4,740 research outputs found
Bridging structure and function in semi-arid ecosystems by integrating remote sensing and ground based measurements
The Southwestern US is projected to continue the current significant warming trend, with increased variability in the timing and magnitude of rainfall events. The effects of these changes in climate are already evident in the form of multi-year droughts which have resulted in the widespread mortality of woody vegetation across the region. Therefore, the need to monitor and model forest mortality and carbon dynamics at the landscape and regional scale is an essential component of regional and global climate mitigation strategies, and critical if we are to understand how the imminent state transitions taking place in forests globally will affect climate forcing and feedbacks. Remote sensing offers the only solution to multitemporal regional observation, yet many challenges exist with employing modern remote sensing solutions in highly stressed vegetation characteristic of semi-arid biomes, making one of the most expansive biomes on the globe also one of the most difficult to accu- rately monitor and model. The goal of this research was to investigate how changes in the structure of semi-arid woodlands following forest mortality impacts ecosystem function, and to determine how this question can be addressed using remotely sensed data sets. I focused primarily on Pinus edulis and Juniperous monosperma (piñon-juniper) woodlands, and took advantage of an existing manipulation experiment where mortality was imposed on all of the large piñon (¡ 7 cm dbh) in a 4 ha PJ woodland in 2009 and the ecosystem functional responses have been quantified using eddy covariance. A nearby intact PJ woodland, also instrumented with eddy covariance, was used as a control for this experiment. I tested the ability of high resolution remote sensing data to mechanistically describe the patterns in overstory mortality and understory green-up in this manipulated woodland by comparing it to the intact woodland, and observed the heterogeneous response of the understory as a function of cover type. I also investigated the relationship between changes in soil water content and the greenness of the canopy, noting that in the disturbed woodland, I observed a decoupling between how the canopy was measured remotely (e.g., via vegetation indices, VI) and photosynthesis. This is significant in that it potentially represents a significant source of error in using existing light use efficiency models of carbon uptake in these disturbed woodlands. This research also suggested that leveraging remote sensing data which measures in the red-edge portion of reflected light can provide increased sensitivity to the low leaf area, ephemeral pulses of greenup that were identified in the disturbed woodland, post-canopy mortality. Given these findings, I developed a hierarchy of simple linear models to test how well vegetation indices acquired through different spatial resolution sensors (Land- sat and RapidEye) were able to predict carbon uptake in both intact and disturbed piñon-juniper woodlands. The vegetation indices used were a moisture sensitive VI, and a red-edge leveraging VI from these sensors, and I compared estimates of carbon uptake derived from these models to the Gross Primary Productivity estimated from tower-based eddy covariance at both the manipulated and intact piñon-juniper sites. I determined that the red-edge VI and the moisture sensitive VI both constrained uncertainty associated with carbon uptake, but that the variability in satellite view angle from scene to scene can impose a significant amount of noise in sparse canopy ecosystems. Finally, given the extent and prevalence of J. monosperma across the region, and its complex growth morphology, I tested the ability of aerial lidar to quantify the biomass of juniper. In this simplified case study, I developed a method- ology to relate the volume of canopy measured via lidar to the equivalent stem area at the root crown. By working in a single species ecosystem, I circumvented many challenges associated with driving allometries remotely, but also present a work-flow that I intend to adapt to more complex systems, namely piñon-juniper woodlands. Together, this work describes and addresses existing challenges with respect to us- ing remote sensing to understand both the structure and function of piñon-juniper woodlands, and how it changes in response to widespread piñon mortality. It provides several new techniques to mitigate the difficulties associated with monitoring mortality / recovery dynamics, predicting canopy function, and determining ecosystem state parameters in these complex, sensitive biomes
Influence of momentum-dependent interactions on balance energy and mass dependence
We aim to study the role of momentum-dependent interactions in transverse
flow as well as in its disappearance. For the present study, central collisions
involving mass between 24 and 394 are considered. We find that
momentum-dependent interactions have different impact in lighter colliding
nuclei compared to heavier colliding nuclei. In lighter nuclei, the
contribution of mean field towards the flow is smaller compared to heavier
nuclei where binary nucleon-nucleon collisions dominate the scene. The
inclusion of momentum-dependent interactions also explains the energy of
vanishing flow in reaction which was not possible with the
static equation of state. An excellent agreement of our theoretical attempt is
found for balance energy with experimental data throughout the periodic table
Disappearance of Transverse Flow in Central Collisions for Heavier Nuclei
For the first time, mass dependence of balance energy only for heavier
systems has been studied. Our results are in excellent agreement with the data
which allow us to predict the balance energy of U+U, for the first time, around
37-39 MeV/nucleon. Also our results indicate a hard equation of state along
with nucleon-nucleon cross-section around 40 mb.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
On the sensitivity of the energy of vanishing flow towards mass asymmetry of colliding nuclei
We demonstrate the role of the mass asymmetry in the energy of vanishing flow
by studying asymmetric reactions throughout the periodic table and over entire
colliding geometry. Our results, which are almost independent of the system
size and as well as of the colliding geometries indicate a sizable effect of
the asymmetry of the reaction on the energy of vanishing flow
Particle-particle correlations and the space-time structure of heavy ion collisions
The present status of the use of two-particle intensity interferometry as a
diagnostic tool to study the space-time dynamics of intermediate energy heavy
ion collisions is examined. Calculations for the two-proton and two-pion
correlation functions are presented and compared to experiment. The
calculations are based on the nuclear Boltzmann-Uehling-Uhlenbeck transport
theory.Comment: 20 pages, Preprint MSUCL-864 (to be published in Progress in Particle
and Nuclear Physics, Vol. 30
Gamow-Teller strength distributions for nuclei in pre-supernova stellar cores
Electron-capture and -decay of nuclei in the core of massive stars
play an important role in the stages leading to a type II supernova explosion.
Nuclei in the f-p shell are particularly important for these reactions in the
post Silicon-burning stage of a presupernova star. In this paper, we
characterise the energy distribution of the Gamow-Teller Giant Resonance (GTGR)
for mid-fp-shell nuclei in terms of a few shape parameters, using data obtained
from high energy, forward scattering (p,n) and (n,p) reactions. The energy of
the GTGR centroid is further generalised as function of nuclear
properties like mass number, isospin and other shell model properties of the
nucleus. Since a large fraction of the GT strength lies in the GTGR region, and
the GTGR is accessible for weak transitions taking place at energies relevant
to the cores of presupernova and collapsing stars, our results are relevant to
the study of important -capture and -decay rates of arbitrary,
neutron-rich, f-p shell nuclei in stellar cores. Using the observed GTGR and
Isobaric Analog States (IAS) energy systematics we compare the coupling
coefficients in the Bohr-Mottelson two particle interaction Hamiltonian for
different regions of the Isotope Table.Comment: Revtex, 28 pages +7 figures (PostScript Figures, uuencoded, filename:
Sutfigs.uu). If you have difficulty printing the figures, please contact
[email protected]. Accepted for publication in Phys. Rev. C, Nov 01,
199
Nuclear Flow Excitation Function
We consider the dependence of collective flow on the nuclear surface
thickness in a Boltzmann--Uehling--Uhlenbeck transport model of heavy ion
collisions. Well defined surfaces are introduced by giving test particles a
Gaussian density profile of constant width. Zeros of the flow excitation
function are as much influenced by the surface thickness as the nuclear
equation of state, and the dependence of this effect is understood in terms of
a simple potential scattering model. Realistic calculations must also take into
account medium effects for the nucleon--nucleon cross section, and impact
parameter averaging. We find that balance energy scales with the mass number as
, where has a numerical value between 0.35 and 0.5, depending on
the assumptions about the in-medium nucleon-nucleon cross section.Comment: 11 pages (LaTeX), 7 figures (not included), MSUCL-884, WSU-NP-93-
Isospin dependence of collective flow in heavy-ion collisions at intermediate energies
Within the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck
(BUU) model using initial proton and neutron densities calculated from the
nonlinear relativistic mean-field (RMF) theory, we compare the strength of
transverse collective flow in reactions and
, which have the same mass number but different neutron/proton
ratios. The neutron-rich system () is found to show
significantly stronger negative deflection and consequently has a higher
balance energy, especially in peripheral collisions. NOTE ADDED IN PROOF: The
new phenomenon predicted in this work has just been confirmed by an experiment
done by G.D. Westfall et al. using the NSCL/MSU radioactive beam facility and a
spartan soccer. A paper by R. Pak et al. is submitted to PRL to report the
experimental result.Comment: Latex file, 9 pages, 4 figures availabe upon request; Phys. Rev.
Lett. (June 3, 1996) in pres
Design, Implementation and First Measurements with the Medipix Neutron Camera in CMS
The Medipix detector is the first device dedicated to measuring mixed-field
radiation in the CMS cavern and able to distinguish between different particle
types. Medipix2-MXR chips bump bonded to silicon sensors with various neutron
conversion layers developed by the IEAP CTU in Prague were successfully
installed for the 2008 LHC start-up in the CMS experimental and services
caverns to measure the flux of various particle types, in particular neutrons.
They have operated almost continuously during the 2010 run period, and the
results shown here are from the proton run between the beginning of July and
the end of October 2010. Clear signals are seen and different particle types
have been observed during regular LHC luminosity running, and an agreement in
the measured flux rate is found with the simulations. These initial results are
promising, and indicate that these devices have the potential for further and
future LHC and high energy physics applications as radiation monitoring devices
for mixed field environments, including neutron flux monitoring. Further
extensions are foreseen in the near future to increase the performance of the
detector and its coverage for monitoring in CMS.Comment: 15 pages, 16 figures, submitted to JINS
Neutrons from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 AMeV
We measured neutron triple-differential cross sections from
multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 \AMeV. The
reaction plane for each collision was estimated from the summed transverse
velocity vector of the charged fragments emitted in the collision. We examined
the azimuthal distribution of the triple-differential cross sections as a
function of the polar angle and the neutron rapidity. We extracted the average
in--plane transverse momentum and the normalized
observable , where is the neutron
transverse momentum, as a function of the neutron center-of-mass rapidity, and
we examined the dependence of these observables on beam energy. These
collective flow observables for neutrons, which are consistent with those of
protons plus bound nucleons from the Plastic Ball Group, agree with the
Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum--dependent
interaction. Also, we calculated the polar-angle-integrated maximum azimuthal
anisotropy ratio R from the value of .Comment: 20 LaTeX pages. 11 figures to be faxed on request, send email to
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