Recently, “phase diagrams” of magnetic reconnection were developed to graphically organize the present knowledge of what type, or phase, of reconnection is dominant in systems with given characteristic plasma parameters. Here, a number of considerations that require caution in using the diagrams are pointed out. First, two known properties of reconnection are omitted from the diagrams: the history dependence of reconnection and the absence of reconnection for small Lundquist number. Second, the phase diagrams mask a number of features. For one, the predicted transition to Hall reconnection should be thought of as an upper bound on the Lundquist number, and it may happen for considerably smaller values. Second, reconnection is never “slow,” it is always “fast” in the sense that the normalized reconnection rate is always at least 0.01. This has important implications for reconnection onset models. Finally, the definition of the relevant Lundquist number is nuanced and may differ greatly from the value based on characteristic scales. These considerations are important for applications of the phase diagrams. This is demonstrated by example for solar flares, where it is argued that it is unlikely that collisional reconnection can occur in the corona.
Received 06 October 2012Accepted 29 November 2012Published online 17 June 2013
P.A.C. gratefully acknowledges support by NSF grant (No. AGS-0953463) and NASA grant (No. NNX10AN08A). This manuscript has been prepared as part of the 2012 US-Japan Workshop on Magnetic Reconnection in Princeton, New Jersey in May, 2012. The authors acknowledge many fruitful conversations at this meeting, especially those with M. Swisdak and H. Ji, and thank H. Karimabadi for pointing out a minor error in Fig. 4.
Article outline: I. INTRODUCTION II. THE PHASE DIAGRAMS III. OMISSIONS FROM PHASE DIAGRAMS A. No reconnection for small S B. History dependence of reconnection IV. INTERPRETATION CAUTIONS A. The “plasmoid induced transition to Hall reconnection” is not a separate phase B. The secondary island to Hall transition is an upper bound on S C. 2D reconnection is always fast! D. The definition of S is nuanced V. IMPLICATIONS FOR SOLAR FLARES VI. DISCUSSION