medRxiv (Cold Spring Harbor Laboratory), Apr 19, 2024
doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by pee... more doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice. leading aid agencies should collaborate to achieve development goals. However, in complex conflicts, governments may have limited control over resources and legitimacy is subsequently questionable. How this study might affect research, practice or policy-This study highlights the need for continuous UN cross-border coordination mechanisms, finding another model than 'the consent model' for cross-border humanitarian aid for northwest Syria to protect local communities from 'aid weaponisation', and support the newly established Aid Fund for Northern Syria (AFNS), expand its coverage to include northeast Syria and improve its policy to include more development health aid and localisation agenda.
Hinode's observations revealed a very dynamic and complex chromosphere. This require revisiting t... more Hinode's observations revealed a very dynamic and complex chromosphere. This require revisiting the assumption that the chromospheric footpoints of solar flares are areas where accelerated particles only lose energy due to collisions. Traditionally electrons are thought to be accelerated in the coronal part of the loop, then travel to the footpoints where they lose their energy and radiate the observed Hard X-ray. Increasing observational evidence challenges this assumption. We review the evidence against this assumption and present the new Local Re-acceleration Thick Target Model (LRTTM) where at the footpoints electrons receive a boost of re-acceleration in addition to the usual collisional loses. Such model may offer an alternative to the standard collisional thick target injection model (TTM) (Brown 1971) of solar HXR burst sources, requiring far fewer electrons and solving some recent problems with the TTM interpretation. We look at the different scenarios which could lead to such re-acceleration and present numerical results from one of them.
We study the evolution of non-linear monochromatic circularly polarised Alfvén waves by solving n... more We study the evolution of non-linear monochromatic circularly polarised Alfvén waves by solving numerically the time-dependent equations of magnetohydrodynamics in one dimension. We find that in a low β plasma the waves 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 β 1 the density enhancement does not interact with the wave and no decay takes place.
A modified Thick Target Model proposed by Brown et al. (2009) and termed the Local Reac-celeratio... more A modified Thick Target Model proposed by Brown et al. (2009) and termed the Local Reac-celeration Thick Target Model (LRTTM) offers a possible solution of these CTTM problems by invoking reacceleration of electrons after injection from a coronal primary acceleration region. Various reacceleration processes are possible, including in current sheet cascades but, provided the acceleration rate offsets collisional losses, electron lifetimes and HXR yield are greatly in-creased, so that the necessary beam density is much reduced, as is the HXR source electron anisotropy. The total beam power input required in the LRTTM is, however, no less than in the CTTM and, furthermore it has predominantly to be released in the chromosphere. This, together with the longer range of reaccelerated electrons, may affect the vertical distribution of beam heating with possible consequences for white light flare production and chromospheric explosive evaporation models.
This Letter presents an analysis of particle acceleration in a model of the complex magnetic fiel... more This Letter presents an analysis of particle acceleration in a model of the complex magnetic field environment in the flaring solar corona. A slender flux tube, initially in hydrodynamic equilibrium, is stressed by random photospheric motions. A three-dimensional MHD code is used to follow the stochastic development of transient current sheets. These processes generate a highly fragmented electric field, through which particles are tracked using a relativistic test particle code. It is shown that both ions and electrons are accelerated readily to relativistic energies in times of order 10 Ϫ2 s for electrons and 10 Ϫ1 s for protons forming power-law distributions in energy. Subject headings: acceleration of particles-Sun: flares
Particle acceleration in large-scale turbulent coronal magnetic fields is considered. Using test ... more Particle acceleration in large-scale turbulent coronal magnetic fields is considered. Using test particle calculations, it is shown that both cellular automata and three dimensional MHD models lead to the production of relativistic particles on sub-second timescales with power law distribution functions. In distinction with the monolithic current sheet models for solar flares, particles gain energy by multiple interactions with many current sheets. Difficulties that need to be addressed, such as feedback between particle acceleration and MHD, are discussed.
We study the evolution of non-linear circularly polarised Alfvén waves by solving numerically the... more We study the evolution of non-linear circularly polarised Alfvén 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 β. In a low β 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 β ≥ 1 the density enhancement does not interact with the wave and no decay takes place, instead the Alfvén wave is reflected against the density enhancement. The reflection zone propagates with the speed 1 n v A. 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 β, and that in particular it is ≤1 for values of β ∼ 1 and ≥1 for β 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 Alfvén waves in the solar corona.
medRxiv (Cold Spring Harbor Laboratory), Apr 19, 2024
doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by pee... more doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice. leading aid agencies should collaborate to achieve development goals. However, in complex conflicts, governments may have limited control over resources and legitimacy is subsequently questionable. How this study might affect research, practice or policy-This study highlights the need for continuous UN cross-border coordination mechanisms, finding another model than 'the consent model' for cross-border humanitarian aid for northwest Syria to protect local communities from 'aid weaponisation', and support the newly established Aid Fund for Northern Syria (AFNS), expand its coverage to include northeast Syria and improve its policy to include more development health aid and localisation agenda.
Hinode's observations revealed a very dynamic and complex chromosphere. This require revisiting t... more Hinode's observations revealed a very dynamic and complex chromosphere. This require revisiting the assumption that the chromospheric footpoints of solar flares are areas where accelerated particles only lose energy due to collisions. Traditionally electrons are thought to be accelerated in the coronal part of the loop, then travel to the footpoints where they lose their energy and radiate the observed Hard X-ray. Increasing observational evidence challenges this assumption. We review the evidence against this assumption and present the new Local Re-acceleration Thick Target Model (LRTTM) where at the footpoints electrons receive a boost of re-acceleration in addition to the usual collisional loses. Such model may offer an alternative to the standard collisional thick target injection model (TTM) (Brown 1971) of solar HXR burst sources, requiring far fewer electrons and solving some recent problems with the TTM interpretation. We look at the different scenarios which could lead to such re-acceleration and present numerical results from one of them.
We study the evolution of non-linear monochromatic circularly polarised Alfvén waves by solving n... more We study the evolution of non-linear monochromatic circularly polarised Alfvén waves by solving numerically the time-dependent equations of magnetohydrodynamics in one dimension. We find that in a low β plasma the waves 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 β 1 the density enhancement does not interact with the wave and no decay takes place.
A modified Thick Target Model proposed by Brown et al. (2009) and termed the Local Reac-celeratio... more A modified Thick Target Model proposed by Brown et al. (2009) and termed the Local Reac-celeration Thick Target Model (LRTTM) offers a possible solution of these CTTM problems by invoking reacceleration of electrons after injection from a coronal primary acceleration region. Various reacceleration processes are possible, including in current sheet cascades but, provided the acceleration rate offsets collisional losses, electron lifetimes and HXR yield are greatly in-creased, so that the necessary beam density is much reduced, as is the HXR source electron anisotropy. The total beam power input required in the LRTTM is, however, no less than in the CTTM and, furthermore it has predominantly to be released in the chromosphere. This, together with the longer range of reaccelerated electrons, may affect the vertical distribution of beam heating with possible consequences for white light flare production and chromospheric explosive evaporation models.
This Letter presents an analysis of particle acceleration in a model of the complex magnetic fiel... more This Letter presents an analysis of particle acceleration in a model of the complex magnetic field environment in the flaring solar corona. A slender flux tube, initially in hydrodynamic equilibrium, is stressed by random photospheric motions. A three-dimensional MHD code is used to follow the stochastic development of transient current sheets. These processes generate a highly fragmented electric field, through which particles are tracked using a relativistic test particle code. It is shown that both ions and electrons are accelerated readily to relativistic energies in times of order 10 Ϫ2 s for electrons and 10 Ϫ1 s for protons forming power-law distributions in energy. Subject headings: acceleration of particles-Sun: flares
Particle acceleration in large-scale turbulent coronal magnetic fields is considered. Using test ... more Particle acceleration in large-scale turbulent coronal magnetic fields is considered. Using test particle calculations, it is shown that both cellular automata and three dimensional MHD models lead to the production of relativistic particles on sub-second timescales with power law distribution functions. In distinction with the monolithic current sheet models for solar flares, particles gain energy by multiple interactions with many current sheets. Difficulties that need to be addressed, such as feedback between particle acceleration and MHD, are discussed.
We study the evolution of non-linear circularly polarised Alfvén waves by solving numerically the... more We study the evolution of non-linear circularly polarised Alfvén 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 β. In a low β 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 β ≥ 1 the density enhancement does not interact with the wave and no decay takes place, instead the Alfvén wave is reflected against the density enhancement. The reflection zone propagates with the speed 1 n v A. 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 β, and that in particular it is ≤1 for values of β ∼ 1 and ≥1 for β 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 Alfvén waves in the solar corona.
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