Turn-on of 2-3 kHz radiation beyond the heliopause

Abstract

The Voyager spacecraft observe radio emissions near 2-3 kHz that appear to be generated when an interplanetary shock nears the heliopause. This paper presents a detailed, qualitative theory for where and why the radiation turns on. The radiation turns on just beyond the heliopause due to the shock moving into a region primed with a strong superthermal electron tail. Lower-hybrid drive produces the tail: ambient electrons are resonantly accelerated by lower hybrid waves driven by a ring-beam instability of pickup protons. The pickup protons originate as neutrals in the inner heliosheath. Strong arguments suggest that the tail exists only beyond the heliopause (within ≈ 20-50 AU of the nose) and that shock acceleration of the tail dramatically enhances the foreshock electron beams, Langmuir waves and radiation, so that the shock triggers observable radiation only when beyond the heliopause.