The image on the left shows the full Sun prior to the transit. The slightly diagonal line indicates Mercury's trajectory near the solar south pole, and the box shows the high-resolution field of view displayed on the right-hand panel. On this panel, one can see a dark dot migrating across the diffuse corona, top to bottom, as time progresses. The apparent diameter of the planetary disk was only about four SXT pixels at the time of the transit. The time sequence of images also shows an X-ray jet, incidentally, and we will do a retrospective nugget on this Yohkoh discovery in some future week.
Transits of Mercury are pretty rare, and observations in soft X-rays (other than this) non-existent. We have been told by R. Noyes that SkyLab also observed a Mercury transit, presumably the one of November 10, 1973, but there may not be any published literature. The Yohkoh observations in fact turned out to be quite crucial in calibrating the SXT and HXT image alignment; see the paper by Wuelser et al. to appear shortly in Solar Physics.
To summarize the work of Wuelser et al., the 1993 transit was used for two purposes. First and most important, it provided an absolute calibration of the Yohkoh spacecraft roll position (ie, rotation around the line of sight to the spacecraft). For high-resolution telescopes this normally requires a stellar reference, and Mercury served admirably to provide a final calibration of the on-board Canopus sensor of Yohkoh. A second objective of the observation was to provide some information on scattered light - the disk of the planet should in principle provide no signal whatsoever, not counting cosmic-ray albedo X-rays as observed by ROSAT from the dark face of the Moon. But, Mercury is too small and fast-moving for this purpose. We look forward to observations of the next transit of Venus (in 2004).
Sorry to be so late in nuggetizing this rather
nice observation, but there is ample time left still to prepare for
the next one - November 15, 1999. A full
discussion for amateur astronomers of transits of Mercury exists on
the Web, but note that it's in Spanish.