For years the field of Space Physics has had a problem, a really big problem for it occurs on the largest spatial scales in Space Physics --- across the entire region under the Sun's influence, the heliosphere. The problem is that the Sun appears to keep opening new magnetic fields into interplanetary space with no obvious way to close them back off again. This state of affairs, without some method for closing the open interplanetary magnetic field (IMF), would lead to an ever growing IMF magnitude in interplanetary space --- a catastrophe that is clearly not observed.
Figure 1 displays a composite picture of the Sun's outermost atmosphere or corona from 30 June 1973. The outer portion was taken in ordinary white light from the ground during a solar eclipse. The superposed soft X-ray image of the denser, near-Sun corona was taken from Skylab on the same day. The structure evident in the images is a consequence of the solar magnetic field that permeates the corona; more and less populated magnetic fields are mapped out as density structures in these images. Coronal holes are low-density regions where the magnetic fields open out into interplanetary space; a large coronal hole is displayed as the dark region near the top of Figure 1. In contrast, bright loop-like structures indicate closed field regions on the Sun that contain high-density plasma. The centers of helmet streamers (closed field loops overlaid by nearly radial high density streamer structures), such as those seen extending outwards to the left and right sides in Figure 1, map out to large reversals of the magnetic field or current sheets in interplanetary space. The solar wind flow from streamers is slower and higher density than it is from coronal holes [e.g., Borrini et al., 1981]. Coronagraph and soft X-ray observations over the past several decades have shown that the solar corona is highly dynamic with open and closed regions evolving over time scales as short as minutes and as long as the 22-year solar cycle.
Because the interplanetary magnetic field is ``frozen'' into the solar wind plasma by its high electrical conductivity, the concept of spaghetti-like magnetic field lines provides a useful paradigm for understanding the magnetic field configuration in interplanetary space. Since the magnetic field is divergence free, all field lines are closed. That is, field lines must connect back to themselves 1) back at the Sun, 2) somewhere far out in the heliosphere, or 3) locally in interplanetary space. Figure 2 utilizes a ``flat heliosphere'' representation to portray the only four fundamentally distinct ways that magnetic field lines in interplanetary space can close. A displays ``open'' field lines that connect between the Sun and the outer heliosphere. A magnetic tongue geometry, where the magnetic field closes to the Sun at both ends is shown in B. C displays a fully disconnected magnetic bottle or plasmoid which closes locally on itself in interplanetary space. Finally, D shows disconnected U-shaped field lines which extend to the outer heliosphere at both ends. The solar wind flows outward (from bottom to top in Figure 2), so that the field lines in B naturally evolve into open field lines (A) while field lines in C and D simply propagate out through the heliosphere and are lost.
Over the past several years, progress has been made in understanding the ways in which open and closed magnetic field regions may evolve from the solar corona out through interplanetary space. This paper examines this progress in understanding in four interrelated areas: 1) the opening of new magnetic fields from the Sun; 2) the rates of opening field lines and implications for unbridled growth of field magnitude; 3) a newly appreciated method for closing off and returning field lines to the Sun; and 4) how a balance between the opening and closing of field lines might be maintained.