3,171 research outputs found
Acceleration of weakly collisional solar-type winds
One of the basic properties of the solar wind, that is the high speed of the
fast wind, is still not satisfactorily explained. This is mainly due to the
theoretical difficulty of treating weakly collisional plasmas. The fluid
approach implies that the medium is collision dominated and that the particle
velocity distributions are close to Maxwellians. However the electron velocity
distributions observed in the solar wind depart significantly from Maxwellians.
Recent kinetic collisionless models (called exospheric) using velocity
distributions with a suprathermal tail have been able to reproduce the high
speeds of the fast solar wind. In this letter we present new developments of
these models by generalizing them over a large range of corona conditions. We
also present new results obtained by numerical simulations that include
collisions. Both approaches calculate the heat flux self-consistently without
any assumption on the energy transport. We show that both approaches - the
exospheric and the collisional one - yield a similar variation of the wind
speed with the basic parameters of the problem; both produce a fast wind speed
if the coronal electron distribution has a suprathermal tail. This suggests
that exospheric models contain the necessary ingredients for the powering of a
transonic stellar wind, including the fast solar one.Comment: Accepted for publication in The Astrophysical Journal Letters
(accepted: 13 May 2005
Nano dust impacts on spacecraft and boom antenna charging
High rate sampling detectors measuring the potential difference between the
main body and boom antennas of interplanetary spacecraft have been shown to be
efficient means to measure the voltage pulses induced by nano dust impacts on
the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463
(2009)). However, rough estimates of the free charge liberated in post impact
expanding plasma cloud indicate that the cloud's own internal electrostatic
field is too weak to account for measured pulses as the ones from the TDS
instrument on the STEREO spacecraft frequently exceeding 0.1 V/m. In this paper
we argue that the detected pulses are not a direct measure of the potential
structure of the plasma cloud, but are rather the consequence of a transitional
interruption of the photoelectron return current towards the portion of the
antenna located within the expanding cloud
Short-term temporal variations of heterotrophic bacterial abundance and production in the open NW Mediterranean Sea
International audienceWe present the vertical and temporal dynamics of total vs. particle-attached bacterial abundance and activity over a 5 week period under summer to autumn transition in NW Mediterranean Sea. By comparison to previous investigations in the same area but during different seasons, we found that total bacterial biomass and production values were consistent with the hydrological conditions of the summer-fall transition. At a weekly time scale, total bacterial biomass and production in the euphotic layers was significantly correlated with phytoplanktonic biomass. At an hourly time scale, total bacterial biomass responded very rapidly to chlorophyll-a fluctuations, suggesting a tight coupling between phytoplankton and bacteria for resource partitioning during summer-autumn transition. In contrast, no influence of diel changes on bacterial parameters was detected. Episodic events such as coastal water intrusions had a significant positive effect on total bacterial abundance and production, whereas we could not detect any influence of short wind events whatever the magnitude. Finally, we show that particle-attached bacteria can represent a large proportion (until 49%) of the total bacterial activity in the euphotic layer but display rapid and sporadic changes at hourly time scales. This study underlines the value of large datasets covering different temporal scales to clarify the biogeochemical role of bacteria in the cycling of organic matter in open seawater
Seasonal to hour variation scales in abundance and production of total and particle-attached bacteria in the open NW Mediterranean Sea (0â1000 m)
We present the vertical and temporal dynamics of total <i>vs.</i> particle-attached bacterial abundance and activity over a 5 week period under summer to autumn transition in NW Mediterranean Sea. At a weekly time scale, total bacterial biomass and production in the euphotic layers was significantly correlated with phytoplanktonic biomass. At an hourly time scale, total bacterial biomass responded very rapidly to chlorophyll <i>a</i> fluctuations, suggesting a tight coupling between phytoplankton and bacteria for resource partitioning during the summer-autumn transition. In contrast, no influence of diel changes on bacterial parameters was detected. Episodic events such as coastal water intrusions had a significant positive effect on total bacterial abundance and production, whereas we could not detect any influence of short wind events whatever the magnitude. Finally, we show that particle-attached bacteria can represent a large proportion (up to 49%) of the total bacterial activity in the euphotic layer but display rapid and sporadic changes at hourly time scales. In the mesopelagic layers, bacterial abundance and production linearly decreased with depth, except some production peaks at 400â750 m. This study underlines the value of large datasets covering different temporal scales to clarify the biogeochemical role of bacteria in the cycling of organic matter in open seawater
Heating of the solar wind with electron and proton effects
We examine the effects of including effects of both protons and electrons on the heating of the fast solar wind through two different approaches. In the ïŹrst approach, we incorporate the electron temperature in an MHD turbulence transport model for the solar wind. In the second approach, we adopt more empirically based methods by analyzing the measured proton and electron temperatures to calculate the heat deposition rates. Overall, we conclude that incorporating separate proton and electron temperatures and heat conduction effects provides an improved and more complete model of the heating of the solar wind
Are we seeing accretion flows in a 250kpc-sized Ly-alpha halo at z=3?
Using MUSE on the ESO-VLT, we obtained a 4 hour exposure of the z=3.12 radio
galaxy MRC0316-257. We detect features down to ~10^-19 erg/s/cm^2/arcsec^2 with
the highest surface brightness regions reaching more than a factor of 100
higher. We find Ly-alpha emission out to ~250 kpc in projection from the active
galactic nucleus (AGN). The emission shows arc-like morphologies arising at
150-250 kpc from the nucleus in projection with the connected filamentary
structures reaching down into the circum-nuclear region. The most distant arc
is offset by 700 km/s relative to circum-nuclear HeII 1640 emission, which we
assume to be at the systemic velocity. As we probe emission closer to the
nucleus, the filamentary emission narrows in projection on the sky, the
relative velocity decreases to ~250 km/s, and line full-width at half maximum
range from 300-700 km/s. From UV line ratios, the emission on scales of 10s of
kpc from the nucleus along a wide angle in the direction of the radio jets is
clearly excited by the radio jets and ionizing radiation of the AGN. Assuming
ionization equilibrium, the more extended emission outside of the axis of the
jet direction would require 100% or more illumination to explain the observed
surface brightness. High speed (>300 km/s) shocks into rare gas would provide
sufficiently high surface brightness. We discuss the possibility that the arcs
of Ly-alpha emission represent accretion shocks and the filamentary emission
represent gas flows into the halo, and compare our results with gas accretion
simulations.Comment: 4 pages, 2 figures, 1 table, A&A letters accepte
On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space
Nano and micro meter sized dust particles travelling through the heliosphere
at several hundreds of km/s have been repeatedly detected by interplanetary
spacecraft. When such fast moving dust particles hit a solid target in space,
an expanding plasma cloud is formed through the vaporisation and ionisation of
the dust particles itself and part of the target material at and near the
impact point. Immediately after the impact the small and dense cloud is
dominated by collisions and the expansion can be described by fluid equations.
However, once the cloud has reached micro-m dimensions, the plasma may turn
collisionless and a kinetic description is required to describe the subsequent
expansion. In this paper we explore the late and possibly collisionless
spherically symmetric unconstrained expansion of a single ionized ion-electron
plasma using N-body simulations. Given the strong uncertainties concerning the
early hydrodynamic expansion, we assume that at the time of the transition to
the collisionless regime the cloud density and temperature are spatially
uniform. We do also neglect the role of the ambient plasma. This is a
reasonable assumption as long as the cloud density is substantially higher than
the ambient plasma density. In the case of clouds generated by fast
interplanetary dust grains hitting a solid target some 10^7 electrons and ions
are liberated and the in vacuum approximation is acceptable up to meter order
cloud dimensions. ..
The Blue Straggler population in the globular cluster M53 (NGC5024): a combined HST, LBT, CFHT study
We used a proper combination of multiband high-resolution and wide field
multi-wavelength observations collected at three different telescopes (HST, LBT
and CFHT) to probe Blue Straggler Star (BSS) populations in the globular
cluster M53. Almost 200 BSS have been identified over the entire cluster
extension. The radial distribution of these stars has been found to be bimodal
(similarly to that of several other clusters) with a prominent dip at ~60'' (~2
r_c) from the cluster center. This value turns out to be a factor of two
smaller than the radius of avoidance (r_avoid, the radius within which all the
stars of ~1.2 M_sun have sunk to the core because of dynamical friction effects
in an Hubble time). While in most of the clusters with a bimodal BSS radial
distribution, r_avoid has been found to be located in the region of the
observed minimum, this is the second case (after NGC6388) where this
discrepancy is noted. This evidence suggests that in a few clusters the
dynamical friction seems to be somehow less efficient than expected.
We have also used this data base to construct the radial star density profile
of the cluster: this is the most extended and accurate radial profile ever
published for this cluster, including detailed star counts in the very inner
region. The star density profile is reproduced by a standard King Model with an
extended core (~25'') and a modest value of the concentration parameter
(c=1.58). A deviation from the model is noted in the most external region of
the cluster (at r>6.5' from the center). This feature needs to be further
investigated in order to address the possible presence of a tidal tail in this
cluster.Comment: 25 pages, 9 figures, accepted for publication on Ap
Deep optical spectroscopy of extended Lyman alpha emission around three radio-quiet z=4.5 quasars
We report the first results of a spectroscopic search for Lyman alpha,
envelopes around three z=4.5 radio-quiet quasars. Our observational strategy
uses the FORS2 spectrograph attached to the UT1 of the Very Large Telescope
(VLT) in the multi-slit mode. This allows us to observe simultaneously the
quasars and several PSF stars. The spectra of the latter are used to remove the
point-like quasar from the data, and to unveil the faint underlying Lyman
alpha, envelopes associated with the quasars with unprecedented depth. We
clearly detect an envelope around two of the three quasars. These envelopes
measure respectively 10" and 13" in extent (i.e. 67 kpc and 87 kpc). This is 5
to 10 times larger than predicted by the models of Haiman & Rees (2001) and up
to 100 times fainter. Our observations better agree with models involing a
clumpy envelope as in Alam & Miralda-Escude (2002) or Chelouche et al. (2008).
We find that the brighter quasars also have the brighter envelopes but that the
extend of the envelopes does not depend on the quasar luminosity. Although our
results are based on only two objects with a detected Lyman alpha, envelope,
the quality of the spatial deblending of the spectra lends considerable hope to
estimate the luminosity function and surface brightness profiles of high
redshift Lyman alpha, envelopes down to F= 2-3 10^{-21} erg/s/cm^2/A. We find
that the best strategy to carry out such a project is to obtain both
narrow-band images and deep slit-spectra.Comment: 8 pages, 5 figures, submitted to A&
Perspective for resonances in p-p collisions with the ALICE detector
The capability of the ALICE detector to reconstruct short-lived resonances
from the very beginning of the LHC p-p program start-up was investigated,
considering about 300,000 minimum bias p-p PYTHIA events at 900 GeV and 10 TeV,
fully reconstructed under the hypothesis of a still misaligned detector. Only
the information from track reconstruction was taken into account and no PID
knowledge was considered. A reliable rho(770) yield estimation was possible in
900 GeV events. Moreover, in 300,000 p-p events at 10 TeV the signal for the
phi-resonance could be extracted. A significance of 9 can be reached if only
resonances with a transverse momentum larger than 1.5 GeV/c are considered.
Moreover, even with this low statistics the phi signal is visible in events
with a multiplicity as high as 100.Comment: 3 pages, 3 figures, Contribution to ICHEP08, 34th International
Conference on High Energy Physics, Philadelphia, 200
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