Radio frequency emissions from a runaway electron avalanche model compared with intense, transient signals from thunderstorms

摘要

We present a one-dimensional model of a runaway electron avalanche in a thunderstorm electric field. Previous simulations have calculated the ionization rates and energy distribution functions for runaway electrons, for various atmospheric values of E/p, through the solution of the modified relativistic Boltzmann equation. We use the field- and pressure-dependent ionization rates in a hydrodynamic macroscopic treatment. The runaway electron avalanche modeled here includes the production of runaway and low-energy electrons, electric field relaxation, electron attachment, and runaway electron loss. The model ambient electric field is established from two disks of charge with a sinusoidally spatially varying charge density of 9 nC/m~3 peak amplitude. The peak ambient electric field from this configuration is 538 kV/m at 5 km. The numerically calculated radio frequency radiation exhibits relativistic effects. We hypothesize that runaway electron avalanches are sources of intense HF/VHF impulses radiated from within electrified clouds. The results from this case study are compared with ground-based and FORTE satellite observations of HF and VHF radiation observed during the rise portion of narrow bipolar pulses (NBP). Given the specified rates and ambient environment, the radiation electric field HF and VHF spectra covering 3–25, 26–48, and 60–66 MHz are in agreement with observations for limited angular ranges. The modeled peak radiation electric field in the time domain is just below one standard deviation from the observed mean for NBPs.