Voyager 2 fine‐scale velocity oscillations at 48 AU

Abstract

Voyager 2 solar wind speeds show unusual variations beginning on day 40 of 1996. These variations are oscillations of approximately 30 km/s with periods of about two days and are unlike any variations heretofore detected in the outer heliosphere. Several possible sources of this behavior are examined. First, this speed feature may be due to a Kelvin‐Helmholtz instability on the boundary between the slow equatorial and fast high‐latitude flows, although this instability has not been observed at other radial distances and latitudes. A second possibility is that these waves are due to instabilities associated with interstellar pick‐up ions; at the ∼48 AU distance where these oscillations 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 resonance conditions and carry sufficient energy density to produce the observed waves. This possibility would require magnetic connection to the shock and the presence of streaming anomalous cosmic ray ions. Plasma data alone cannot determine whether the necessary conditions for any of these possibilities exist. Further progress requires collaborative studies using all of the available particle and field data from Voyager 2.