Coronal Plumes and Their Relationship to Network Activity

Using Skylab extreme-ultraviolet spectroheliograms, we address the question of what lies under a coronal plume. Plumes and their base areas, both inside polar coronal holes and within lower latitude holes near central meridian, are identified in the Mg IX λ368 emission line. While some (usually spike-shaped) plumes show a strongly enhanced Mg IX core, others (sheetlike in appearance) are characterized by a much more diffuse base which may extend over several supergranules. The base areas are found to contain collections of compact (although not always intense) Ne VII λ465 features, whose locations in turn generally coincide with enhancements in the He II λ304 network inside the coronal hole. Bright plumes always show intense network features within their base areas, but the converse does not hold: not every Ne VII or He II bright point has an associated Mg IX plume. By comparing the locations of plumelike Mg IX "haze" in a lower latitude hole with a simultaneous high-resolution magnetogram, we infer that coronal plumes occur near regions of mixed magnetic polarity. We suggest a mechanism for plume formation, whereby small bipoles within a coronal hole reconnect with unipolar flux concentrations located at network junctions.