Observed Constraint on Proton-Proton Relative Velocities in the Solar Wind

From June 1994 to December 1995 the Ulysses spacecraft was within 3 AU of the Sun and traveled from high southern to high northern heliospheric latitudes. For this period, SWOOPS instrument data obtained when the magnetic field was approximately aligned with the radial direction from the Sun have been analyzed. In the fast wind at high latitudes, two resolvable proton components are typically present. Statistical studies of the relative densities and the fieldaligned velocity difference, vo, of the two components are carried out, considering the dependence on distance from the Sun, the local Alfvén speed, and the type of solar wind flow. The observed values of vo/vA, where vA is the local Alfvén speed, are less than the upper bound determined from the linear theory for electromagnetic protonproton instabilities. This result is good evidence that these microinstabilities constrain the relative streaming of the two components in the solar wind.