3,496 research outputs found
Security and Privacy Issues in Cloud Computing
Cloud computing transforming the way of information technology (IT) for consuming and managing, promising improving cost efficiencies, accelerate innovations, faster time-to-market and the ability to scale applications on demand (Leighton, 2009). According to Gartner, while the hype grew ex-ponentially during 2008 and continued since, it is clear that there is a major shift towards the cloud computing model and that the benefits may be substantial (Gartner Hype-Cycle, 2012). However, as the shape of the cloud computing is emerging and developing rapidly both conceptually and in reality, the legal/contractual, economic, service quality, interoperability, security and privacy issues still pose significant challenges. In this chapter, we describe various service and deployment models of cloud computing and identify major challenges. In particular, we discuss three critical challenges: regulatory, security and privacy issues in cloud computing. Some solutions to mitigate these challenges are also proposed along with a brief presentation on the future trends in cloud computing deployment
Diffusion of Elements in the Interstellar Medium in Early-Type Galaxies
We consider the role of diffusion in the redistribution of elements in the
hot interstellar medium (ISM) of early-type galaxies. It is well known that
gravitational sedimentation can affect significantly the abundances of helium
and heavy elements in the intracluster gas of massive galaxy clusters. The
self-similarity of the temperature profiles and tight mass--temperature
relation of relaxed cool-core clusters suggest that the maximum effect of
sedimentation take place in the most massive virialized objects in the
Universe. However, Chandra and XMM-Newton observations demonstrate more complex
scaling relations between the masses of early-type galaxies and other
parameters, such as the ISM temperature and gas mass fraction. An important
fact is that early-type galaxies can show both decreasing and increasing radial
temperature profiles. We have calculated the diffusion based on the observed
gas density and temperature distributions for 13 early-type galaxies that
belonging to the different environments and cover a wide range of X-ray
luminosities. To estimate the maximum effect of sedimentation and thermal
diffusion, we have solved the full set of Burgers' equations for a
non-magnetized ISM plasma. The results obtained demonstrate a considerable
increase of the He/H ratio within one effective radius for all galaxies of our
sample. For galaxies with a flat or declining radial temperature profile the
average increase of the helium abundance is 60\% in one billion years of
diffusion. The revealed effect can introduce a significant bias in the metal
abundance measurements based on X-ray spectroscopy and can affect the evolution
of stars that could be formed from a gas with a high helium abundance.Comment: 15 pages, 9 figures, 3 tables, published in Astronomy Letters, 2017,
Volume 43, Issue 5, pp.285-30
Electric field generation by the electron beam filamentation instability: Filament size effects
The filamentation instability (FI) of counter-propagating beams of electrons
is modelled with a particle-in-cell simulation in one spatial dimension and
with a high statistical plasma representation. The simulation direction is
orthogonal to the beam velocity vector. Both electron beams have initially
equal densities, temperatures and moduli of their nonrelativistic mean
velocities. The FI is electromagnetic in this case. A previous study of a small
filament demonstrated, that the magnetic pressure gradient force (MPGF) results
in a nonlinearly driven electrostatic field. The probably small contribution of
the thermal pressure gradient to the force balance implied, that the
electrostatic field performed undamped oscillations around a background
electric field. Here we consider larger filaments, which reach a stronger
electrostatic potential when they saturate. The electron heating is enhanced
and electrostatic electron phase space holes form. The competition of several
smaller filaments, which grow simultaneously with the large filament, also
perturbs the balance between the electrostatic and magnetic fields. The
oscillations are damped but the final electric field amplitude is still
determined by the MPGF.Comment: 14 pages, 10 plots, accepted for publication in Physica Script
- …