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 earlier work of L. F. Burlaga et al. ( Journal of Geophysical Research, 99 , 21,511, 1994) by analyzing pressure‐balanced structures observed by Voyager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1994. The pickup proton temperature is high in the region of the distant heliosphere that we considered: (5.4 ± 0.1) × 10 6 K at 39–41 AU and (6.0 ± 0.4) × 10 6 K at ≈43 AU. The density of the pickup protons is (1.6 ± 0.3) × 10 −4 cm −3 at 39–41 AU and (1.2 ± 0.2) × 10 −4 cm −3 at ≈43 AU. The ratio of the pickup 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 pickup proton pressures are relatively high because of their high temperatures. The pickup proton pressure was (11 ± 2) × 10 −14 erg cm −3 at 39–41 AU and (9 ± 1) × 10 −14 erg cm −3 at 43 AU. There is a possible decrease in P i with increasing distance from the Sun. The pickup proton pressure is an order of magnitude greater than the solar wind proton pressure: P i / P swp = 10 ± 2 at 39–41 AU and 8 ± 2 at 43 AU. Our results support the hypothesis of Burlaga et al. that the pickup proton pressure is more important than the solar wind thermal pressure in the dynamics of the distant heliosphere. The ratio of the pickup ion pressure to the magnetic pressure is P i /( B 2 /8π) = 1.7 ± 0.3 at 39–41 AU and 1.7 ± 0.72 at ≈43 AU. These results are compared with a model.