2,706 research outputs found
On the Early Holocene: Foraging to Early Agriculture
We consider a world in which the mode of food production, foraging or agriculture, is endogenous, and in which technology grows exogenously. Within a model of coalition formation, we allow individuals to rationally form cooperative communities (bands) of foragers or farmers. At the lowest levels of technology, equilibrium entails the grand coalition of foragers, a cooperative structure which avoids over-exploitation of the environment. But at a critical state of technology, the cooperative structure breaks down through an individually rational splintering of the band. At this stage, there can be an increase in work and through the over-exploitation of the environment, a food crisis. In the end, technological growth may lead to a one-way transition from foraging to agriculture.Foraging, Agriculture, Transition, Coalition Formation, Cooperation
From Foraging to Agriculture
We consider a world in which the mode of food production, foraging or agriculture, is endogenous, and in which technology grows exogenously. Using a recent model of coalition formation, we allow individuals to rationally form cooperative communities (bands) of foragers or farmers. At the lowest levels of technology, equilibrium entails the grand coalition of foragers, a cooperative structure which avoids over-exploitation of the environment. But at a critical state of technology, the cooperative structure breaks down through an individually rational splintering of the band. At this stage there can be an increase in work and, through the over-exploitation of the environment, a food crisis. In the end, technological growth leads to a one-way transition from foraging to agriculture.
The collisional relaxation of electrons in hot flaring plasma and inferring the properties of solar flare accelerated electrons from X-ray observations
X-ray observations are a direct diagnostic of fast electrons produced in
solar flares, energized during the energy release process and directed towards
the Sun. Since the properties of accelerated electrons can be substantially
changed during their transport and interaction with the background plasma, a
model must ultimately be applied to X-ray observations in order to understand
the mechanism responsible for their acceleration. A cold thick target model is
ubiquitously used for this task, since it provides a simple analytic
relationship between the accelerated electron spectrum and the emitting
electron spectrum in the X-ray source, with the latter quantity readily
obtained from X-ray observations. However, such a model is inappropriate for
the majority of solar flares in which the electrons propagate in a hot
megaKelvin plasma, because it does not take into account the physics of
thermalization of fast electrons. The use of a more realistic model, properly
accounting for the properties of the background plasma, and the collisional
diffusion and thermalization of electrons, can alleviate or even remove many of
the traditional problems associated with the cold thick target model and the
deduction of the accelerated electron spectrum from X-ray spectroscopy, such as
the number problem and the need to impose an ad hoc low energy cut-off.Comment: 6 pages, 14th Annual International Astrophysics Conference Tampa
proceeding
A Classification Scheme For Turbulent Acceleration Processes In Solar Flares
We establish a classification scheme for stochastic acceleration models
involving low-frequency plasma turbulence in a strongly magnetized plasma. This
classification takes into account both the properties of the accelerating
electromagnetic field, and the nature of the transport of charged particles in
the acceleration region. We group the acceleration processes as either
resonant, non-resonant or resonant-broadened, depending on whether the particle
motion is free-streaming along the magnetic field, diffusive or a combination
of the two. Stochastic acceleration by moving magnetic mirrors and adiabatic
compressions are addressed as illustrative examples. We obtain expressions for
the momentum-dependent diffusion coefficient , both for general forms of
the accelerating force and for the situation when the electromagnetic force is
wave-like, with a specified dispersion relation . Finally,
for models considered, we calculate the energy-dependent acceleration time, a
quantity that can be directly compared with observations of the time profile of
the radiation field produced by the accelerated particles, such as during solar
flares.Comment: 45 pages, submitted to Astrophysical Journa
Suppression of parallel transport in turbulent magnetized plasmas and its impact on the non-thermal and thermal aspects of solar flares
The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent RHESSI observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We then evaluate the effect of these modified transport coefficients on the flare coronal temperature that can be attained, on the post-impulsive-phase cooling of heated coronal plasma, and on the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. We also discuss the possible ways in which anomalous transport processes have an impact on the required overall energy associated with accelerated electrons in solar flares
The role of diffusion in the transport of energetic electrons during solar flares
The transport of the energy contained in suprathermal electrons in solar
flares plays a key role in our understanding of many aspects of flare physics,
from the spatial distributions of hard X-ray emission and energy deposition in
the ambient atmosphere to global energetics. Historically the transport of
these particles has been largely treated through a deterministic approach, in
which first-order secular energy loss to electrons in the ambient target is
treated as the dominant effect, with second-order diffusive terms (in both
energy and angle) being generally either treated as a small correction or even
neglected. We here critically analyze this approach, and we show that spatial
diffusion through pitch-angle scattering necessarily plays a very significant
role in the transport of electrons. We further show that a satisfactory
treatment of the diffusion process requires consideration of non-local effects,
so that the electron flux depends not just on the local gradient of the
electron distribution function but on the value of this gradient within an
extended region encompassing a significant fraction of a mean free path. Our
analysis applies generally to pitch-angle scattering by a variety of
mechanisms, from Coulomb collisions to turbulent scattering. We further show
that the spatial transport of electrons along the magnetic field of a flaring
loop can be modeled rather effectively as a Continuous Time Random Walk with
velocity-dependent probability distribution functions of jump sizes and
occurrences, both of which can be expressed in terms of the scattering mean
free path.Comment: 11 pages, to be published in Astrophysical Journa
Heating and cooling of coronal loops with turbulent suppression of parallel heat conduction
Using the "enthalpy-based thermal evolution of loops" (EBTEL) model, we
investigate the hydrodynamics of the plasma in a flaring coronal loop in which
heat conduction is limited by turbulent scattering of the electrons that
transport the thermal heat flux. The EBTEL equations are solved analytically in
each of the two (conduction-dominated and radiation-dominated) cooling phases.
Comparison of the results with typical observed cooling times in solar flares
shows that the turbulent mean free-path lies in a range
corresponding to a regime in which classical (collision-dominated) conduction
plays at most a limited role. We also consider the magnitude and duration of
the heat input that is necessary to account for the enhanced values of
temperature and density at the beginning of the cooling phase and for the
observed cooling times. We find through numerical modeling that in order to
produce a peak temperature ~K and a 200~s cooling time
consistent with observations, the flare heating profile must extend over a
significant period of time; in particular, its lingering role must be taken
into consideration in any description of the cooling phase. Comparison with
observationally-inferred values of post-flare loop temperatures, densities, and
cooling times thus leads to useful constraints on both the magnitude and
duration of the magnetic energy release in the loop, as well as on the value of
the turbulent mean free-path .Comment: 16 pages, 4 figures, to be published in The Astrophysical Journa
From the Crescent City to Jaguar Hill: New Orleans' business interest in the Tehuantepec National Railroad of Mexico, 1849-1861
M.A.--University of Oklahoma, 2013Includes bibliographical references
Excited states of neutral donor bound excitons in GaN
We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygenâthe key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200âÎźeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task.EC/H2020/749565/EU/Heat Transport and its Effects on the Performance of Nanostructured, Photonic Materials/PhotoHeatEffectDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement
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