LIMITS ON THE ASYMMETRY OF SOLAR-WIND TEMPERATURE IN THE OUTER HELIOSPHERE

Interstellar pickup ions, if they are thermalized, could produce a lon gitudinal asymmetry in solar wind temperature in the outer heliosphere . Between 1978 and mid-1992, the Pioneer 10 and Voyager 2 spacecraft w ere well situated to search for such an asymmetry. Pioneer 10 was head ed downstream with respect to the local interstellar medium while Voya ger 2 was headed upstream. Both spacecraft were at large heliocentric distances; by mid-1992, Pioneer 10 was at 55 AU, while Voyager 2 was a t 37 AU. Measurements from these spacecraft and the Pioneer Venus Orbi ter are complicated by solar cycle effects but suggest an upper Limit of 5000 K for any asymmetry that might be present. These measurements are also consistent with no asymmetry. There is also some suggestion t hat the radial profile of solar wind temperature in the vicinity of th e solar equator was steeper during the declining phase of the last sol ar cycle. The solar wind temperature has increased at Voyager 2 after mid-1992, but Voyager 2 has also been moving to higher heliographic la titudes, where such an increase should be expected from the well-known latitudinal gradient in solar wind temperature that has been observed by Pioneer 11 and Ulysses.