Plasma waves associated with the termination shock

Voyager 1 crossed the termination shock of the heliosphere on December 16, 2004 and provided our first opportunity to examine this unique boundary and associated processes. A common wave phenomenon that is often observed upstream of shocks in the solar wind are Langmuir waves, or electron plasma oscillations. Langmuir waves were observed sporadically by Voyager 1 for nearly a year prior to the termination shock crossing. The Langmuir waves are thought to be produced by a bump-on-tail electron distribution consisting of beams of electrons coming from the shock, and are usually produced by a velocity selection effect caused by the convection of the solar wind. In fact, evidence for such electron beams was found in conjunction with Langmuir waves observed on the day prior to the termination shock crossing. Voyager 2, which trails Voyager 1 in heliocentric distance by about 19 AU, is now approaching the termination shock and promises a second opportunity with which to observe the region near this boundary. We assess the usefulness of Langmuir waves as a predictor of an impending shock crossing, using the Voyager I experience as a guide.