258 research outputs found
The effect of shock wave properties on the release timings of solar energetic particles
Context. Fast and wide coronal mass ejections (CMEs) and CME-driven shock waves are capable of accelerating solar energetic particles (SEPs) and releasing them in very distant locations in the solar corona and near-Sun interplanetary space. SEP events have a variety of characteristics in their release times and particle anisotropies. In some events, specifics of the SEP release times are thought to be difficult to reconcile with the scenario that a propagating shock wave is responsible for the SEP release. Aims. Despite the apparent difficulties posed by the shock scenario, many studies have not considered the properties of the propagating shock waves when making a connection with SEP release. This could probably resolve some of the issues and would help us to delve into and understand more important issues such as the effect of the shock acceleration efficiency on the observed characteristics of the SEP timings and the role of particle transport. This study aims to approach these issues from the shock wave perspective and elucidate some of these aspects. Methods. We constructed a simple 2D geometrical model to describe the propagation and longitudinal extension of a disturbance. We used this model to examine the longitudinal extension of the wave front from the eruption site as a function of time, to calculate the connection times as a function of the longitudinal separation angle, and to determine the shock parameters at any connection point. We examined how the kinematic and geometric properties of the disturbance could affect the timings of the SEP releases at different heliolongitudes. Results. We show that the extension of a wave close to the solar surface may not always indicate when a magnetic connection is established for the first time. The first connection times depend on both the kinematics and geometry of the propagating wave. A shock-related SEP release process can produce a large event-to-event variation in the relationship between the connection and release times and the separation angle to the eruption site. The evolution of the shock geometry and shock strength at the field lines connected to an observer are important parameters for the observed characteristic of the release times.Peer reviewe
Latitudinal gradients of galactic cosmic rays during the 2007 solar minimum
Ulysses, launched in 1990 October in the maximum phase of solar cycle 22, completed its third out-of-ecliptic orbit in 2008 February. This provides a unique opportunity to study the propagation of cosmic rays over a wide range of heliographic latitudes during different levels of solar activity and different polarities in the inner heliosphere. Comparison of the first and second fast latitude scans from 1994 to 1995 and from 2000 to 2001 confirmed the expectation of positive latitudinal gradients at solar minimum versus an isotropic Galactic cosmic ray distribution at solar maximum. During the second scan in mid-2000, the solar magnetic field reversed its global polarity. From 2007 to 2008, Ulysses made its third fast latitude scan during the declining phase of solar cycle 23. Therefore, the solar activity is comparable in 2007-2008 to that from 1994 to 1995, but the magnetic polarity is opposite. Thus, one would expect to compare positive with negative latitudinal gradients during these two periods for protons and electrons, respectively. In contrast, our analysis of data from the Kiel Electron Telescope aboard Ulysses results in no significant latitudinal gradients for protons. However, the electrons show, as expected, a positive latitudinal gradient of ~0.2% per degree. Although our result is surprising, the nearly isotropic distribution of protons in 2007-2008 is consistent with an isotropic distribution of electrons from 1994 to 1995
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
New results on source and diffusion spectral features of Galactic cosmic rays: I- B/C ratio
In a previous study (Maurin et al., 2001), we explored the set of parameters
describing diffusive propagation of cosmic rays (galactic convection,
reacceleration, halo thickness, spectral index and normalization of the
diffusion coefficient), and we identified those giving a good fit to the
measured B/C ratio. This study is now extended to take into account a sixth
free parameter, namely the spectral index of sources. We use an updated version
of our code where the reacceleration term comes from standard minimal
reacceleration models. The goal of this paper is to present a general view of
the evolution of the goodness of fit to B/C data with the propagation
parameters. In particular, we find that, unlike the well accepted picture, and
in accordance with our previous study, a Kolmogorov-like power spectrum for
diffusion is strongly disfavored. Rather, the analysis points towards
along with source spectra index . Two
distinct energy dependences are used for the source spectra: the usual
power-law in rigidity and a law modified at low energy, the second choice being
only slightly preferred. We also show that the results are not much affected by
a different choice for the diffusion scheme. Finally, we compare our findings
to recent works, using other propagation models. This study will be further
refined in a companion paper, focusing on the fluxes of cosmic ray nuclei.Comment: 32 pages, 13 figures, accepted in A&
Numerical Studies of Cosmic Ray Injection and Acceleration
A numerical scheme that incorporates a thermal leakage injection model into a
combined gas dynamics and cosmic ray (CR, hereafter) diffusion-convection code
has been developed. The particle injection is followed numerically by filtering
the diffusive flux of suprathermal particles across the shock to the upstream
region according to a velocity-dependent transparency function that controls
the fraction of leaking particles. We have studied CR injection and
acceleration efficiencies during the evolution of CR modified planar shocks for
a wide range of initial shock Mach numbers, , assuming a Bohm-like
diffusion coefficient. The injection process is very efficient when the
subshock is strong, leading to fast and significant modification of the shock
structure. As the CR pressure increases, the subshock weakens and the injection
rate decreases accordingly, so that the subshock does not disappear. Although
some fraction of the particles injected early in the evolution continue to be
accelerated to ever higher energies, the postshock CR pressure reaches an
approximate time-asymptotic value due to a balance between fresh
injection/acceleration and advection/diffusion of the CR particles away from
the shock. We conclude that the injection rates in strong parallel shocks are
sufficient to lead to rapid nonlinear modifications to the shock structures and
that self-consistent injection and time-dependent simulations are crucial to
understanding the non-linear evolution of CR modified shocks.Comment: 28 pages, To appear in ApJ November 1, 2002 issu
Optically levitated nanoparticle as a model system for stochastic bistable dynamics
Nano-mechanical resonators have gained an increasing importance in nanotechnology owing to their contributions to both fundamental and applied science. Yet, their small dimensions and mass raises some challenges as their dynamics gets dominated by nonlinearities that degrade their performance, for instance in sensing applications. Here, we report on the precise control of the nonlinear and stochastic bistable dynamics of a levitated nanoparticle in high vacuum. We demonstrate how it can lead to efficient signal amplification schemes, including stochastic resonance. This work contributes to showing the use of levitated nanoparticles as a model system for stochastic bistable dynamics, with applications to a wide variety of fields.inancial support from the ERC- QnanoMECA (Grant No. 64790), the Spanish Ministry of Economy and Competitiveness, under grant FIS2016-80293-R and through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0522), Fundació Privada CELLEX and from the CERCA Programme/Generalitat de Catalunya. J.G. has been supported by H2020-MSCA-IF-2014 under REA grant Agreement No. 655369. L.R. acknowledges support from an ETH Marie Curie Cofund Fellowship
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
Uncoupling of EGFR–RAS signaling and nuclear localization of YBX1 in colorectal cancer
The transcription factor YBX1 can act as a mediator of signals transmitted via
the EGFR–RAS–MAPK axis. YBX1 expression has been associated with tumor
progression and prognosis in multiple types of cancer. Immunohistochemical
studies have revealed dependency between YBX1 expression and individual EGFR
family members. We analyzed YBX1 and EGFR family proteins in a colorectal
cancer (CRC) cohort and provide functional analyses of YBX1 in the context of
EGFR–RAS–MAPK signaling. Immunohistochemistry for YBX1 and EGFR family
receptors with two antibodies for YBX1 and EGFR were performed and related to
clinicopathological data. We employed Caco2 cells expressing an inducible
KRASV12 gene to determine effects on localization and levels of YBX1. Mouse
xenografts of Caco2-KRASV12 cells were used to determine YBX1 dynamics in a
tissue context. The two different antibodies against YBX1 showed discordant
immunohistochemical stainings in cell culture and clinical specimens.
Expression of YBX1 and EGFR family members were not correlated in CRC.
Analysis of Caco2 xenografts displayed again heterogeneity of YBX1 staining
with both antibodies. Our results suggest that YBX1 is controlled via complex
regulatory mechanisms involving tumor stroma interaction and signal
transduction processes. Our study highlights that YBX1 antibodies have
different specificities, advocating their use in a combined manner
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