| Summary: | The Skylab experiments acquired a wealth of observations applicable to investigating problems associated with the mass and energy flow in the solar atmosphere. In this review we discuss some of the observations of quiet regions, coronal holes and active regions and Illustrate the fundamental role that magnetic fields play in defining the structure and the mass and energy flow in these regions. Figure 1 (Altschuler et al., 1976) illustrates three major classes of structure: (1) strong field, magnetically closed areas typified by active regions, (2) weak field, open regions which, according to present evidence, appear to be associated primarily with coronal holes, and (3) weak field regions which appear to be magnetically closed on a large scale and appear to be associated primarily with normal quiet areas. Some of these latter structures can be seen in the X-ray photograph presented in Figure 2 (Vaiana et al., 1973a). Also evident in this figure are large coronal holes near the east limb at the north pole.
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