PICKUP PROTONS AND PRESSURE-BALANCED STRUCTURES FROM 39 TO 43 AU - VOYAGER-2 OBSERVATIONS DURING 1993 AND 1994

The pressure of interstellar pickup protons in the distant heliosphere can be determined by analyzing pressure-balanced structures, observed on a scale of a few hundredths of an AU. This paper extends the earli er work of L. F. Burlaga et al. (Journal of Geophysical Research, 99, 21,511, 1994) by analyzing pressure-balanced structures observed by Vo yager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1 994. The pickup proton temperature is high in the region of the distan t heliosphere that we considered: (5.4 +/- 0.1) x 10(6) K at 39-41 AU and (6.0 +/- 0.4) x 10(6) K at approximate to 43 AU. The density of th e pickup protons is (1.6 +/- 0.3) x 10(-4) cm(-3) at 39-41 AU and (1.2 +/- 0.2) x 10(-4) cm(-3) at approximate to 43 AU. The ratio of the pi ckup proton density to the solar wind proton density (N-i/N) is small, only 0.03 +/- 0.01 during both 1993 and 1994. Nevertheless, the picku p proton pressures are relatively high because of their high temperatu res. The pickup at 43 AU. There is a possible decrease in Pi with incr easing distance from the Sun. The pickup proton pressure is an order o f magnitude greater than the solar wind proton pressure: P-i/P-swp = 1 0 +/- 2 at 39-41 AU and 8 +/- 2 at 43 AU. Our results support the hypo thesis of Burlaga et al. that the pickup proton pressure is more impor tant than the solar wind thermal pressure in the dynamics of the dista nt heliosphere. The ratio of the pickup ion pressure to the magnetic p ressure is P-i/(B-2/8 pi) = 1.7 +/- 0.3 at 39-41 AU and 1.7 +/- 0.72 a t approximate to 43 AU. These results are compared with a model.