularly sensitive to the precise choice
of the halo mass function, lens triaxiality, the inner slope of the halo
density profile and the mass-concentration relation. Using the distributions so
obtained,we study the occurrence probability of the large ER of MACS J0717.5,
finding that this system is not in tension with LCDM. We also find that the GEV
distribution can be used to fit very accurately the sampled distributions and
that all of them can be described by a Frechet distribution. With a multitude
of effects that strongly influence the distribution of the single largest ER,
it is more than doubtful that the standard LCDM cosmology can be ruled out on
the basis of a single observation. If, despite the large uncertainties in the
underlying assumptions, one wanted to do so, a much larger ER (> 100 arcsec)
than that of MACS J0717.5 would have to be observed.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics, minor corrections to match the accepted version, added
discussion of the distribution of the largest Einstein radii for the MACS
survey area, extended Fig.
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