1,369 research outputs found
Pulsar wind nebulae in supernova remnants
A spherically symmetric model is presented for the interaction of a pulsar
wind with the associated supernova remnant. This results in a pulsar wind
nebula whose evolution is coupled to the evolution of the surrounding supernova
remnant. This evolution can be divided in three stages. The first stage is
characterised by a supersonic expansion of the pulsar wind nebula into the
freely expanding ejecta of the progenitor star. In the next stage the pulsar
wind nebula is not steady; the pulsar wind nebula oscillates between
contraction and expansion due to interaction with the reverse shock of the
supernova remnant: reverberations which propagate forward and backward in the
remnant. After the reverberations of the reverse shock have almost completely
vanished and the supernova remnant has relaxed to a Sedov solution, the
expansion of the pulsar wind nebula proceeds subsonically. In this paper we
present results from hydrodynamical simulations of a pulsar wind nebula through
all these stages in its evolution. The simulations were carried out with the
Versatile Advection Code.Comment: 10 pages, 9 figures, submitted to Astronomy and Astrophysic
Het einde van duurzame armoede?
Om verschillende redenen stellen bekende individualiseringssociologen als
Ulrich Beck en Antony Giddens dat armoede in laat-moderne samenlevingen
een steeds tijdelijker fenomeen wordt en dat het daarmee eerlijker verspreid
raakt over de hele bevolking. Vroeger of later, zo betogen zij, komt iedereen wel
een korte periode in aanraking met armoede. In dit artikel onderzoeken Achterberg
en Snel de houdbaarheid van deze vertijdelijkingsthese voor Nederland
aan het einde van de vorige eeuw
Analytical model of the interaction force between a rectangular coil and a cuboidal permanent magnet
Diges
Nonlinear shock acceleration beyond the Bohm limit
We suggest a physical mechanism whereby the acceleration time of cosmic rays
by shock waves can be significantly reduced. This creates the possibility of
particle acceleration beyond the knee energy at ~10^15eV. The acceleration
results from a nonlinear modification of the flow ahead of the shock supported
by particles already accelerated to the knee momentum at p ~ p_*. The particles
gain energy by bouncing off converging magnetic irregularities frozen into the
flow in the shock precursor and not so much by re-crossing the shock itself.
The acceleration rate is thus determined by the gradient of the flow velocity
and turns out to be formally independent of the particle mean free path
(m.f.p.). The velocity gradient is, in turn, set by the knee-particles at p ~
p_* as having the dominant contribution to the CR pressure. Since it is
independent of the m.f.p., the acceleration rate of particles above the knee
does not decrease with energy, unlike in the linear acceleration regime. The
reason for the knee formation at p ~ p_* is that particles with are
effectively confined to the shock precursor only while they are within limited
domains in the momentum space, while other particles fall into
``loss-islands'', similar to the ``loss-cone'' of magnetic traps. This
structure of the momentum space is due to the character of the scattering
magnetic irregularities. They are formed by a train of shock waves that
naturally emerge from unstably growing and steepening magnetosonic waves or as
a result of acoustic instability of the CR precursor. These losses steepen the
spectrum above the knee, which also prevents the shock width from increasing
with the maximum particle energy.Comment: aastex, 13 eps figure
A current driven instability in parallel, relativistic shocks
Recently, Bell has reanalysed the problem of wave excitation by cosmic rays
propagating in the pre-cursor region of a supernova remnant shock front. He
pointed out a strong, non-resonant, current-driven instability that had been
overlooked in the kinetic treatments, and suggested that it is responsible for
substantial amplification of the ambient magnetic field. Magnetic field
amplification is also an important issue in the problem of the formation and
structure of relativistic shock fronts, particularly in relation to models of
gamma-ray bursts. We have therefore generalised the linear analysis to apply to
this case, assuming a relativistic background plasma and a monoenergetic,
unidirectional incoming proton beam. We find essentially the same non-resonant
instability noticed by Bell, and show that also under GRB conditions, it grows
much faster than the resonant waves. We quantify the extent to which thermal
effects in the background plasma limit the maximum growth rate.Comment: 8 pages, 1 figur
The Evolution of Hydrocarbons in Saturn's Northern Storm Region
The massive storm at 40N on Saturn that began in December 2010 has produced significant and lasting effects in the northern hemisphere on temperature and species abundances (Fletcher et aL 2011). The northern storm region was observed on several occasions between March 2011 and April 2012 by Cassini's Composite Infrared Spectrometer (CIRS) at a spectral resolution (0.5/cm) which permits the study of trace species in Saturn's stratosphere. During this time period, stratospheric temperatures in regions referred to as "beacons" (warm regions at specific longitudes at the latitude of the storm) became significantly warmer than pre-storm values of 140K, peaking near 220K, and subsequently cooling. These warm temperatures led to greatly enhanced infrared emission due to C4H2, C3H4, C2H2, and C2H6 in the stratosphere as well as the first detection of C2H4 on Saturn in the thermal infrared (Hesman et al. 2012). Using CH4 as a thermometer of Saturn's stratosphere in the beacon regions, we can derive the mixing ratios of each of these molecules. The most common hydrocarbons (C2H2 and C2H6) serve as dynamical tracers on Saturn and their abundances may constrain vertical motion in the stratosphere. All of these hydrocarbons are products of methane photolysis. Since many of the photochemical reactions that produce heavier hydrocarbons such as C4H2 and C3H4 are temperature sensitive, the beacon region provides a natural laboratory for studying these reactions on Saturn. We will discuss the time evolution of the abundances of each of these hydrocarbons from their pre-storm values, through the period of maximum heating , and during the period of cooling that is taking place in Saturn's stratosphere
Trace Species Identified in Saturn's Northern Storm Region
The massive storm at 40degN on Saturn that began in December 2010 has produced significant and lasting effects in the northern hemisphere on temperature and species abundances [I}. The northern storm region was observed at 0.5/cm spectral resolution in March 2011 by Cassini's Composite Infrared Spectrometer (CIRS). Temperatures in the stratosphere as high as 190 K were derived from CIRS spectra in warm regions referred to as "beacons". Other longitudes exhibit cold temperatures in the upper troposphere. These unusual conditions allow us to identify rare species such as C4H2, C3H4, and CO2 in the stratosphere, as well as to measure changes in the abundance of phosphine (PH3) in the troposphere. Phosphine is a disequilibrium species whose abundance is a tracer of upwelling from the deep atmosphere
Environments for Magnetic Field Amplification by Cosmic Rays
We consider a recently discovered class of instabilities, driven by cosmic
ray streaming, in a variety of environments. We show that although these
instabilities have been discussed primarily in the context of supernova driven
interstellar shocks, they can also operate in the intergalactic medium and in
galaxies with weak magnetic fields, where, as a strong source of helical
magnetic fluctuations, they could contribute to the overall evolution of the
magnetic field. Within the Milky Way, these instabilities are strongest in warm
ionized gas, and appear to be weak in hot, low density gas unless the injection
efficiency of cosmic rays is very high.Comment: 9 pages, 8 figures; Accepted to Ap
Intercomparison of carbonate chemistry measurements on a cruise in northwestern European shelf seas
Four carbonate system variables were measured in surface waters during a cruise aimed at investigating ocean acidification impacts traversing northwestern European shelf seas in the summer of 2011. High-resolution surface water data were collected for partial pressure of carbon dioxide (pCO2; using two independent instruments) and pH using the total pH scale (pHT), in addition to discrete measurements of total alkalinity and dissolved inorganic carbon. We thus overdetermined the carbonate system (four measured variables, two degrees of freedom), which allowed us to evaluate the level of agreement between the variables on a cruise whose main aim was not intercomparison, and thus where conditions were more representative of normal working conditions. Calculations of carbonate system variables from other measurements generally compared well with direct observations of the same variables (Pearson’s correlation coefficient always greater than or equal to 0.94; mean residuals were similar to the respective accuracies of the measurements). We therefore conclude that four of the independent data sets of carbonate chemistry variables were of high quality. A diurnal cycle with a maximum amplitude of 41 μatm was observed in the difference between the pCO2 values obtained by the two independent analytical pCO2 systems, and this was partly attributed to irregular seawater flows to the equilibrator and partly to biological activity inside the seawater supply and one of the equilibrators. We discuss how these issues can be addressed to improve carbonate chemistry data quality on future research cruises
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