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ACE News #85 - Nov 9, 2004
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The SWEPAM and MAG experiments on ACE have obtained the first direct evidence that magnetic reconnection occurs in the solar wind near 1 AU. Reconnection describes a process in which the frozen-in field condition of magnetohydrodynamics is violated in such a manner that pairs of magnetic field lines merge to produce topological changes in the field. The reconnection process generally favors oppositely directed magnetic fields at thin current sheets and plays a central role in many models of space, solar, astrophysical and laboratory plasma phenomena. As illustrated in the figure on the left above, field-line kinks on newly merged magnetic field lines propagate as pairs of Alfven waves into the incoming flow. Dashed blue arrows marked A1 and A2, which pass through the kinks on adjacent merged field lines, define the boundaries of the reconnection exhaust. As plasma enters into the exhaust region it encounters one of the bounding Alfven waves and is accelerated away from the reconnection site. Since Alfven waves propagating parallel (anti-parallel) to the magnetic field, B, produce anti-correlated (correlated) variations in B and flow velocity, V, one expects to observe anti-correlated changes in V and B along A1 and correlated variation in V and B along A2.
The figure on the right above shows ACE solar wind magnetic field and proton (black) and alpha particle (red) flow velocity components (in r,t,n coordinates) in the 12:00 13:00 interval on 23 November 1997. Dotted blue lines mark the boundaries of a brief (~15 min) accelerated flow event that occurred at a relatively sharp shear in B. As predicted by the reconnection exhaust model, ACE observed anti-correlated changes in V and B as it entered the accelerated flow region and correlated variations in V and B as it exited that region, indicating that the accelerated flow was associated with a reconnection site that was convecting with the solar wind flow and that initially lay sunward of the spacecraft. Only a limited number (5) of additional reconnection exhaust events have been identified in the ACE data. This suggests that reconnection in the solar wind near 1 AU is a relatively rare phenomenon. All reconnection events identified in the ACE data 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 km/s - 204 km/s). They also were all associated with coronal mass ejections.
Contributed by Jack Gosling and Ruth Skoug of Los Alamos National Laboratory, Dave McComas of Southwest Research Institute and Chuck Smith of the University of New Hampshire. Address comments or questions to firstname.lastname@example.org.
Last modified 9 Nov 2004, by