SOLAR-WIND CONDITIONS IN THE OUTER HELIOSPHERE AND THE DISTANCE TO THE TERMINATION SHOCK

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We us e these observations to discuss the probable location and motion Of th e termination shock of the solar wind. A least squares fit of proton r am pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67 x 10(-8) dynes cm-2) R-2.00+/-0.02, where R i s measured in astronomical units. Assuming that the interstellar press ure is due to a 5 muG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., fo r a 7 muG field, the termination shock will be located on average at 6 4 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of d ays), due primarily to large variations in solar wind density at a giv en radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the t ermination shock as it responds to these ram pressure changes. The res ults of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.