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 againstthe 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 highspeed 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 apossible source ofnon-recurrent interplanetary disturbances, and transient holes suggest the formation of open magnetic fields related to these events.

The effect of the high-speed stream following the corotating interaction region on the geomagnetic activities

“…However, the relation between the coronal-hole area and the observed maximum velocity is not good (see Table 1). This result supports the recent result by Watari et al (1995), and might suggest saturation, because all high-speed streams selected here exceed 700 km/s at their maximum speed. Usually a high-speed stream associated with coronal holes does not exceed 850 km/s at Earth.…”

“…Nolte et al (1976) noted that the velocity of the solar wind from coronal holes is roughly proportional to the area of the holes, and the holes located near the equator have more chance of aecting the Earth than polar ones. However, the recent analysis by Watari et al (1995) showed that the relation between coronal-hole area and solar-wind velocity is more complicated than that derived by Nolte et al (1976).…”

[ITAL]Yohkoh[/ITAL] SXT Observations of X-Ray “Dimming” Associated with a Halo Coronal Mass Ejection

“…Transient coronal holes seen with SXT have been interpreted in terms of flux ropes in several previous studies (e.g., Watari et al 1995;Kozuka et al 1995;Manoharan et al 1996;Smith et al 1997). There were no corresponding direct observations of CMEs for these previous studies, but the event reported by Smith et al resulted in an interplanetary magnetic cloud that is consistent with our interpretation in terms of a flux rope.…”

The birth of coronal holes

“…At the same time answers to key questions which are related to the originating (the birth) of coronal holes till now are not obtained. It is known a little works in which the birth of CHs were investigated (Solodina et al 1977;Bohlin & Sheeley 1978;Watari et al 1995). The objective of this paper is to study the birth of non-polar CHs.…”