The interaction of relativistic electron beams with the near-earth space environment

A model based on several established analytical computational techniques has been developed to study the interaction of relativistic (E {dollar}sim{dollar} MeV) electron beams with the Earth's upper atmosphere and ionosphere. The emphasis is on the analysis of active experiments involving beams launched from a satellite in Low Earth Orbit (LEO) or from a suborbital sounding rocket. The Beam-Atmosphere Interaction (BAI) forms a subset of physical phenomena associated with the injection of charged particle beams from a spacecraft. The present study extends the analysis of the BAI from the keV range of past experiments, and it is motivated in part by the recent advances in technology which allow MeV electron beams to be launched from spacecraft. The model is designed to accept beam and environmental parameters as input, such as beam current, energy, and mean divergence, and to compute quantities of interest resulting from the relativistic BAI as output, such as ionization and bremsstrahlung emissions. The BAI is examined by first computing the electron beam energy loss using the Continuous Slowing Down Approximation (CSDA) and the beam cross sectional area by using the envelope equations which describe beam dynamics in the paraxial approximation. These results are used to complete a first-order stability analysis associated with the Beam-Plasma Interaction (BPI) and to calculate secondary electron fluxes resulting from electron-impact ionization. With a steady-state relativistic electron beam source, secondary electrons will cascade in energy until an equilibrium is reached. Model results for beam energies from 1 to 100 MeV are in reasonable agreement with previously established values of the collisional range and fractional energy loss due to radiative processes. The stability analysis shows that beams of lower current and higher energy and divergence are less susceptible to instability, and that the Earth's magnetic field plays a significant role in stability against certain transverse modes. As a sample of practical application of the model, bremsstrahlung fluxes incident on detectors onboard a satellite in LEO were compared with those incident on balloon detectors. Future potential applications include analysis of stratospheric odd-nitrogen production from relativistic electron precipitation events and ionospheric modification due to sprite propagation.