THE DEPENDENCE OF MAGNETIC RECONNECTION ON PLASMA β AND MAGNETIC SHEAR: EVIDENCE FROM SOLAR WIND OBSERVATIONS

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, theta, across the current sheet and the difference in the plasma beta values on the two sides of the current sheet, Delta beta. For low Delta beta, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large Delta beta: Events with shears as low as 11 degrees were observed for Delta beta < 0.1, but for Delta beta > 1.5 only events with theta > 100 degrees were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high beta plasmas at low magnetic shears due to super-Alfvenic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-Delta beta dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.