Project Description
Understanding the complex multiscale process of magnetic reconnection can open new insight into energy conversion in astrophysical processes and help in the interpretation of observations like particle spectra originating from astrophysical jets. The plasmoid instability triggers magnetic reconnection in collisional and collisionless current sheets. We intend to study fast plasmoid instability onset in collisionless current sheets numerically, to complement ongoing theoretical investigations. We focus on collisionless physics for two reasons: firstly, most astrophysical plasmas are collisionless. Moreover, while tearing instability onset has been extensively studied theoretically in the collisional regime, studies in collisionless plasmas have been limited to specific regimes. Plasmoid instability theory tells us that a critical aspect ratio (current sheet length to thickness) exists, above which plasmoid instability---and hence reconnection---onset is fast. We intend to identify this critical aspect ratio numerically, for the ranges of parameters expected for astrophysical jet plasma. As part of this project we simulate current sheets with aspect ratio both below and above the critical aspect ratio, to highlight the transition from fast to slow plasmoid instability. Especially, the latter simulations are very expensive computationally, and require that the numerical methods used satisfy certain characteristics, such as excellent energy conservation and long-time stability. The method should also guarantee qualitatively correct results when microscale phenomena are underresolved.
Our goal is to shed light on the question to what extend kinetic processes matter in relativistic particle accelleration.
We develop a novel scale-bridging approach and study how complete a moment description is in order to learn about the effectiveness of various closure procedures. The overall goal is to derive an efficient numerical framework that adaptively includes necessary kinetic information with a minimum effort. With this tool we will study reconnection phenomena in relativistic jets. In particular, adaptively choosing the level of kinetics in the simulation should enable us to address the onset problem in slow plasmoid instabilities.
