Origin and development of transient coronal holes

Solar transient coronal holes (TCHs) are short-lived (less than or equal to2 days) regions of dimmed X-ray intensity sometimes observed in association with coronal mass ejections. These features, first discovered from Skylab observations, can occur in magnetic unipolar regions on either side of the X-ray posteruptive arcades. They have been suggested as the magnetically open footpoints of associated transient flux ropes observed at 1 AU. We have used images from the Yohkoh Soft X-ray Telescope (SXT) to study the development of 19 TCH events obtained in a survey of 9 years of Yohkoh observations. We find that the boundaries of the TCHs are never static. The boundaries closer to the magnetic neutral line generally move away from it as the closed-loop X-ray arcades expand. In addition, previously closed coronal loops at the ends of the arcades often continue to expand and open on the outer boundaries of the TCHs. These processes typically last for hours. The arcade brightenings do not extend into the full areas of the TCHs. The TCHs tend to disappear only by a net contraction of the boundaries, rather than by brightening within their boundaries. The location of a TCH appears to coincide with a large-scale curvature of the magnetic neutral line or the occurrence of a nearby active region at one end of the coronal eruption. This distinguishes the formation of TCHs from the arcade development, suggesting that there is no requirement for a pair of TCHs or even any TCH to be formed in an eruptive event. The moving magnetic boundaries, uniformly dark interiors, and short lifetimes of TCHs pose significant problems for the interpretation that TCHs are footpoints of interplanetary magnetic flux ropes.