Soft X-Ray Coronal Holes Observed by the Yohkoh SXT

Abstract

Many coronal holes have been observed by the Yohkoh Soft X-ray Telescope (SXT). These holes appear as dark regions in the soft X-ray images. It is now widely accepted that coronal holes are sources of high-speed solar wind. However, the size of coronal holes has not been clearly defined in previous works, and the relationship between soft X-ray coronal holes and high-speed solar wind is still not fully understood. In this paper we check the results from Skylab observations against the Yohkoh SXT data. We examined the boundaries of soft X-ray coronal holes imaged by the SXT. We then analyzed the characteristics of coronal holes associated with high-speed solar wind at 1 AU and compared the results with Skylab data. Faster solar wind seems to blow from larger and darker coronal holes. Coronal holes at relatively high latitudes (more than 30°), which extend from polar coronal holes, also produce high-speed streams, as coronal holes located around the solar equator. The manifestation of coronal holes and the sudden changes in their boundaries are frequently associated with large-scale coronal disturbances (LCDs) such as filament eruptions or flares. LCDs maybe a possible source of non-recurrent interplanetary disturbances, and transient holes suggest the formation of open magnetic fields related to these events.