Recent Voyager Observations of Radio Emissions and Plasma Waves in the Outer Heliosphere

During the past year or so, two new types of observations have been obtained from the plasma wave instrument on the Voyager spacecraft. First, starting on February 12-13, 2004, again on August 29 to September 1, 2004, and several times since then, the plasma wave instrument on Voyager 1, which is now at about 95 AU (Astronomical Units), began detecting sporadic narrowband emissions at frequencies of a few hundred Hz. These emissions usually occur in only a single frequency channel, typically 178 or 311 Hz, and in one case 562 Hz, and have field strengths ranging up to about 1 microvolt per meter. The simple and most straightforward interpretation of these emissions is that they are electron plasma oscillations of the type that have been predicted to occur in the region upstream of the termination shock (upstream electron plasma oscillations are a common feature of planetary bow shocks). The frequencies are consistent with the electron plasma frequency computed from a 1/R2 extrapolation to 95 AU of the long term average plasma density measured by the plasma instrument on Voyager 2. If this interpretation is correct, these observations indicate that Voyager 1 is still upstream of the termination shock, but is now close enough for low energy electrons from the shock to occasionally reach the spacecraft, thereby exciting the electron plasma oscillations. Second, starting in early September 2004, another new heliospheric radio emission event was detected. This event is characterized by an emission frequency that gradually drifted upward from about 2.4 to 3.0 kHz over a period of several months, very similar to several earlier events of this type. Radio emission events of this type are thought to be produced when the shock or shocks associated with an intense period of solar activity interact with the heliosphere. The causative solar event is not yet certain, but one possibility is that it is related to the intense series of solar flares that occurred in late October and early November 2003.