Electrostatic waves observed at and near the solar wind termination shock by Voyager 2

Upstream electron plasma oscillations and broadband bursts of electrostatic waves have been observed in the vicinity of the solar wind termination shock by the Voyager 2 plasma wave instrument. The upstream electron plasma oscillations were first detected on August 1, 2007, at a radial distance of 83.4 astronomical units (AU). These oscillations continued sporadically for about a month until, starting on August 31, three well-defined bursts of broadband electrostatic waves, similar to those observed at planetary bow shocks, were observed at a heliocentric radial distance of about 83.7 AU. Two of these broadband bursts corresponded to shock crossings identified in the magnetometer and plasma data, and one did not. During the crossings labeled TS-3 and TS-4 by the magnetometer and plasma teams, the broadband electrostatic bursts corresponded almost exactly with steep ramps in the magnetic field strength. By scaling the frequencies by the upstream electron plasma frequency and the spectral densities by the upstream plasma energy density, we have shown that the normalized spectrum is very similar to those observed at the bow shocks of outer planets. Plasma simulations based on the observed upstream parameters suggest that the broadband waves are driven by a beam-plasma instability caused by the reflection of solar wind ions in the magnetic field ramp.