Identification of trailing edge solar wind stream interfaces: A comparisonof Ulysses plasma and composition measurements

Measurements of the specific entropy argument of the solar wind protons, T/ n(gamma-1), reveal that nearly every occurrence of a high-speed stream seen at Ulysses in 1992-1993 is characterized by an abrupt interface at its tra iling edge. These observations, made by the solar wind plasma instrument (S WOOPS), at a heliocentric range of 4.5 to 5 AU show that there is a discont inuous drop in specific entropy at the interface from a high value in the h igh-speed wind to a lower value in the slow interstream wind. This interfac e is coincident with, but much more abrupt than, compositional changes meas ured by the Solar Wind Ion Composition Spectrometer (SWICS) [Geiss et al., 1995]. These results suggest that a relatively thin interface can be identi fied which separates two plasmas of distinctly different origins as determi ned by the compositional measurements. A superposed epoch analysis performe d on seventeen events reveal the interface is characterized by (1) an abrup t drop in entropy by a factor of similar to 1/3 due to an enhancement in de nsity along with gradually declining temperature, (2) a distinct drop in th e alpha/proton ratio from a value of similar to 5%, typical of the fast win d, to similar to 4% characteristic of the slow solar wind, and (3) relative changes in Mg10+/O6+ at the interface which are as large as the variations in the total Mg/O ratio and the freezing-in temperature derived from O7+/O 6+. The specific entropy argument, a combination of commonly measured solar wind parameters, gives a strong signature of the trailing edge interface w hich is preserved as far out in the heliosphere as 5 AU and may provide use ful information regarding the coronal origin of solar wind streams.