Title: Electron Acceleration in the Atmosphere and Space

Author: Caitano L. da Silva

Abstract: Mechanisms of relativistic electron acceleration in plasmas are a subject of intense investigation. Some interesting examples are: unsteady magnetic reconnection in the solar wind leading to betatron and Fermi acceleration, runaway electron beam instabilities in tokamaks, and relativistic acceleration in the wakefields of high-power lasers. This talk is a brief research update on two projects aimed at understanding electron acceleration in the atmosphere and space. 

The first one is aimed at understanding the mechanism of whistler-mode chorus excitation in the Earth’s radiation belts, and its subsequent interaction with trapped energetic electrons (10s of keV to a few MeV) leading to local energization and precipitation into the atmosphere. We address this problem with a hybrid fluid/particle-in-cell simulation approach. We discuss the contrasting characteristics between chorus at dawn and at noon magnetic local time. We further quantify wave-particle interactions with test particle simulations, pointing out the role of cyclotron and Landau resonances in pitch-angle and energy diffusion. We show how chorus can efficiently accelerate ~100 keV electrons.

The second project is aimed at understanding how strong quasi-electrostatic fields at tips of growing lightning channels can accelerate electrons from the thermal population up to 10-100s keV. These so-called runaway electrons are accelerated despite the strong friction force they experience propagating through atmospheric pressure air. There are only a handful of ground observations of bremsstrahlung X-rays produced by lightning runaway electrons. This is simply due to the fact that the mean free path of an 100 keV X-ray photon in air is 50 m. We address this problem with a controlled laboratory experiment, where 100 kV fast impulse voltages are applied to a blunt rod with 5 mm diameter producing streamer corona discharges, mimicking the lightning tip. The streamers accelerate electrons up to 100 keV energies. Upon collision with the anode target they produce bremsstrahlung X-rays unveiling the characteristics of the source.