Direct evidence for magnetic reconnection in the solar wind near 1 AU

We have obtained direct evidence for local magnetic reconnection in the solar wind using solar wind plasma and magnetic field data obtained by the Advanced Composition Explorer (ACE). The prime evidence consists of accelerated ion flow observed within magnetic field reversal regions in the solar wind. Here we report such observations obtained in the interior of an interplanetary coronal mass ejection (ICME) or at the interface between two ICMEs on 23 November 1997 at a time when the magnetic field was stronger than usual. The observed plasma acceleration was consistent with the Walen relationship, which relates changes in flow velocity to density‐weighted changes in the magnetic field vector. Pairs of proton beams having comparable densities and counterstreaming relative to one another along the magnetic field at a speed of ∼1.4 V A , where V A was the local Alfven speed, were observed near the center of the accelerated flow event. We infer from the observations that quasi‐stationary reconnection occurred sunward of the spacecraft and that the accelerated flow occurred within a Petschek‐type reconnection exhaust region bounded by Alfven waves and having a cross section width of ∼4 × 10 5 km as it swept over ACE. The counterstreaming ion beams resulted from solar wind plasma entering the exhaust region from opposite directions along the reconnected magnetic field lines. We have identified a limited number (five) of other accelerated flow events in the ACE data that are remarkably similar to the 23 November 1997 event. All such events identified occurred at thin current sheets associated with moderate to large changes in magnetic field orientation (98°–162°) in plasmas characterized by low proton beta (0.01–0.15) and high Alfven speed (51–204 km/s). They also were all associated with ICMEs.