24,061 research outputs found
Mainstreaming Underutilized Indigenous and Traditional Crops into Food Systems: A South African Perspective
Business as usual or transformative change? While the global agro-industrial food system is credited with increasing food production, availability and accessibility, it is also credited with giving birth to ‘new’ challenges such as malnutrition, biodiversity loss, and environmental degradation. We reviewed the potential of underutilized indigenous and traditional crops to bring about a transformative change to South Africa’s food system. South Africa has a dichotomous food system, characterized by a distinct, dominant agro-industrial, and, alternative, informal food system. This dichotomous food system has inadvertently undermined the development of smallholder producers. While the dominant agro-industrial food system has led to improvements in food supply, it has also resulted in significant trade-offs with agro-biodiversity, dietary diversity, environmental sustainability, and socio-economic stability, especially amongst the rural poor. This challenges South Africa’s ability to deliver on sustainable and healthy food systems under environmental change. The review proposes a transdisciplinary approach to mainstreaming underutilized indigenous and traditional crops into the food system, which offers real opportunities for developing a sustainable and healthy food system, while, at the same time, achieving societal goals such as employment creation, wellbeing, and environmental sustainability. This process can be initiated by researchers translating existing evidence for informing policy-makers. Similarly, policy-makers need to acknowledge the divergence in the existing policies, and bring about policy convergence in pursuit of a food system which includes smallholder famers, and where underutilized indigenous and traditional crops are mainstreamed into the South African food system
Ellerman bombs and UV bursts: transient events in chromospheric current sheets
Ellerman bombs (EBs) and UV bursts are both brightenings related to flux
emergence regions and specifically to magnetic flux of opposite polarity that
meet in the photosphere. These two reconnection-related phenomena, nominally
formed far apart, occasionally occur in the same location and at the same time,
thus challenging our understanding of reconnection and heating of the lower
solar atmosphere. We consider the formation of an active region, including long
fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic
flux sheet, injected ~Mm below the photosphere, is studied as it pierces
the photosphere and interacts with the preexisting ambient field. Specifically,
we aim to study whether EBs and UV bursts are generated as a result of such
flux emergence and examine their physical relationship. The Bifrost radiative
magnetohydrodynamics code was used to model flux emerging into a model
atmosphere that contained a fairly strong ambient field, constraining the
emerging field to a limited volume wherein multiple reconnection events occur
as the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the D RH code and the fully 3D MULTI3D code. The formation
of UV bursts and EBs at intensities and with line profiles that are highly
reminiscent of observed spectra are understood to be a result of the
reconnection of emerging flux with itself in a long-lasting current sheet that
extends over several scale heights through the chromosphere. Synthetic
diagnostics suggest that there are no compelling reasons to assume that UV
bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally
formed at opposite ends of a long current sheet that resides in an extended
bubble of cool gas.Comment: 10 pages, 8 figures, accepted by A&
The role of damped Alfven waves on magnetospheric accretion models of young stars
We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa
A note on the cylindrical collapse of counter-rotating dust
We find analytical solutions describing the collapse of an infinitely long
cylindrical shell of counter-rotating dust. We show that--for the classes of
solutions discussed herein--from regular initial data a curvature singularity
inevitably develops, and no apparent horizons form, thus in accord with the
spirit of the hoop conjecture.Comment: 8 pages, LaTeX, ijmpd macros (included), 1 eps figure; accepted for
publication in Int. J. Mod. Phys.
Exact edge singularities and dynamical correlations in spin-1/2 chains
Exact formulas for the singularities of the dynamical structure factor,
S^{zz}(q,omega), of the S=1/2 xxz spin chain at all q and any anisotropy and
magnetic field in the critical regime are derived, expressing the exponents in
terms of the phase shifts which are known exactly from the Bethe ansatz
solution. We also study the long time asymptotics of the self-correlation
function . Utilizing these results to supplement very
accurate time-dependent Density Matrix Renormalization Group (DMRG) for short
to moderate times, we calculate S^{zz}(q,omega) to very high precision.Comment: 4 pages, 1 figure, 1 table, published versio
Density-density propagator for one-dimensional interacting spinless fermions with non-linear dispersion and calculation of the Coulomb drag resistivity
Using bosonization-fermionization transformation we map the
Tomonaga-Luttinger model of spinless fermions with non-linear dispersion on the
model of fermionic quasiparticles whose interaction is irrelevant in the
renormalization group sense. Such mapping allows us to set up an expansion for
the density-density propagator of the original Tomonaga-Luttinger Hamiltonian
in orders of the (irrelevant) quasiparticle interaction. The lowest order term
in such an expansion is proportional to the propagator for free fermions. The
next term is also evaluated. The propagator found is used for calculation of
the Coulomb drug resistivity in a system of two capacitively coupled
one-dimensional conductors. It is shown that is proportional to for
both free and interacting fermions. The marginal repulsive in-chain interaction
acts to reduce as compared to the non-interacting result. The correction to
due to the quasiparticle interaction is found as well. It scales as
at low temperature.Comment: 5 pages, 1 eps figure; the new version of the e-print corrects an
error, which exists in the original submission; fortunately, all important
conclusions of the study remain vali
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