1,733 research outputs found
Simulating spatial variability of cereal yields from historical yield maps and satellite imagery
[Abstract]: The management of spatial variability of crop yields relies on the availability of affordable and accurate spatial data. Yield maps are a direct measure of the crop yields, however, costs and difficulties in collection and processing to generate yield maps results in poor availability of such data in Australia. In this study, we used historical mid-season normalised difference vegetation index (NDVI), generated from Landsat imagery over 4 years. Using linear regression model, the NDVI was compared to the actual yield map from a 257 ha paddock. The difference between actual and predicted yield showed that 77% and 93% of the paddock area had an error of <20% and <30%, respectively. The linear model obtained in the paddock was used to simulate crop yield for an adjoining paddock of 162 ha. On an average of 4 years, the difference between actual and simulated yield showed that 87% of the paddock had an error of <20%. However, this error varied from season to season. Paddock area with <20% error increased exponentially with decreasing in-crop rainfall between anthesis and crop maturity. Furthermore, the error in simulating crop yield also varied with the soil constraints. Paddock zones with high concentrations of subsoil chloride and surface soil exchangeable sodium percentage generally had higher percent of error in simulating crop yields. Satellite imagery consistently over-predicted cereal yields in areas with subsoil constraints, possibly due to chloride-induced water stress during grain filling. The simulated yield mapping methodology offers an opportunity to identify within-field spatial variability using satellite imagery as a surrogate measure of biomass. However, the ability to successfully simulate crop yields at farm scale or regional scale requires wider evaluation across different soil types and climatic conditions
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
Warping and Precession of Accretion Disks Around Magnetic Stars: Nonlinear Evolution
The inner region of the accretion disk around a magnetized star (T Tauri
star, white dwarf or neutron star) is subjected to magnetic torques that induce
warping and precession of the disk. These torques arise from the interaction
between the stellar field and the induced electric currents in the disk. We
carry out numerical simulations of the nonlinear evolution of warped, viscous
accretion disks driven by the magnetic torques. We show that the disk can
develop into a highly warped steady state in which the disk attains a fixed
(warped) shape and precesses rigidly. The warp is most pronounced at the disk
inner radius (near the magnetosphere boundary). As the system parameters (such
as accretion rate) change, the disk can switch between a completely flat state
(warping stable) and a highly warped state. The precession of warped disks may
be responsible for a variety of quasi-periodic oscillations or radiation flux
variabilities observed in many different systems, including young stellar
objects and X-ray binaries.Comment: 16 pages, 7 figures; extended parameter searches, changes in
discussion; accepted for publication in Ap
Humanitarianism, race, and denial: The International Committee of the Red Cross and Kenyaâs Mau Mau rebellion, 1952-60
This article explores the humanitarian principles of universality, impartiality, and neutrality, and the moral boundaries of humanitarian action in the past. It does so through a case study of the International Committee of the Red Cross (ICRC) and Kenyaâs Mau Mau rebellion (1952-60). Drawing on the vocabulary of denial, and on the spaces in which something is both âknownâ and ânot-knownâ, it asks why the ICRC were reluctant to respond to allegations of torture in Kenyaâs detention camps and prisons, and explores reasons for its later positive assessment of detention conditions. The article advances two arguments about the ICRC and Mau Mau. Firstly, that ideas about race and cultural difference helped provide a moral justification for the avoidance of politically-sensitive decisions about Kenya and may have fed into the down-playing of evidence of abuse. Secondly, that the ICRC became entangled in the British Governmentâs project of denial. By drawing on the language of detention and interrogation developed by the British, the ICRC fed into and confirmed an official version of events. In doing so, the article reveals the uneasy relationship between key humanitarian principles and denial, and highlights how the impartiality of classical humanitarian actors like the ICRC could be invoked by powers committing abuses and cover-ups
On the evolution of anomalous X-ray pulsars and soft gamma ray repeaters with fallback disks
We show that the period clustering of anomalous X-ray pulsars (AXPs) and soft
gamma-ray repeaters (SGRs), their X-ray luminosities, ages and statistics can
be explained with fallback disks with large initial specific angular momentum.
The disk evolution models are developed by comparison to self-similar
analytical models. The initial disk mass and angular momentum set the viscous
timescale. An efficient torque, with (1 - w^2) dependence on the fastness
parameter w leads to period clustering in the observed AXP-SGR period range
under a wide range of initial conditions. The timescale t_0 for the early
evolution of the fallback disk, and the final stages of fallback disk
evolution, when the disk becomes passive, are the crucial determinants of the
evolution. The disk becomes passive at temperatures around 100 K, which
provides a natural cutoff for the X-ray luminosity and defines the end of
evolution in the observable AXP and SGR phase. This low value for the minimum
temperature for active disk turbulence indicates that the fallback disks are
active up to a large radius greater than ~10^{12} cm. We find that transient
AXPs and SGRs are likely to be older than their persistent cousins. A fallback
disk with mass transfer rates corresponding to the low quiescent X-ray
luminosities of the transient sources in early evolutionary phases would have a
relatively lower initial mass, such that the mass-flow rate in the disk is not
sufficient for the inner disk to penetrate into the light cylinder of the young
neutron star, making mass accretion onto the neutron star impossible. The
transient AXP phase therefore must start later. The model results imply that
the transient AXP/SGRs, although older, are likely to be similar in number to
persistent sources (abridged).Comment: 42 pages, 22 figures. Accepted for publication in the Astrophysical
Journa
Highly-symmetric travelling waves in pipe flow
The recent theoretical discovery of finite-amplitude travelling waves in pipe
flow has re-ignited interest in the transitional phenomena that Osborne
Reynolds studied 125 years ago. Despite all being unstable, these waves are
providing fresh insight into the flow dynamics. Here we describe two new
classes of highly-symmetric travelling waves (possessing rotational,
shift-&-reflect and mirror symmetries) and report a new family of
mirror-symmetric waves which is the first found in pipe flow not to have
shift-&-reflect symmetry. The highly-symmetric waves appear at lower Reynolds
numbers than the originally-discovered non-mirror-symmetric waves found by
Faisst & Eckhardt 2003 and Wedin & Kerswell 2004 and have much higher wall
shear stresses. The first M-class comprises of the various
discrete-rotationally-symmetric analogues of the mirror-symmetric wave found in
Pringle & Kerswell (2007) and have a distinctive double layer structure of fast
and slow streaks across the pipe radius. The second N-class has the more
familiar separation of fast streaks to the exterior and slow streaks to the
interior and looks the precursor to the class of non-mirror-symmetric waves
already known.Comment: 16 pages, 8 figures, for Phil Trans theme issue on pipe flo
Megamaser Disks in Active Galactic Nuclei
Recent spectroscopic and VLBI-imaging observations of bright extragalactic
water maser sources have revealed that the megamaser emission often originates
in thin circumnuclear disks near the centers of active galactic nuclei (AGNs).
Using general radiative and kinematic considerations and taking account of the
observed flux variability, we argue that the maser emission regions are clumpy,
a conclusion that is independent of the detailed mechanism (X-ray heating,
shocks, etc.) driving the collisionally pumped masers. We examine scenarios in
which the clumps represent discrete gas condensations (i.e., clouds) and do not
merely correspond to velocity irregularities in the disk. We show that even two
clouds that overlap within the velocity coherence length along the line of
sight could account (through self-amplification) for the entire maser flux of a
high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and
we suggest that cloud self-amplification likely contributes also to the flux of
the background-amplifying ``systemic'' features in these objects. Analogous
interpretations have previously been proposed for water maser sources in
Galactic star-forming regions. We argue that this picture provides a natural
explanation of the time-variability characteristics of extragalactic megamaser
sources and of their apparent association with Seyfert 2-like galaxies. We also
show that the requisite cloud space densities and internal densities are
consistent with the typical values of nuclear (broad emission-line region-type)
clouds.Comment: 55 pages, 7 figures, AASTeX4.0, to appear in The Astrophysical
Journal (1999 March 1 issue
Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution
Magnetic connections to the plunging region can exert stresses on the inner
edge of an accretion disk around a black hole. We recompute the relativistic
corrections to the thin-disk dynamics equations when these stresses take the
form of a time-steady torque on the inner edge of the disk. The additional
dissipation associated with these stresses is concentrated relatively close
outside the marginally stable orbit, scaling as r to the -7/2 at large radius.
As a result of these additional stresses: spin-up of the central black hole is
retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs
at a/M=0.94; the disk spectrum is extended toward higher frequencies; line
profiles (such as Fe K-alpha) are broadened if the line emissivity scales with
local flux; limb-brightening, especially at the higher frequencies, is
enhanced; and the returning radiation fraction is substantially increased, up
to 58%. This last effect creates possible explanations for both synchronized
continuum fluctuations in AGN, and polarization rises shortward of the Lyman
edge in quasars. We show that no matter what additional stresses occur, when
a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound
on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Magnetically Driven Warping, Precession and Resonances in Accretion Disks
The inner region of the accretion disk onto a rotating magnetized central
star (neutron star, white dwarf or T Tauri star) is subjected to magnetic
torques which induce warping and precession of the disk. The origin of these
torques lies in the interaction between the (induced) surface current on the
disk and the horizontal magnetic field (parallel to the disk) produced by the
inclined magnetic dipole. Under quite general conditions, there exists a
magnetic warping instability in which the magnetic torque drives the disk plane
away from the equatorial plane of the star toward a state where the disk normal
vector is perpendicular to the spin axis. Viscous stress tends to suppress the
warping instability at large radii, but the magnetic torque always dominates as
the disk approaches the magnetosphere boundary. The magnetic torque also drives
the tilted inner disk into retrograde precession around the stellar spin axis.
Moreover, resonant magnetic forcing on the disk can occur which may affect the
dynamics of the disk. The magnetically driven warping instability and
precession may be related to a number observational puzzles, including: (1)
Spin evolution (torque reversal) of accreting X-ray pulsars; (2) Quasi-periodic
oscillations in low-mass X-ray binaries; (3) Super-orbital periods in X-ray
binaries; (4) Photometric period variations of T Tauri stars.Comment: 39 pages including 1 ps figure; Published version; ApJ, 524,
1030-1047 (1999
On the Nature of Part Time Radio Pulsars
The recent discovery of rotating radio transients and the quasi-periodicity
of pulsar activity in the radio pulsar PSR B193124 has challenged the
conventional theory of radio pulsar emission. Here we suggest that these
phenomena could be due to the interaction between the neutron star
magnetosphere and the surrounding debris disk. The pattern of pulsar emission
depends on whether the disk can penetrate the light cylinder and efficiently
quench the processes of particle production and acceleration inside the
magnetospheric gap. A precessing disk may naturally account for the
switch-on/off behavior in PSR B193124.Comment: 9 pages, accepted to ApJ
- âŠ