For the opening and closing of magnetic fields from the Sun to maintain some sort of equilibrium, neither growing nor shrinking without bound, there must be some interaction or feedback between these two processes. One likely interaction [ McComas et al., 1989; 1991; McComas, 1994] is through magnetic pressure; the expansion of newly opened field regions must enhance the magnetic pressure which will compress the open field lines, including those above helmet streamers, elsewhere around the Sun. Ultimately, when enough pressure has built up, reconnection may be initiated, which will reduce the number of open field lines and hence the magnetic pressure. In this way, the opening and closing of the IMF could maintain a sort of rough equilibrium.
Direct evidence for at least part of this process is provided by the 27 June 1988 coronal disconnection event. In this case, an impulsive compression, indicated by the deflection of the streamers in the corona, preceded and appeared to initiate the coronal disconnection event [ McComas et al., 1991]. Another line of supporting evidence is provided by numerical simulations [ Linker et al., 1992]. These authors used a 2-D MHD simulation of the corona to examine the effect of increasing the overall magnetic pressure around an equilibrium helmet streamer. They found that increased field pressure led to reconnection of the previously stable streamer configuration which continued until that pressure was sufficiently reduced to again achieve equilibrium.
The opening and closing processes described here could also explain the evolution of magnetic structures back in the corona. Coronal structures such as coronal holes, streamers, and loops, appear, disappear, and evolve on a wide range of time scales, from minutes to years. The global coronal structure also changes from simple and dipolar at solar minimum to very complicated at solar maximum. Both of these sorts of reconfigurations seem to be easier to explain by opening and closing processes than by motions of magnetic footprints across huge distances on the Sun.
Acknowledgments. Many valuable discussions with and comments on this manuscript by J.T. Gosling as well as suggestions on wording by J. Gustafson are gratefully acknowledged. This work was carried out under the auspices of the United States Department of Energy.