VOYAGER 2 FINE-SCALE VELOCITY OSCILLATIONS AT 48 AU

Voyager 2 solar wind speeds show unusual variations beginning on day 4 0 of 1996. These variations are oscillations of approximately 30 km/s with periods of about two days and are unlike any variations heretofor e detected in the outer heliosphere. Several possible sources of this behavior are examined. First, this speed feature may be due to a Kelvi n-Helmholtz instability on the boundary between the slow equatorial an d fast high-latitude flows, although this instability has not been obs erved at other radial distances and latitudes. A second possibility is that these waves are due to instabilities associated with interstella r pick-up ions; at the similar to 48 AU distance where these oscillati ons are observed, the pick-up pressure far exceeds the pressure of the thermal solar wind proton population. A third possibility is that the waves are an upstream signature of the termination shock, similar to waves observed upstream of planetary bow shocks. Anomalous cosmic ray protons with 100 MeV energies satisfy the necessary cyclotron resonanc e conditions and carry sufficient energy density to produce the observ ed waves. This possibility would require magnetic connection to the sh ock and the presence of streaming anomalous cosmic ray ions. Plasma da ta alone cannot determine whether the necessary conditions for any of these possibilities exist. Further progress requires collaborative stu dies using all of the available particle and field data from Voyager 2 .