It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.
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.
It is possible to reproduce the configuration of the neutral line on the solar source surface by the axial dipole at the center of the Sun and a few fictitious dipoles on the photosphere. In this paper we attempt to identify the nature of such fictitious dipoles in the photospheric magnetic fields. It is shown that large‐scale photospheric dipole fields can be identified clearly at the locations indicated by the fictitious dipoles when the photospheric field is very simple. They are found to be active regions.
Comets P/Brorsen‐Metcalf and C/Okazaki‐Levy‐Rudenko appeared successively in 1989 and displayed various disturbances of their magnetospheres. We observed two cases of a phenomenon in which a disconnection event of the cometary plasma tail (on August 13 and November 16, 1989) was followed by a terrestrial magnetic storm (on August 14 and November 17). A survey of solar flares suggests that an identical solar flare excited successively the cometary magnetosphere and the Earth's magnetosphere in each case. The average velocities of the shock front in interplanetary space were estimated by using the magnetospheric disturbances of both the comets and the Earth together with the assumed responsible solar flare. It is speculated from the results that the propagation of the shock front associated with the flare is not symmetric with respect to the radial axis from the flare region of the Sun.
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