Conditions for plasma emission in the solar wind and very local interstellar medium (VLISM)

Abstract

Plasma emission at radio frequencies is important in many regions of the solar system and very local interstellar medium (VLISM). This emission, induced by an unstable beam of electrons, can occur owing to either nonlinear three‐wave decay interactions involving Langmuir waves, ion sound waves, and transverse waves or STI. Here recently obtained conditions for decay dominating STI and expressions for the combined efficiencies of decay and STI processes are applied to several phenomena: (1) type II and III solar radio bursts, both in the corona and in the solar wind, and (2) emission associated with global merged interaction region (GMIR) shocks which propagate through the solar system, and inner and outer heliosheath plasmas. The shock‐associated electron distributions are computed using a shock drift acceleration model. It is found that decay should dominate STI for type II and III bursts and that the conditions for decay dominating STI reduce the emission predicted by decay processes by ≲ a factor of 2. Similar results are found for GMIR related emission in the solar wind. In the portion of the VLISM where the electron distribution is primed with a superthermal tail the conditions are less severe; however, outside the primed region the conditions are important.