Towards a quantitative theory for 2–3 kHz radio emission from beyond the heliopause

Abstract

Radio emissions observed at 2–3 kHz by the Voyager spacecraft are associated with global merged interaction regions (GMIRs) reaching the vicinity of the heliopause. A recent theory predicts that the radiation turns on when the GMIR shock enters a region in the outer heliosheath primed with a superthermal electron tail, itself produced by “lower-hybrid drive” (LHD) associated with pickup ions. Here we combine this theory with a recent theory for type II solar radio bursts, resulting in an analytic quantitative theory for the 2–3 kHz radiation. The resulting theory treats electron reflection and acceleration at the GMIR shock, formation of electron beams in the foreshock, generation of Langmuir waves, and conversion of Langmuir energy into radiation at the fundamental and harmonic of the electron plasma frequency. Predictions include the characteristics of electron beams and the power input into radiation throughout the foreshock, as well as the flux observed by a distant observer. For nominal shock and plasma parameters, we show that the new theory predicts: (1) fluxes of fundamental radiation of order those observed, (2) the superthermal electron tail produced by LHD increases the fundamental flux by ≈2 orders of magnitude compared with predictions that neglect the tail, and (3) fundamental emission dominates harmonic emission. While issues remain, these results are significant progress towards a viable theory for the 2–3 kHz emissions.