Petschek-type reconnection exhausts in the solar wind well beyond 1 AU: Ulysses

We have identified 91 Petschek-type exhausts associated with local, quasi-stationary magnetic reconnection in the solar wind in plasma and magnetic field data from the Ulysses spacecraft obtained over a wide range of heliocentric distances (1.4-5.4 AU) and latitudes (S79°-N65°). The characteristic signature of an exhaust was a brief (minutes) interval of accelerated or decelerated plasma flow within a bifurcated current sheet in which changes in magnetic field and flow velocity were correlated at one edge and anticorrelated at the other. Transitions from outside to inside the exhausts were always slow-mode-like, the exhausts appearing to an observer as encounters with closely spaced, forward-reverse, slow-mode wave (shock) pairs. The exhausts almost universally occurred in low-speed or interplanetary coronal mass ejection plasma having low proton β(< 1 and often ≪ 1) at relatively large shears (90°-180°) in the heliospheric magnetic field. Magnetic field strength decreases within the exhausts were highly variable from event to event and were roughly correlated with the magnitude of the local magnetic shears; this indicates the presence of substantial guide fields in at least some of these events. Many of the exhausts occurred at current sheets separating plasma having distinctly different Alfvén speeds and were thus asymmetric. Local exhaust widths at Ulysses varied up to a maximum of ∼2 × 10⁶ km but statistically did not vary significantly with heliocentric distance. We offer several possible explanations of this observation.