kglobal, A Power Law Producing Model for Energetic Electron Acceleration in Macroscale Systems

Abstract:

We have developed a new computational model, {\it kglobal}, to explore superthermal electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of superthermal electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers and does not need to resolve any kinetic scales. We use guiding center equations for macro-particle electrons that provide self-consistent feedback on the magnetic field and ion fluid through their anisotropic pressure tensor. Additionally, in order to ensure charge neutrality, we include a fluid electron species as well. The result is a code with a MHD backbone that allows us to study superthermal electron energization while conserving the total amount of energy. This code has accurately simulated Alfv\'en waves, magnetohydrodynamic waves, the firehose instability, and Landau damping of electron acoustic modes. Here we present results from macroscale multi-island magnetic reconnection simulations demonstrating the generation of a power law spectrum of energetic electrons in addition to a heated thermal component. We also show how the exponent of the power law and the contribution of superthermal electrons to the total energy decreases with the strength of the guide field, becoming almost negligble with a guide field equal to the reconnecting field. This model is capable of bridging the orders of magnitude gap between PIC simulations and global systems, such as the solar corona.