484 research outputs found
A Model for the Stray Light Contamination of the UVCS Instrument on SOHO
We present a detailed model of stray-light suppression in the spectrometer
channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO
spacecraft. The control of diffracted and scattered stray light from the bright
solar disk is one of the most important tasks of a coronagraph. We compute the
fractions of light that diffract past the UVCS external occulter and
non-specularly pass into the spectrometer slit. The diffracted component of the
stray light depends on the finite aperture of the primary mirror and on its
figure. The amount of non-specular scattering depends mainly on the
micro-roughness of the mirror. For reasonable choices of these quantities, the
modeled stray-light fraction agrees well with measurements of stray light made
both in the laboratory and during the UVCS mission. The models were constructed
for the bright H I Lyman alpha emission line, but they are applicable to other
spectral lines as well.Comment: 19 pages, 5 figures, Solar Physics, in pres
Be Stars: Rapidly Rotating Pulsators
I will show that Be stars are, without exception, a class of rapidly rotating
stars, which are in the majority of cases pulsating stars as well, while none
of them does possess a large scale (i.e. with significant dipolar contribution)
magnetic field.Comment: Review talk given at "XX Stellar Pulsation Conference Series: Impact
of new instrumentation and new insights in stellar pulsations", Granada, 5-9
September 2011, in press in AIP Conf. Se
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Atmospheric Heating and Wind Acceleration: Results for Cool Evolved Stars based on Proposed Processes
A chromosphere is a universal attribute of stars of spectral type later than
~F5. Evolved (K and M) giants and supergiants (including the zeta Aurigae
binaries) show extended and highly turbulent chromospheres, which develop into
slow massive winds. The associated continuous mass loss has a significant
impact on stellar evolution, and thence on the chemical evolution of galaxies.
Yet despite the fundamental importance of those winds in astrophysics, the
question of their origin(s) remains unsolved. What sources heat a chromosphere?
What is the role of the chromosphere in the formation of stellar winds? This
chapter provides a review of the observational requirements and theoretical
approaches for modeling chromospheric heating and the acceleration of winds in
single cool, evolved stars and in eclipsing binary stars, including physical
models that have recently been proposed. It describes the successes that have
been achieved so far by invoking acoustic and MHD waves to provide a physical
description of plasma heating and wind acceleration, and discusses the
challenges that still remain.Comment: 46 pages, 9 figures, 1 table; modified and unedited manuscript;
accepted version to appear in: Giants of Eclipse, eds. E. Griffin and T. Ake
(Berlin: Springer
Effective K-factors for gg -> H -> WW -> lnu lnu at the LHC
A simulation of the search for the Standard Model Higgs boson at the LHC, in
the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD
corrections are taken into account by using a reweighting procedure, which
allows us to combine event rates obtained with the PYTHIA Monte Carlo program
with the most up-to-date theoretical predictions for the transverse-momentum
spectra of the Higgs signal and its corresponding WW background. With this
method the discovery potential for Higgs masses between 140 and 180 GeV is
recalculated and the potential statistical significance of this channel is
found to increase considerably. For a Higgs mass of 165 GeV a
signal-to-background ratio of almost 2:1 can be obtained. A statistical
significance of five standard deviations might already be achieved with an
integrated luminosity close to 0.4 fb^{-1}. Using this approach, an
experimental effective K-factor of about 2.04 is obtained for the considered
Higgs signature, which is only about 15 % smaller than the theoretical
inclusive K-factor.Comment: 16 pages, 8 eps figures, comparison of jet veto efficiencies between
PYTHIA and HERWIG added, to appear on JHE
Kinetic Turbulence
The weak collisionality typical of turbulence in many diffuse astrophysical
plasmas invalidates an MHD description of the turbulent dynamics, motivating
the development of a more comprehensive theory of kinetic turbulence. In
particular, a kinetic approach is essential for the investigation of the
physical mechanisms responsible for the dissipation of astrophysical turbulence
and the resulting heating of the plasma. This chapter reviews the limitations
of MHD turbulence theory and explains how kinetic considerations may be
incorporated to obtain a kinetic theory for astrophysical plasma turbulence.
Key questions about the nature of kinetic turbulence that drive current
research efforts are identified. A comprehensive model of the kinetic turbulent
cascade is presented, with a detailed discussion of each component of the model
and a review of supporting and conflicting theoretical, numerical, and
observational evidence.Comment: 31 pages, 3 figures, 99 references, Chapter 6 in A. Lazarian et al.
(eds.), Magnetic Fields in Diffuse Media, Astrophysics and Space Science
Library 407, Springer-Verlag Berlin Heidelberg (2015
Difficult Scenarios for NMSSM Higgs Discovery at the LHC
We identify scenarios not ruled out by LEP data in which NMSSM Higgs
detection at the LHC will be particularly challenging. We first review the
`no-lose' theorem for Higgs discovery at the LHC that applies if Higgs bosons
do not decay to other Higgs bosons - namely, with L=300 fb^-1, there is always
one or more `standard' Higgs detection channel with at least a 5 sigma signal.
However, we provide examples of no-Higgs-to-Higgs cases for which all the
standard signals are no larger than 7 sigma implying that if the available L is
smaller or the simulations performed by ATLAS and CMS turn out to be overly
optimistic, all standard Higgs signals could fall below 5 sigma even in the
no-Higgs-to-Higgs part of NMSSM parameter space. In the vast bulk of NMSSM
parameter space, there will be Higgs-to-Higgs decays. We show that when such
decays are present it is possible for all the standard detection channels to
have very small significance. In most such cases, the only strongly produced
Higgs boson is one with fairly SM-like couplings that decays to two lighter
Higgs bosons (either a pair of the lightest CP-even Higgs bosons, or, in the
largest part of parameter space, a pair of the lightest CP-odd Higgs bosons). A
number of representative bench-mark scenarios of this type are delineated in
detail and implications for Higgs discovery at various colliders are discussed.Comment: 31 pages, 5 figure
Heavy-Tailed Distribution of Cyber-Risks
With the development of the Internet, new kinds of massive epidemics,
distributed attacks, virtual conflicts and criminality have emerged. We present
a study of some striking statistical properties of cyber-risks that quantify
the distribution and time evolution of information risks on the Internet, to
understand their mechanisms, and create opportunities to mitigate, control,
predict and insure them at a global scale. First, we report an exceptionnaly
stable power-law tail distribution of personal identity losses per event, , with . This result is
robust against a surprising strong non-stationary growth of ID losses
culminating in July 2006 followed by a more stationary phase. Moreover, this
distribution is identical for different types and sizes of targeted
organizations. Since , the cumulative number of all losses over all events
up to time increases faster-than-linear with time according to
, suggesting that privacy, characterized by personal
identities, is necessarily becoming more and more insecure. We also show the
existence of a size effect, such that the largest possible ID losses per event
grow faster-than-linearly as with the organization size . The
small value of the power law distribution of ID losses is
explained by the interplay between Zipf's law and the size effect. We also
infer that compromised entities exhibit basically the same probability to incur
a small or large loss.Comment: 9 pages, 3 figure
Application of Genetic Programming to High Energy Physics Event Selection
We review genetic programming principles, their application to FOCUS data
samples, and use the method to study the doubly Cabibbo suppressed decay D+ ->
K+ pi+ pi- relative to its Cabibbo favored counterpart, D+ -> K- pi+ pi+. We
find that this technique is able to improve upon more traditional analysis
methods. To our knowledge, this is the first application of the genetic
programming technique to High Energy Physics data.Comment: 39 page
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
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