1,937 research outputs found
3D Structures on Relativistic Jets
The properties of wave-like helically twisted normal mode structures on
steady relativistic jets are summarized. Wave speeds are a function of the
wavelength and less than the jet speed. However, normal mode interference can
lead to both stationary and superluminal phase effects. A maximum pressure
fluctuation criterion suggested by numerical simulations of axisymmetric
relativistic jets is used to find the maximum asymmetric jet distortions and
velocity fluctuations. Cyclic transverse velocity fluctuation can lead to
variation in the flow direction on the order of the relativistic beaming angle.
Resulting variation in the Doppler boost factor can lead to significant
brightness asymmetries as helical structures twist around the jet beam. Growth
of these structures is reduced as the jet density, Lorentz factor or Mach
number are increased. Maximum jet distortion is reduced as the Lorentz factor
increases and this suggests a reduction in mass entrainment or other non-linear
disruptive processes that influence the morphological development of radio
sources.Comment: to appear in Life Cycles of Radio Galaxies, ed. J. Biretta et al.,
New Astronomy Reviews; 8 pages, including 3 figure
Anatomy of helical relativistic jets: The case of S5 0836+710
Helical structures are common in extragalactic jets. They are usually
attributed in the literature to periodical phenomena in the source (e.g.,
precession). In this work, we use VLBI data of the radio-jet in the quasar S5
0836+710 and hypothesize that the ridge-line of helical jets like this
corresponds to a pressure maximum in the jet and assume that the helically
twisted pressure maximum is the result of a helical wave pattern. For our
study, we use observations of the jet in S5 0836+710 at different frequencies
and epochs. The results show that the structures observed are physical and not
generated artificially by the observing arrays. Our hypothesis that the
observed intensity ridge-line can correspond to a helically twisted pressure
maximum is confirmed by our observational tests. This interpretation allows us
to explain jet misalignment between parsec and kiloparsec scales when the
viewing angle is small, and also brings us to the conclusion that
high-frequency observations may show only a small region of the jet flow
concentrated around the maximum pressure ridge-line observed at low
frequencies. Our work provides a potential explanation for the apparent
transversal superluminal speeds observed in several extragalactic jets by means
of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa
Population and Reproductive Health in National Adaptation Programs of Action
This paper reviews 41 National Adaptation Programmes of Action (NAPAs) submitted by Least Developed Countries (LDCs) to the United Nations Framework Convention on Climate Change (UNFCCC), and identifies the range of interventions included in countries' priority adaptation actions. The review found near-universal recognition among the NAPAs of the importance of population considerations as a central pillar in climate change adaptation
The role of Kelvin-Helmholtz instability in the internal structure of relativistic outflows. The case of the jet in 3C 273
Relativistic outflows represent one of the best-suited tools to probe the
physics of AGN. Numerical modelling of internal structure of the relativistic
outflows on parsec scales provides important clues about the conditions and
dynamics of the material in the immediate vicinity of the central black holes
in AGN. We investigate possible causes of the structural patterns and
regularities observed in the parsec-scale jet of the well-known quasar 3C 273.
We present here the results from a 3D relativistic hydrodynamics numerical
simulation based on the parameters given for the jet by Lobanov & Zensus
(2001), and one in which the effects of jet precession and the injection of
discrete components have been taken into account. We compare the model with the
structures observed in 3C 273 using very long baseline interferometry and
constrain the basic properties of the flow. We find growing perturbation modes
in the simulation with similar wavelengths to those observed, but with a
different set of wave speeds and mode identification. If the observed longest
helical structure is produced by the precession of the flow, longer precession
periods should be expected. Our results show that some of the observed
structures could be explained by growing Kelvin-Helmholtz instabilities in a
slow moving region of the jet. However, we point towards possible errors in the
mode identification that show the need of more complete linear analysis in
order to interpret the observations. We conclude that, with the given viewing
angle, superluminal components and jet precession cannot explain the observed
structures.Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages.
Higher resolution plots available on request to [email protected] and
at http://www.mpifr-bonn.mpg.de/staff/mperuch
The space shuttle orbiter remote manipulator positioning mechanism
The major subassemblies of the Manipulator Positioning Mechanism (MPM) are described and illustrated. The Space Shuttle Orbiter design provides that the MPM may be mounted on either left or right sides of the payload bay, or both sides if two are flown. This MPM is provided for the purpose of securing the remote arm in stowed position during lift-off, boost, and landing. It also provides the deploy, latch and unlatch capabilities of the Remote Manipulator System arm
Ground-water resources of the lower Hillsboro Canal area, Southeastern Florida
This study was done to determine the amount and kinds of water being
produced from the lower Hillsboro Canal Area in Palm Beach and
Broward counties.
All of the potable ground water being produced from the
Biscayne aquifer is developed from the canal through infiltration.
Rainfall in the area is the ultimate source for all of the water.
Careful control and management will allow the development of
large quantities of water from the canal toward Lake Okeechobee, but
a fresh water head must be maintained along the contact of fresh water
with sea water to prevent salt water intrusion. (PDF contains 51 pages.
Linking Population, Fertility and Family Planning with Adaptation to Climate Change: Views from Ethiopia
As global climate change unfolds, its effects are being felt disproportionately in the world's poorest countries and among the groups of people least able to cope. Many of the countries hardest hit by the effects of climate change also face rapid population growth, with their populations on track to double by 2050.Population Action International (PAI) and Miz-Hasab Research Center (MHRC), in collaboration with the Joint Global Change Research Institute (JGCRI), studied which groups are most vulnerable, what community members say they need to adapt, and the role of family planning and reproductive health in increasing resilience to climate change impacts.The study was carried out in 2008-2009 in peri-urban and rural areas of two regions in Ethiopia: the Oromia region and the Southern Nations, Nationalities and People's (SNNP) region
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