Local re-acceleration and a modified thick target model of solar flare electrons

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (${\cal E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local ${\cal E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfv\'{e}n speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.Comment: 8 pages, 2 figure

Structuring and support by Alfven waves around prestellar cores

Observations of molecular clouds show the existence of starless, dense cores, threaded by magnetic fields. Observed line widths indicate these dense condensates to be embedded in a supersonically turbulent environment. Under these conditions, the generation of magnetic waves is inevitable. In this paper, we study the structure and support of a 1D plane-parallel, self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is injected in its central plane. Dimensional analysis shows that the solution must depend on three dimensionless parameters. To study the nonlinear, turbulent evolution of such a slab, we use 1D high resolution numerical simulations. For a parameter range inspired by molecular cloud observations, we find the following. 1) A single source of energy injection is sufficient to force persistent supersonic turbulence over several hydrostatic scale heights. 2) The time averaged spatial extension of the slab is comparable to the extension of the stationary, analytical WKB solution. Deviations, as well as the density substructure of the slab, depend on the wave-length of the injected wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with increasing plasma beta. 4) Good spatial resolution is mandatory, making similar simulations in 3D currently prohibitively expensive.Comment: 13 pages, 8 figures, accepted for publication in A&A. The manuscript with full color, high-resolution, figures can be downloaded from http://www.astro.phys.ethz.ch/papers/folini/folini_p_nf.htm

Thermalisation of self-interacting solar flare fast electrons

Most theoretical descriptions of the production of solar flare bremsstrahlung radiation assume the collision of dilute accelerated particles with a cold, dense target plasma, neglecting interactions of the fast particles with each other. This is inadequate for situations where collisions with this background plasma are not completely dominant, as may be the case in, for example, low-density coronal sources. We aim to formulate a model of a self-interacting, entirely fast electron population in the absence of a dense background plasma, to investigate its implications for observed bremsstrahlung spectra and the flare energy budget. We derive approximate expressions for the time-dependent distribution function of the fast electrons using a Fokker-Planck approach. We use these expressions to generate synthetic bremsstrahlung X-ray spectra as would be seen from a corresponding coronal source. We find that our model qualitatively reproduces the observed behaviour of some flares. As the flare progresses, the model's initial power-law spectrum is joined by a lower energy, thermal component. The power-law component diminishes, and the growing thermal component proceeds to dominate the total emission over timescales consistent with flare observations. The power-law exhibits progressive spectral hardening, as is seen in some flare coronal sources. We also find that our model requires a factor of 7 - 10 fewer accelerated electrons than the cold, thick target model to generate an equivalent hard X-ray flux. This model forms the basis of a treatment of self-interactions among flare fast electrons, a process which affords a more efficient means to produce bremsstrahlung photons and so may reduce the efficiency requirements placed on the particle acceleration mechanism. It also provides a useful description of the thermalisation of fast electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this version clarifies arguments around Eqs. (11) and (20

Propagation of non-linear circularly polarised Alfven waves in a homogeneous medium

We study the evolution of non-linear circularly polarised Alfven waves by solving numerically the time-dependent equations of magnetohydrodynamics (MHD) in one dimension. We examine the behaviour of the waves and find that different physical mechanisms are relevant in different ranges of beta. In a low beta plasma the wave may undergo a parametric decay. This is because the wave excites a density enhancement that travels slower than the wave itself and thus interacts with the wave. When beta is greater or equal to one the density enhancement does not interact with the wave and no decay takes place, instead the Alfven wave is reflected against the density enhancement. The reflection zone propagates with the speed 1/n of the Alfven speed. Because of that the magnetic flux is conserved which results in an amplification of the oscillating magnetic field by a factor 1/n. We find that n depends on beta, and that in particular it is less or equal to one for values of beta ~ 1 and is greater or equal to one for beta >> 1. We discuss the relevance of these mechanisms to the acceleration of the solar wind, and the triggering of MHD turbulence in the polar wind region. In particular these simulations can explain the presence of inward propagating Alfven waves in the solar corona

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

The role and scope of practice of midwives in humanitarian settings:a systematic review and content analysis

Abstract Background Midwives have an essential role to play in preparing for and providing sexual and reproductive health (SRH) services in humanitarian settings due to their unique knowledge and skills, position as frontline providers and geographic and social proximity to the communities they serve. There are considerable gaps in the international guidance that defines the scope of practice of midwives in crises, particularly for the mitigation and preparedness, and recovery phases. We undertook a systematic review to provide further clarification of this scope of practice and insights to optimise midwifery performance. The review aimed to determine what SRH services midwives are involved in delivering across the emergency management cycle in humanitarian contexts, and how they are working with other professionals to deliver health care. Methods Four electronic databases and the websites of 33 organisations were searched between January and March 2017. Papers were eligible for inclusion if they were published in English between 2007 and 2017 and reported primary research pertaining to the role of midwives in delivering and performing any component of sexual and/or reproductive health in humanitarian settings. Content analysis was used to map the study findings to the Minimum Initial Service Package (MISP) for SRH across the three phases of the disaster management cycle and identify how midwives work with other members of the health care team. Results Fourteen studies from ten countries were included. Twelve studies were undertaken in conflict settings, and two were conducted in the context of the aftermath of natural disasters. We found a paucity of evidence from the research literature that examines the activities and roles undertaken by midwives across the disaster management cycle. This lack of evidence was more apparent during the mitigation and preparedness, and recovery phases than the response phase of the disaster management cycle. Conclusion Research-informed guidelines and strategies are required to better align the scope of practice of midwives with the objectives of multi-agency guidelines and agreements, as well as the activities of the MISP, to ensure that the potential of midwives can be acknowledged and optimised across the disaster management cycle

Dynamics of circularly polarised Alfvén waves in a stratified medium

We present a numerical study of the propagation of circularly polarised Alfvén waves in a plane-parallel stratified medium. Because of the stratification there is a global gradient in the magnetic pressure of the wave, which accelerates the plasma and supports it against gravity. The spatial distribution of the wave force is determined by the amplitude of the Alfvén wave which in its turn is set by the influences of dissipation, reflection and parametric decay on the wave. The Alfvén wave is partially reflected off the smooth density gradient. The relative amplitude of the reflected wave is proportional to the background magnetic field strength and is independent of the absolute amplitude of the wave. At high amplitudes strong backward propagating Alfvén waves are generated through the parametric decay of the Alfvén wave, and by reflection off density fluctuations generated by the wave front