The detection by the Roentgen satellite (ROSAT) x-ray telescope of a shadow in the 1/4-kiloelectron volt (C band, 0.1 to 0.284 kiloelectron volt) cosmic diffuse background is reported. The location and morphology of the local minimum in x-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance approximately 600 parsecs); it then follows that there is significant 1/4-kiloelectron volt x-ray emission at a large distance (>400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and, if it indicates the existence of a hot galactic corona, it must be patchy in nature.
A detailed study of the quasi-periodical post-flare variations on November 6, 1980 in X-rays, UV lines, microwaves, and metric waves confirms that these variations were predominantly thermal phenomena and occurred solely in the corona. Only the short-lived impulsive components that preceded all or most of the individual variations were of non-thermal character and penetrated down to the transition layer. The chromosphere (in Hc~) did not participate in any part of these events, in contrast to a flare that appeared at the same place a few hours later. However, the X-ray emission of these variations was so strong that the transition layer and the chromosphere definitely should have been enhanced through heat conduction along the magnetic field lines. The expected heat flux at the top of the chromosphere coming from some of these coronal brightenings was 60-80% of the flux expected in the flare at 17 : 26 which gave rise to a 2B flare in Ha (Figure 8). Therefore, we suggest that the variations were produced in a coronal plasmoid with closed field lines completely detached from the lower atmospheric layers (Figure 9b). We also give reasons why such a detached plasmoid can be expected to be formded in the very late phase (some 4-5 hr after the onset) of a major two-ribbon flare.
We report observations of the solar radio radius at wavelengths between 1.2 and 11 cm performed with the Bonn 100 m-telescope. In combination with former measurements of the centre-tolimb variation of the solar brightness these observations are discussed in terms of atmospheric models. We consider the solar disk to be covered by arches at low latitudes, while at the poles coronal holes are located. The temperature dependence on height is taken from EUV-line intensities, hydrostatic equilibrium is adopted, spicules are assumed to be responsible for the relatively low brightening. The interpretation of our measurements demands certain values of the brightness temperature of spicules as a function of wavelength within a modest interval.
On July 24, 1973, a coronal hole was observed at 10.69 GHz (2.8 cm) with the Bonn 100-m telescope. The difference of the brightness temperature between outside and inside the hole was about 400 to 500 K. It is shown that this lack of emission can be explained by usually adopted values of the electron density at the bottom of the corona.
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