Open magnetic structures on the sun

High-resolution harmonic analysis of the solar magnetic field has been used succesfully to calculate the geometry of open magnetic field lines in the solar corona. Comparison of the loci of open-field-line footpoints with solar X-ray photographs shows that all the coronal holes during two solar rotations are successfully represented, including details of their evolution. Some open magnetic configurations derived in the calculations precede by up to one solar rotation the manifestation of coincident dark areas on the X-ray photographs. The only other areas that contribute open field lines to the corona are separations between active-region loop systems. By varying the radius at which field lines are forced to be open in the calculation, it is possible to reproduce more closely the surface configuration of particular coronal holes. Comparison of the size of X-ray holes with the fraction of the solar surface covered by open field lines leads to the conclusion that a significant part of the area of coronal holes must contain closed magnetic fields. Comparison of open field lines which lie in the equatorial plane of the sun with solar-wind data indicates that eventual high-speed solar-wind streams are associated with those parts of open magnetic structures that diverge the least.