Study of the Cyclical Variations of Coronal Holes and Their Relation to Open Magnetic Fields

摘要

This contract was a study of possible variations in the observed flux from coronal holes over a solar cycle and their relationship to solar magnetic fields. The variation of X-ray emission from coronal holes was measured and compared with data on photospheric magnetic field strength and models of open field structures from holes. The data set consisted of reduced X-ray images from AS&E rocket flights and digitized photospheric magnetic field strength data from Mt. Wilson and Kitt peak observatories over the period 1974 to 1981. The results are as follows. We found that soft X-ray emission from coronal holes could be detected in the rocket data, confirming earlier Skylab results. Also, this emission appeared to vary systematically over an 8-year period, from a minimum in mid-1974 to a maximum about one year after sunspot activity maximum. This conclusion is strengthened by three other observations: (1) HeI 10830 A coronal holes exhibited decreased contrast during this same period; (2) the particular holes that we measured were typical of those at each phase of the cycle in terms of their evolutionary and interplanetary characteristics; and (3) the X-ray hole emission followed a similar trend in Harvey et al.'s (1982) data whereby the photospheric field strength of near-equatorial holes increased systematically by a factor of three between 1975 and 1980. Further, our own measurements of field strengths within the X-ray hole boundaries were sparse but generally in agreement with the Harvey et al. trend.