5.3 The Reconnection/Loop-Opening (RLO) solar wind idea

It is clear from observations of the Sun’s highly dynamical “magnetic carpet” (Schrijver et al., 1997Jump To The Next Citation PointTitle and Schrijver, 1998Hagenaar et al., 1999) that much of coronal heating is driven by the continuous interplay between the emergence, separation, merging, and cancellation of small-scale magnetic elements. Reconnection seems to be the most likely channel for the injected magnetic energy to be converted to heat (e.g., Priest and Forbes, 2000). Only a small fraction of the photospheric magnetic flux is in the form of open flux tubes connected to the heliosphere (Close et al., 2003). Thus, the idea has arisen that the dominant source of energy for open flux tubes is a series of stochastic reconnection events between the open and closed fields (e.g., Fisk et al., 1999Ryutova et al., 2001Fisk, 2003Schwadron and McComas, 2003Feldman et al., 2005Schwadron et al., 2006Schwadron and McComas, 2008Fisk and Zhao, 2009).

The natural appeal of the RLO idea is evident from the fact that open flux tubes are always rooted in the vicinity of closed loops (e.g., Dowdy et al., 1986) and that all layers of the solar atmosphere seem to be in continual motion with a wide range of timescales. In fact, observed correlations between the lengths of closed loops in various regions, the electron temperature in the low corona, and the wind speed at 1 AU (Feldman et al., 1999Gloeckler et al., 2003) are highly suggestive of a net transfer of Poynting flux from the loops to the open-field regions that may be key to understanding the macroscopic structure of the solar wind. The proposed RLO reconnection events may also be useful in generating energetic particles and cross-field diffusive transport throughout the heliosphere (e.g., Fisk and Schwadron, 2001).

Testing the RLO idea using theoretical models seems to be more difficult than testing the WTD idea because of the complex multi-scale nature of magnetic reconnection. It can be argued that one needs to create fully three-dimensional models of the coronal magnetic field (arising from multiple magnetic elements on the surface) to truly assess the full range of closed/open flux interactions. The idea of modeling the coronal field via a collection of discrete magnetic sources (referred to in various contexts as “magneto-chemistry,” “tectonics,” or “magnetic charge topology”) has been used extensively to study the evolution of the closed-field corona (e.g., Longcope, 1996Schrijver et al., 1997Longcope and Kankelborg, 1999Sturrock et al., 1999Priest et al., 2002Beveridge et al., 2003Barnes et al., 2005Parnell, 2007Ng and Bhattacharjee, 2008), but applications to open fields and the solar wind have been rarer (see, however, Fisk, 2005Tu et al., 2005).

In order to develop the RLO paradigm to the point where it can be tested more quantitatively, several key questions remain to be answered. For example, how much magnetic flux actually opens up in the magnetic carpet? Also, what is the time and space distribution of reconnection-driven energy addition into the (transiently) open flux tubes? Lastly, how is the reconnection energy distributed into various forms (e.g., bulk kinetic energy in “jets,” thermal energy, waves, turbulence, and energetic particles) that each affect the accelerating solar wind in different ways? Combinations of simulations, analytic scaling relations, and observations are needed to make further progress.

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