44,360 research outputs found
Generation of Suprathermal Electrons by Collective Processes in Collisional Plasma
The ubiquity of high-energy tails in the charged particle velocity
distribution functions observed in space plasmas suggests the existence of an
underlying process responsible for taking a fraction of the charged particle
population out of thermal equilibrium and redistributing it to suprathermal
velocity and energy ranges. The present Letter focuses on a new and fundamental
physical explanation for the origin of suprathermal electron distribution
function in a highly collisional plasma. This process involves a newly
discovered electrostatic bremsstrahlung emission that is effective in a plasma
in which binary collisions are present. The steady-state electron velocity
distribution function dictated by such a process corresponds to a Maxwellian
core plus a quasi-inverse power-law tail, which is a feature commonly observed
in many space plasma environment. In order to demonstrate this, the system of
self-consistent particle- and wave- kinetic equations are numerically solved
with an initially Maxwellian electron velocity distribution and Langmuir wave
spectral intensity, which is a state that does not reflect the presence of
electrostatic bremsstrahlung process, and hence not in force balance. The
electrostatic bremsstrahlung term subsequently drives the system to a new
force-balanced steady state. After a long integration period it is demonstrated
the initial Langmuir fluctuation spectrum is modified, which in turn distorts
the initial Maxwellian electron distribution into a velocity distribution that
resembles the said core-suprathermal velocity distribution. Such a mechanism
may thus be operative at the coronal source region, which is characterized by
high collisionality.Comment: 7 pages, 2 figures. Published at: The Astrophysical Journal Letters,
Volume 849, Number 2, L30. url: https://doi.org/10.3847/2041-8213/aa956
Heating and Cooling Dynamics of Carbon Nanotubes Observed by Temperature-Jump Spectroscopy and Electron Microscopy
Microscopy imaging indicates that in situ carbon nanotubes (CNTs) irradiation with relatively low dosages of infrared radiation results in significant heating of the tubes to temperatures above 1300 K. Ultrafast temperature-jump experiments reveal that CNTs laser-induced heating and subsequent cooling in solution take tens and hundreds of picoseconds, respectively. Given the reported transient behavior, these observations suggest novel ways for a T-jump methodology, unhindered by the requirement for excitation of water in the study of biological structures. They also provide the rate information needed for optimization of photothermal therapy that invokes infrared irradiation to selectively heat and annihilate cancer cells
A Unified Picture of the FIP and Inverse FIP Effects
We discuss models for coronal abundance anomalies observed in the coronae of
the sun and other late-type stars following a scenario first introduced by
Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with
the partially neutral gas. Instead of considering wave heating of ions in this
location, we explore the effects on the upper chromospheric plasma of the wave
ponderomotive forces. These can arise as upward propagating waves from the
chromosphere transmit or reflect upon reaching the chromosphere-corona
boundary, and are in large part determined by the properties of the coronal
loop above. Our scenario has the advantage that for realistic wave energy
densities, both positive and negative changes in the abundance of ionized
species compared to neutrals can result, allowing both FIP and Inverse FIP
effects to come out of the model. We discuss how variations in model parameters
can account for essentially all of the abundance anomalies observed in solar
spectra. Expected variations with stellar spectral type are also qualitatively
consistent with observations of the FIP effect in stellar coronae.Comment: 25 pages, 4 figures, submitted to Ap
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
Communications software performance prediction
Software development can be costly and it is important that confidence in a software system be established as early as possible in the design process. Where the software supports communication services, it is essential that the resultant system will operate within certain performance constraints (e.g. response time). This paper gives an overview of work in progress on a collaborative project sponsored by BT which aims to offer performance predictions at an early stage in the software design process. The Permabase architecture enables object-oriented software designs to be combined with descriptions of the network configuration and workload as a basis for the input to a simulation model which can predict aspects of the performance of the system. The prototype implementation of the architecture uses a combination of linked design and simulation tools
Non-Markovian generalization of the Lindblad theory of open quantum systems
A systematic approach to the non-Markovian quantum dynamics of open systems
is given by the projection operator techniques of nonequilibrium statistical
mechanics. Combining these methods with concepts from quantum information
theory and from the theory of positive maps, we derive a class of correlated
projection superoperators that take into account in an efficient way
statistical correlations between the open system and its environment. The
result is used to develop a generalization of the Lindblad theory to the regime
of highly non-Markovian quantum processes in structured environments.Comment: 10 pages, 1 figure, replaced by published versio
- …