Coronal holes and high‐speed wind streams

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.