Non-Isobaric Thermal Instability

Multiphase media have very complex structure and evolution. Accurate numerical simulations are necessary to make advances in our understanding of this rich physics. Because simulations can capture both the linear and nonlinear evolution of perturbations with a relatively wide range of sizes, it is important to thoroughly understand the stability of condensation and acoustic modes between the two extreme wavelength limits of isobaric and isochoric instability as identified by Field. Partially motivated by a recent suggestion that large non-isobaric clouds can shatter into tiny cloudlets, we revisit the linear theory to survey all possible regimes of thermal instability. We uncover seven regimes in total, one of which allows three unstable condensation modes. Using the code Athena++, we determine the numerical requirements to properly evolve small amplitude perturbations of the entropy mode into the nonlinear regime. Our 1D numerical simulations demonstrate that for a typical AGN cooling function, the nonlinear evolution of a single eigenmode in an isobarically unstable plasma involves increasingly larger amplitude oscillations in cloud size, temperature, and density as the wavelength increases. Such oscillations are the hallmark behavior of non-isobaric multiphase gas dynamics and may be observable as correlations between changes in brightness and the associated periodic redshifts and blueshifts in systems that can be spatially resolved. Intriguingly, we discuss regimes and derive characteristic cloud sizes for which the saturation process giving rise to these oscillations can be so energetic that the cloud may indeed break apart. However, we dub this process splattering instead of shattering, as it is a different fragmentation mechanism that is triggered when the cloud suddenly lands on the stable cold branch of the equilibrium curve

Synthetic absorption lines for a clumpy medium: a spectral signature for cloud acceleration in AGN?

There is increasing evidence that the highly ionized multiphase components of AGN disk winds may be due to thermal instability. The ions responsible for forming the observed X-ray absorption lines may only exist in relatively cold clumps that can be identified with the so-called 'warm absorbers'. Here we calculate synthetic absorption lines for such warm absorbers from first principles by combining 2D hydrodynamic solutions of a two-phase medium with a dense grid of photoionization models to determine the detailed ionization structure of the gas. Our calculations reveal that cloud disruption, which leads to a highly complicated velocity field (i.e. a clumpy flow), will only mildly affect line shapes and strengths when the cold gas becomes highly mixed but not depleted. Prior to complete disruption, clouds which are optically thin to the driving UV resonance lines will cause absorption at an increasingly blueshifted line of sight velocity as they are accelerated. This behavior will imprint an identifiable signature on the line profile if warm absorbers are enshrouded in an even broader absorption line produced by a high column of intercloud gas. Interestingly, we show that it is possible to develop a spectral diagnostic for cloud acceleration by differencing the absorption components of a doublet line, a result which can be qualitatively understood using a simple partial covering model. Our calculations also permit us to comment on the spectral differences between cloud disruption and ionization changes driven by flux variability. Notably, cloud disruption offers another possibility for explaining absorption line variability.Comment: 14 pages, 7 figures, to appear in MNRA

01-03 "Community Control in a Global Economy: Lessons from Mexico's Economic Integration Process"

The North American Free Trade Agreement appeared to promise economic growth for Mexico and improved living conditions for its people. While the Mexican economy has recovered significantly from its post-NAFTA collapse, there is mounting evidence that many of the pre-NAFTA warnings of worsening poverty and deteriorating environmental conditions were true, if exaggerated. However one interprets the statistics, there is little doubt that the economic integration process, which began a full decade before NAFTA took effect, has created a significant restructuring of the Mexican economy, with some of the country's most vulnerable residents facing the harshest conditions. How have those most affected by the economic integration process responded to the challenges and opportunities posed by globalization? Based on a collaborative research project between U.S. and Mexican researchers, the authors provide an overview of the existing English-language research on the subject and suggest a research agenda to assess adaptive strategies and to draw from those experiences lessons for the construction of future trade agreements.

The saturation mechanism of thermal instability

The nonlinear outcome of plasma instabilities range from a gentle reconfiguration of the initial state to an explosive one, and a non-disruptive outcome in between can nevertheless still be a route to turbulence. The literature on thermal instability (TI) reveals that even for a simple homogeneous plasma, all these possibilities can occur, depending on whether the condensations that form evolve in an isobaric or nonisobaric manner. Here we derive several general identities from the evolution equation for entropy that reveals the mechanism by which TI saturates: whenever the boundary of the instability region (the Balbus contour) is crossed, a dynamical change is triggered that causes the comoving time derivative of the pressure to change sign. This temporal event implies that the gas pressure force reverses direction, slowing the continued growth of the condensation. For isobaric evolution, this `pressure reversal' occurs nearly simultaneously for every fluid element in the condensation and a steady state is quickly reached. For nonisobaric evolution, the condensation is no longer in mechanical equilibrium and the contracting gas rebounds with greater force during the expansion phase that accompanies gas reaching the equilibrium curve. The cloud then pulsates because the return to mechanical equilibrium becomes wave-mediated as a result of the pressure reversal occurring at different times for different locations in the cloud core. We show that both the contraction rebound event and the subsequent pulsation behavior follow analytically from an analysis of the new identities.Comment: 19 pages, 2 figures. Submitted as a contribution to the research topic "Thermal Imbalance and Multiphase Plasmas Across Scales: From the Solar Corona to the Intracluster Medium

Performance of private sector health care: implications for universal health coverage

Although the private sector is an important health-care provider in many low-income and middle-income countries, its role in progress towards universal health coverage varies. Studies of the performance of the private sector have focused on three main dimensions: quality, equity of access, and efficiency. The characteristics of patients, the structures of both the public and private sectors, and the regulation of the sector influence the types of health services delivered, and outcomes. Combined with characteristics of private providers-including their size, objectives, and technical competence-the interaction of these factors affects how the sector performs in different contexts. Changing the performance of the private sector will require interventions that target the sector as a whole, rather than individual providers alone. In particular, the performance of the private sector seems to be intrinsically linked to the structure and performance of the public sector, which suggests that deriving population benefit from the private health-care sector requires a regulatory response focused on the health-care sector as a whole