Plasma Emission at Shocks by the Eigenmode-Antenna Mechanism

Abstract

Planetary bow shocks, interplanetary shocks, and other heliospheric shocks are remotely observable at radio frequencies due to plasma emission. In this process, shocks are a source of energetic electron beams which excite electrostatic plasma oscillations (Langmuir waves) near the shock. Langmuir waves are then converted into electromagnetic emission at the local plasma frequency (fp) and its harmonic (2fp). While the production of electron beams by shocks and the subsequent generation of Langmuir waves are well understood, the mechanism which converts electrostatic Langmuir waves into electromagnetic radiation at fp and 2fp remains a subject of debate. A conversion mechanism is presented based on the idea that a substantial fraction of Langmuir waves are localized as eigenmodes of ambient plasma density fluctuations. Thus localized, the fp and 2fp currents associated with Langmuir waves radiate as antennas. Evidence, in the form of observations from the STEREO and WIND spacecraft, is presented for the localization of Langmuir waves by density structures and for radio frequency emission by the eigenmode‐antenna mechanism.