JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. A4, PAGES 6561-6573, APRIL 1, 1994 Solar wind velocity and temperature in the outer heliosphere P. It. Gazis San Jose State University Foundation, NASA Ames Research Center, Moffett Field, California A. Barnes and J. D. Mihalov NASA Ames Research Center, Moffett Field, California A. J. Lazarus Center for Space Research, Massachusetts Institute of Technology, Cambridge Abstract. At the end of 1992, the Pioneer 10, Pioneer 11, and Voyager2 spacecraft were at heliocentric distances of 56.0, 37.3, and 39.0 AU and heliographic latitudes of 3.3øN, 17.4øN, and 8.6øS,respectively. Pioneer 11 and Voyager2 are at similar celestial longitudes,while Pioneer 10 is on the oppositeside of the Sun. All three spacecrafthave working plasma analyzers,so intercomparison of data from thesespacecraft provides important information about the global character of the solar wind in the outer heliosphere.The averagedsolar wind speedcontinuedto exhibit its well-known variation with solar cycle: Even at heliocentricdistances greater than 50 AU, the average speedis highest during the declining phase of the solar cycle and lowest near solar minimum. There was a strong latitudinal gradient in solar wind speedbetween3 ø and 17øN during the last solar minimum, but this gradient has sincedisappeared.The solar wind temperature declined with increasing heliocentric distance out to a heliocentric distance of at least 20 AU; this decline appeared to continue at larger heliocentric distances, but temperatures in the outer heliosphere weresurprisingly high. While Pioneer 10 and Voyager 2 observedcomparablesolar wind temperatures, the temperature at Pioneer 11 was significantly higher, which suggests the existence of a large-scale variation of temperature with hellographic longitude. There was also somesuggestion that solar wind temperatures were higher near solar minimum. Introduction The radial variation of solar wind parametersin the outer heliosphere has important implications for our understand- ing of the various processes that govern the evolution of the solar wind. Stream interactions; heating due to shocks, waves,and turbulence; the anomalous componentof galac- tic cosmic rays; and interstellar pickup ions are expected to affect the global structure of the solar wind. Direct observation of the radial dependence of solar wind parameters began with measurements from the Mariner 2 spacecraft [Neugebauer and Snyder, 1962]. Our knowledge was extended into the outer heliosphere by measurements from the Pioneer10 and 11 [Collardand Wolfe, 1974;Smith and Wolfe, 1979; Mihalov and Wolfe, 1978, 1979; Co!lard et al., 1982; Smith and Barnes, 1983; Kayset et al., 1984] and Voyager1 and 2 spacecraft [Gazis and Lazarus,1982; Gazis1984]. Theseobservations showed that the average solar wind speed did not vary with heliocentric distance but doesshowa solar cycledependence; it was higher dur- ing solar minimum and lower during solar maximum. The average proton density varied as1•-•; there was no sign of any large-scale convergence or divergence of flow. The pro- ton temperature dropped slowly with increasingdistance- as1• -a with/3 reported to be between 0.5 and0.7- which Copyright 1994 by the American Geophysical Union. Paper number 93JA03144. 0148-022 ? / 94 / 93 J A-03144505.00 is much more slowly than would be expected for the adia- batic expansion of a perfect gas. Mihalov [1983]reported that this temperature gradient was steeper when the solar wind temperature was higher. l•ecentobservations by Lazarus and Belcher [1988],Gazis et al. [1988],and Smith et al. [1988] have extended our knowledge out to heliocentric distances of nearly 50 AU and appeared to confirm the trends established by these earlier observations, with one important exception: The ra- dial temperature profile appeared to be somewhat flatter than reported by earlier observers. Indeed, there seems to have been a general trend throughout the 1970s and 1980s for successive observers to report flatter temperature pro- files as the relevant spacecraftmoved farther from the Sun. We have reexamined solar wind speed and temperature measurements from the Pioneer, Voyager, and IMP space- craft,both individually and through the intercomparison of measurements from different spacecraft. We have also examined new data that have become available since 1988. Our results suggest that the radial profile and temporal vari- ation of the solar wind speed is consistent with the reports of previous observersbut that the solar wind temperature may behave in a more complicated fashion. This behavior has been obscured until now by the limited spatial extent of previous observations. Observations The Pioneer and Voyager spacecraft are uniquely placed to conduct observations of the outer heliosphere. The long 6561