20,411 research outputs found
Selfsimilar time dependent shock structures
Diffusive shock acceleration as an astrophysical mechanism for accelerating charged particles has the advantage of being highly efficient. This means however that the theory is of necessity nonlinear; the reaction of the accelerated particles on the shock structure and the acceleration process must be self-consistently included in any attempt to develop a complete theory of diffusive shock acceleration. Considerable effort has been invested in attempting, at least partially, to do this and it has become clear that in general either the maximum particle energy must be restricted by introducing additional loss processes into the problem or the acceleration must be treated as a time dependent problem (Drury, 1984). It is concluded that stationary modified shock structures can only exist for strong shocks if additional loss processes limit the maximum energy a particle can attain. This is certainly possible and if it occurs the energy loss from the shock will lead to much greater shock compressions. It is however equally possible that no such processes exist and we must then ask what sort of nonstationary shock structure develops. The ame argument which excludes stationary structures also rules out periodic solutions and indeed any solution where the width of the shock remains bounded. It follows that the width of the shock must increase secularly with time and it is natural to examine the possibility of selfsimilar time dependent solutions
Stress Corrosion Cracking of titanium alloys - SCC of aluminum alloys, polarization of titanium alloys in hydrogen chloride and correlation of titanium and aluminum SCC behavior Quarterly progress report, 1 Oct. 1968 - 31 Mar. 1969
Stress corrosion cracking of titanium alloys and aluminum alloy
Magnetic fields in ring galaxies
Many galaxies contain magnetic fields supported by galactic dynamo action.
However, nothing definitive is known about magnetic fields in ring galaxies.
Here we investigate large-scale magnetic fields in a previously unexplored
context, namely ring galaxies, and concentrate our efforts on the structures
that appear most promising for galactic dynamo action, i.e. outer star-forming
rings in visually unbarred galaxies. We use tested methods for modelling
galactic dynamos, taking into account the available
observational information concerning ionized interstellar matter in ring
galaxies. Our main result is that dynamo drivers in ring galaxies are strong
enough to excite large-scale magnetic fields in the ring galaxies studied. The
variety of dynamo driven magnetic configurations in ring galaxies obtained in
our modelling is much richer than that found in classical spiral galaxies. In
particular, various long-lived transients are possible. An especially
interesting case is that of NGC 4513 where the ring counter-rotates with
respect to the disc. Strong shear in the region between the disc and the ring
is associated with unusually strong dynamo drivers for the counter-rotators.
The effect of the strong drivers is found to be unexpectedly moderate. With
counter-rotation in the disc, a generic model shows that a steady mixed parity
magnetic configuration, unknown for classical spiral galaxies, may be excited,
although we do not specifically model NGC 4513. We deduce that ring galaxies
constitute a morphological class of galaxies in which identification of
large-scale magnetic fields from observations of polarized radio emission, as
well as dynamo modelling, may be possible. Such studies have the potential to
throw additional light on the physical nature of rings, their lifetimes and
evolution.Comment: 25 pages, 12 figure
Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants
The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium
When the working day is through: The end of work as identity?
This article seeks to present a counter-case to the ‘end of work thesis’ advocated by writers such as Beck, Sennett and Bauman. It argues that work remains a significant locus of personal identity and that the depiction by these writers of endemic insecurity in the workplace is inaccurate and lacks empirical basis. The article draws upon case study data to illustrate how, across a range of workplaces, work remains an importance source of identity, meaning and social affiliation
Money in monetary policy design: monetary cross-checking in the New-Keynesian model
In the New-Keynesian model, optimal interest rate policy under uncertainty is formulated without reference to monetary aggregates as long as certain standard assumptions on the distributions of unobservables are satisfied. The model has been criticized for failing to explain common trends in money growth and inflation, and that therefore money should be used as a cross-check in policy formulation (see Lucas (2007)). We show that the New-Keynesian model can explain such trends if one allows for the possibility of persistent central bank misperceptions. Such misperceptions motivate the search for policies that include additional robustness checks. In earlier work, we proposed an interest rate rule that is near-optimal in normal times but includes a cross-check with monetary information. In case of unusual monetary trends, interest rates are adjusted. In this paper, we show in detail how to derive the appropriate magnitude of the interest rate adjustment following a significant cross-check with monetary information, when the New-Keynesian model is the central bank’s preferred model. The cross-check is shown to be effective in offsetting persistent deviations of inflation due to central bank misperceptions. Keywords: Monetary Policy, New-Keynesian Model, Money, Quantity Theory, European Central Bank, Policy Under Uncertaint
Opposite magnetic polarity of two photospheric lines in single spectrum of the quiet Sun
We study the structure of the photospheric magnetic field of the quiet Sun by
investigating weak spectro-polarimetric signals. We took a sequence of Stokes
spectra of the Fe I 630.15 nm and 630.25 nm lines in a region of quiet Sun near
the disk center, using the POLIS spectro-polarimeter at the German VTT on
Tenerife. The line cores of these two lines form at different heights in the
atmosphere. The 3 noise level of the data is about 1.8 . We present co-temporal and co-spatial Stokes- profiles of the Fe I
630 nm line pair, where the two lines show opposite polarities in a single
spectrum. We compute synthetic line profiles and reproduce these spectra with a
two-component model atmosphere: a non-magnetic component and a magnetic
component. The magnetic component consists of two magnetic layers with opposite
polarity: the upper one moves upwards while the lower one moves downward.
In-between, there is a region of enhanced temperature. The Stokes- line pair
of opposite polarity in a single spectrum can be understood as a magnetic
reconnection event in the solar photosphere. We demonstrate that such a
scenario is realistic, but the solution may not be unique.Comment: 4 pages, 5 figures, accepted in Astronomy & Astrophysics Letter
Coupled-channels effects in elastic scattering and near-barrier fusion induced by weakly bound nuclei and exotic halo nuclei
The influence on fusion of coupling to the breakup process is investigated
for reactions where at least one of the colliding nuclei has a sufficiently low
binding energy for breakup to become an important process. Elastic scattering,
excitation functions for sub-and near-barrier fusion cross sections, and
breakup yields are analyzed for Li+Co. Continuum-Discretized
Coupled-Channels (CDCC) calculations describe well the data at and above the
barrier. Elastic scattering with Li (as compared to Li) indicates
the significant role of breakup for weakly bound projectiles. A study of
He induced fusion reactions with a three-body CDCC method for the
He halo nucleus is presented. The relative importance of breakup and
bound-state structure effects on total fusion is discussed.Comment: 29 pages, 9 figure
Measurement of transparency ratios for protons from short-range correlated pairs
Nuclear transparency, Tp(A), is a measure of the average probability for a
struck proton to escape the nucleus without significant re-interaction.
Previously, nuclear transparencies were extructed for quasi-elastic A(e,e'p)
knockout of protons with momentum below the Fermi momentum, where the spectral
functions are well known. In this paper we extract a novel observable, the
transparency ratio, Tp(A)/T_p(12C), for knockout of high-missing-momentum
protons from the breakup of short range correlated pairs (2N-SRC) in Al, Fe and
Pb nuclei relative to C. The ratios were measured at momentum transfer Q^2 >
1.5 (GeV/c)^2 and x_B > 1.2 where the reaction is expected to be dominated by
electron scattering from 2N-SRC. The transparency ratios of the knocked-out
protons coming from 2N-SRC breakup are 20 - 30% lower than those of previous
results for low missing momentum. They agree with Glauber calculations and
agree with renormalization of the previously published transparencies as
proposed by recent theoretical investigations. The new transparencies scale as
A^-1/3, which is consistent with dominance of scattering from nucleons at the
nuclear surface.Comment: 6 pages, 4 figure
Detection of microgauss coherent magnetic fields in a galaxy five billion years ago
Magnetic fields play a pivotal role in the physics of interstellar medium in
galaxies, but there are few observational constraints on how they evolve across
cosmic time. Spatially resolved synchrotron polarization maps at radio
wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies
that are believed to grow from a seed field via a dynamo effect. To directly
test and characterize this theory requires magnetic field strength and geometry
measurements in cosmologically distant galaxies, which are challenging to
obtain due to the limited sensitivity and angular resolution of current radio
telescopes. Here, we report the cleanest measurements yet of magnetic fields in
a galaxy beyond the local volume, free of the systematics traditional
techniques would encounter. By exploiting the scenario where the polarized
radio emission from a background source is gravitationally lensed by a
foreground galaxy at z = 0.439 using broadband radio polarization data, we
detected coherent G magnetic fields in the lensing disk galaxy as seen 4.6
Gyrs ago, with similar strength and geometry to local volume galaxies. This is
the highest redshift galaxy whose observed coherent magnetic field property is
compatible with a mean-field dynamo origin.Comment: 29 pages, 5 figures (including Supplementary Information). Published
in Nature Astronomy on August 28, 201
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