THE SOLAR-WIND VELOCITY DETERMINED FROM VOYAGER-1 AND VOYAGER-2 - LOW-ENERGY CHARGED-PARTICLE MEASUREMENTS IN THE OUTER HELIOSPHERE

The Voyager 1 and 2 spacecraft are now exploring the outer heliosphere with active field and particle instruments. The primary instrument-fo r the critical plasma bulk flow parameter is the plasma science (PLS) instrument. Unfortunately, on Voyager 1, the spacecraft most likely to first encounter the termination shock and other heliospheric boundari es, the PLS instrument is no longer able to measure plasma flows. In t his paper we demonstrate that the Low-Energy Charged Particle (LECP) d etector may, under certain conditions, be used to determine the solar wind plasma flow velocities. We show that higher time resolution radia l speeds (daily averages) are obtainable in the downstream regions nea r interplanetary transient shocks where the ion activity is sufficient ly above background levels and where the ion distributions are suffici ently isotropized. Longer-term (26-day) averaged speeds are obtainable during lower counting periods, such as during 1980 and 1992, when cor otating shocks were prevalent. The Voyager 2 LECP results are in good agreement with known PLS determinations of bulk flow speed from the co ld plasma. Using large transient shocks seen during 1989 and 1991 as e xamples, Voyager 1 calculations indicate slower solar wind speeds near these events despite the higher heliolatitude location of Voyager 1 d uring this period. During 1992, when Ulysses entered the southern high -speed stream region, Voyager 1 apparently did not see high-speed stre ams at similar northern latitudes. Given the expected activity near th e termination shock, our determination of the solar wind speed should serve as an indicator of a Voyager 1 crossing of this shock. The LECP measurements would provide the only determination of bulk flow speed a nd deviations from the nominal solar wind flow speed at this and other boundaries or shocks encountered by Voyager 1.