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WG5: Theory
Gordon Emslie
15 years of RHESSI X-ray imaging and spectroscopy observations have improved our understanding of solar flares and the production, acceleration, and transport of high-energy particles, as well as the transport of energy through the solar atmosphere. Other instruments (e.g. MinXSS, IRIS, Hinode/EIS) complement our understanding of these processes, and provide insight in ways that any single instrument cannot do alone. Further, the next decade will look beyond RHESSI with new X-ray observations and diagnostics from the next generation of missions (e.g., MinXSS-2, Solar Orbiter/STIX, FOXSI) bringing a new set of challenges and questions, which will require the development of theory, models and diagnostic tools. This working group welcomes all contributions that aim to address these challenges, as well as weaknesses in our current understanding of the solar flare process.
Non-Local Heat Conduction Effects in Active Region, Flaring and Post-Flare Loops | |
A G Emslie, N H Bian, E P Kontar | Talk |
It is, of course, a given that gradients of electron temperature lead to the transport of energy by heat conduction. However, solution of the pertinent electron-transport Fokker-Planck diffusion equation via a Legendre polynomial expansion, or equivalently via a continuous time random walk analysis, shows that in general the heat flux at a particular point in space is determined not just by the temperature gradient at that point, but rather by a convolution of the temperature gradient over a finite region with a kernel that has the form of a bi-exponential function. Somewhat surprisingly, the kernel has a characteristic width equal to several (approximately 7) mean free paths, so that nonlocal effects can be important even in situations where the temperature scale length is quite long compared to the collisional mean free path. We explore the consequences of non-local effects in determining the temperature profiles of static coronal loops and in modelling the transport of thermal energy in flaring and post-flare loops. |
Electron acceleration at slow mode shocks in the magnetic reconnection region | |
G. Mann, A. Warmuth, and H. Oenel | Talk |
During solar flares a substantial part of electrons are accelerated up to high energies, i.e. beyond 20 keV, as observed by RHESSI. Magnetic reconnection in the corona is thought to be the primary process of solar flares. In the standard (or CSHKP-) model, slow mode shocks appear in the vicinity of the reconnection region. They separate the in- from the out-flow region. At the slow mode shocks, magnetic field energy is transformed into kinetic energy of particles, i.e. electrons, protons, and heavy ions. We discuss in which way electrons are accelerated up to high energies at slow mode shocks under typical plasma parameters in the flare region. We find that electron acceleration at slow mode shocks is efficient if the Aflven velocity in th inflow region becomes greater than 3000 km/s. The theoretical results are compared with RHESSI measurements. |
Test Particle Simulations at Tearing Null Point Current Sheets | |
Ross Pallister, David Pontin | Talk |
Shearing of a magnetic null-point in the Solar corona can lead to the formation of a current sheet, which can be a source of non-thermal acceleration. The formation of flux ropes can destabilise this sheet, leading to tearing and quasi-turbulent dynamics. We have written a test-particle simulation code to model the full motions of protons in such a system, with electric and magnetic field values derived from a 3D MHD simulation of sheet tearing performed by Wyper and Pontin (2014). We present preliminary results of these test-particle simulations at different stages of sheet-tearing, giving an overview of how the acceleration profiles of the protons change over the course of the tearing. We also examine features in the acceleration profiles and trajectories that may warrant further investigation, as well as discuss further code development. |
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