Coherent Cerenkov radiation from the Spacelab 2 electron beam

Abstract

During the Spacelab 2 mission, a spacecraft called the Plasma Diagnostics Package (PDP) was released from the space shuttle to investigate the surrounding plasma environment. During an interval when the shuttle and PDP were magnetically connected, a continuous 1‐keV/50‐mA electron beam was ejected along a field line from an electron generator on board the shuttle. As the PDP flew by the beam, the PDP plasma wave instrument detected intense whistler mode radiation originating from the beam. It is believed that coherent Cerenkov radiation from bunches of beam electrons is responsible for the whistler mode radiation, where an electrostatic beam‐plasma instability forms the coherently radiating bunches. In this paper a detailed model of the coherent Cerenkov emission process is presented. A one‐dimensional computer simulation of the beam is used to model the expected phase space structure of the electrons, and power emitted from Cerenkov radiation is computed using an analytical expression. The calculated power from the modeled 200‐m beam segment is ∼5 × 10−8 W/Hz, which can easily account for the measured whistler mode wave power. The inclusion of coherent effects in the beam increases the wave powers by nearly 90 dB above incoherent power levels. These calculations demonstrate that a spontaneous emission process, alone, can account for the observed whistler mode wave powers.