Voyager 2 Observations of Langmuir Waves Upstream of the Termination Shock

Electrons from a collisionless shock beamed upstream into the supersonic flow are often unstable to Langmuir waves, also called electron plasma oscillations. These were expected to be precursors to crossings of the termination shock and Voyager 1 observed these occasionally for about 10 months prior to that spacecraft encountering the shock on Dec. 16, 2004. Analysis of the solar wind conditions leading up to the Voyager 1 crossing suggested increasing pressure in the supersonic wind probably resulting in the spacecraft chasing the shock for a good portion of the 10-month interval during which the Langmuir waves were observed. Beginning on Aug. 1, 2007, the Voyager 2 plasma wave instrument recorded Langmuir waves at 311 Hz in the most extensive such activity since the Neptune encounter. Additional activity was observed briefly on August 26 and extensively on Aug. 29-30, 2007. In this most recent event, some wave activity was also found in the 562 Hz channel. For a plasma frequency at the center of the 311 Hz channel, the electron density is about 0.0012 cm-3. A plasma frequency of 562 Hz corresponds to a density of about 0.0039 cm-3. Measurements of the plasma density by the Voyager 2 plasma instrument are consistent with this range of plasma frequencies. The Langmuir waves appear to occur when large fluxes of ~20 keV ions are observed by the Voyager 2 LECP instrument. While the ions are not directly responsible for the Langmuir waves, this association strongly suggests a magnetic connection to the termination shock, especially with an abrupt energy cutoff in the ion anisotropies, as is observed. The plasma instrument indicated decreasing solar wind pressure during August 2007 and even earlier. Hence, contrary to the situation with the Voyager 1 crossing, it may be that the termination shock is moving inward and a crossing by Voyager 2 will occur on a time scale much shorter than the 10-month upstream interval observed by Voyager 1.