No abstract

Coronal holes are regions of unusually low density and low temperature in the solar corona. During a 9‐month Skylab solar workshop, 50 participants established some of the basic properties of coronal holes and their associated high‐speed wind streams using a combination of Skylab, satellite, and ground‐based observations. The holes have been identified now as Bartel's M regions, i.e., sources of high‐speed wind streams that produce recurrent geomagnetic variations. Throughout the Skylab period the polar caps of the sun were coronal holes, and at lower latitudes the most persistent and recurrent holes were equatorial extensions of the polar caps. The holes rotated ‘rigidly’ at the equatorial synodic rate. They formed in regions of unipolar photospheric magnetic field, and their internal magnetic fields diverged rapidly with increasing distance from the sun. The geometry of the magnetic field in the inner corona seems to control both the physical properties of the holes and the global distribution of high‐speed wind streams in the heliosphere. The diverging field lines in well‐established holes act as Laval nozzle which produces supersonic flow and depressed densities at low altitudes. The latitude variation of the divergence of the coronal magnetic field lines produces corresponding variations in wind speed, in agreement with interplanetary scintillation measurements and multisatellite observations. During the years of declining solar activity the global field of the corona approximates a perturbed dipole. The divergence of field lines in each hemisphere produces a high‐speed wind near the poles and low‐speed wind in a narrow belt that coincides with the magnetic neutral sheet. The width and magnetic polarity of recurrent wind streams, as measured near earth, has been predicted successfully on the basis of the global coronal magnetic field geometry, at least during the Skylab period. The analysis of electron density measurements within a polar hole indicates that solar wind is accelerated principally in the region between 2 and 5 Rs and that mechanical wave pressure (possibly Alfvén wave) may be responsible for the acceleration of the wind. Phenomenological models for the birth and decay of coronal holes have been proposed. A global pattern of formation, with a systematic eastward drift of successive hole appearances, suggests that the diverging magnetic fields of the coronal holes arise through dynamo action in the deep convection zone. However, attempts to explain the birth and rigid rotation of holes through dynamo action have been only partially successful. More observational data exist on the solar cycle variation of wind streams than on coronal holes. The polar holes shrink, and the volume occupied by the neutral sheet may increase near solar maximum. The 11‐year variation of cosmic ray intensities at the earth may result from cyclic variation of open field regions associated with coronal holes.

We present observations of the early stages of formation of a Ðlament channel when a compact activity complex emerged in a previously quiet, near-equatorial area. In a few hours, and while Ñux was rising rapidly in one bipolar component in the complex, Ha Ðne structure overlying a polarity inversion zone organized into a conspicuous pattern of parallel Ðbrils enclosing the trailing end of the new activity complex. Yet it took another 4 days for a stable Ðlament to form inside that pattern. It did so at a place where migrating positive polarity Ñux from the new activity complex contacted the negative polarity Ñux in a plage of an adjacent decaying bipolar active region. In contrast, no Ðlament formed along an existing channel inside the adjacent decaying region ; the opposite-polarity Ñuxes on the borders of the existing channel showed no signs of convergence. We attribute the Ðbril-aligning forces in the new channel to the horizontal component of an extended nonpotential magnetic Ðeld caused by currents in the multipolar activity complex. The channel is, in this view, an elementary part of the magnetic topology of an activity complex. We propose that the later formation of the Ðlament in the new channel requires an additional and separate process. A plausible candidate for this second step is the development of a current sheet at the site of converging magnetic Ñux.