New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.Comment: This is an article for a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

Between state and market: The relationship between non-profit housing organisations and the state in three national contexts

This paper presents the results of an international collaborative study5 of non-profit housing; one of the main alternatives to public housing developed in many countries. The study involved researchers6 in three countries using a common methodology to engage the leaders of non-profit organisations themselves in the research process. The paper draws on earlier work on the hybridisation of social housing provision and the competing logics of state, market and third sector. It explores the hypothesis that despite the rhetoric of independence from government and the discourse of social enterprise, the relationship of non-profits with the state has been one of the key influences on the scope for action and forms of hybridity in each country studied. It uses a modified Delphi methodology to collect information from leaders of non-profit housing organisations in three countries about how their organisations are negotiating hybrid influences on their strategy and operations. This method involves collation of responses to scaled surveys from panels of 20-31 organisations in each country, playing back the results to the participants and follow up depth interviews with samples of respondents to explore and interpret the meanings of their responses7. The paper is intended to provoke comments8 on the approach by displaying preliminary findings in relation to state influences on hybridity; reflecting on methodology and drawing out some tentative conclusions in relation to the paper’s hypothesis. Contrary to Koppell’s (2001) warning in another domain of housing hybrids, it appears that state influence and control over social housing hybrids remains a key influence on their scope and actionOTB ResearchArchitecture and The Built Environmen

Between state and market: Non-profit housing organisations in three national contexts

This paper presents the results of an international collaborative study5 of non-profit housing involving researchers6 in three countries using a common methodology to engage the leaders of non-profit organisations themselves in the research process. The paper draws on earlier work on the hybridisation of social housing provision and the competing logics of state, market and third sector. It explores the hypothesis that despite the rhetoric of independence from government and the discourse of social enterprise, the relationship with the state has been one of the key influences on scope for action and hybridity in each country studied. It uses a modified Delphi methodology to collect information from leaders of non-profit housing organisations in three countries about how their organisations are negotiating these hybrid influences on their strategy and operations. This method involves collation of responses to scaled surveys from panels of 20-31 organisations in each country, playing back the results to the participants and follow up depth interviews with samples of respondents to explore and interpret the meanings of their responses7. The paper is intended to elicit comments on the approach by displaying preliminary findings in relation to state influences on hybridity; reflecting on methodology and drawing out some tentative conclusions in relation to the paper’s hypothesis. Contrary to Koppell’s (2001) warning in another domain of housing hybrids, it appears that state influence and control over social housing hybrids remains a key influence on their scope and action.OTB ResearchArchitecture and The Built Environmen

The old and the new: Comparing strategic positioning of third sector housing organisations in the Netherlands and Australia

Housing Quality and Process InnovationOTB Research Institute for the Built Environmen