There was no independently funded guest investigator program for SXT. As mentioned elsewhere in these notes, this situation not only reduced the data analysis effort directly because of lack of funding, but also probably contributed to the wrong impression regarding data policy.
It is difficult now to attend a solar session of any major meeting, on any continent, and not see Yohkoh data being used. There are two aspects to this. First, the Yohkoh movie of the soft X-ray corona, some 50 images per day, nicely characterizes the solar origins of space weather. The movie essentially provides maps to the origins of the Geostationary Operational Environmental Satellite (GOES) soft X-ray photometry, provided by the National Oceanic and Atmospheric Administration, long used to characterize the time development of coronal magnetic activity. Second, the movie of routine images itself, but even more so the special data sets covering flares and other activity, form the basis for a great deal of unique Yohkoh research. In the sense that science is fundamentally international and no one nation's property, the solar community worldwide benefits a great deal from the existence of the Yohkoh data.
However, national programs exist at least partly because of national interests, so it makes sense to inquire about narrow U.S. benefits. One main measure of the benefit of a “big science” program, of which Yohkoh represents a small example, might be the training of graduate students, especially in terms of their participation in the instrumentation. Because the lead U.S. institutions involved in Yohkoh are large commercial or public laboratories, students were not involved to a great extent in the instrumentation. However the SXT team made a special point to incorporate universities (specifically Stanford, Berkeley, and Hawaii, and now Montana State) directly into the project, so that observational Ph.D. theses based on Yohkoh data or related theory did happen. Probably as many Yohkoh-based Ph.D. theses have been written outside Japan as inside, including several in Europe from groups not connected in any formal way with the instrument groups. A separate plus on the U.S. ledger, of course, has to do with the successful development of high-technology instrumentation mainly from U.S. sources. This contributes to the development of optics, detectors, and software technology, for example.
The era of a simple U.S.-Japanese collaboration on a small space mission seems to have ended, because ISAS no longer schedules small missions. This is a flip-flop in comparison with the U.S. programs, which, starting about the time Yohkoh began development, saw a renewal of interest in flexible small flight opportunities. So the cultures of space science in the two countries seem to have traded places to a certain extent. This is also reflected in the speed of development of the programs, with the U.S. space programs now happening on astonishingly short development schedules (e.g., TRIANA, HESSI, or many other Small Explorers (SMEXs) and University Explorers (UNEXs)).
For larger missions, such as Solar-B (in phase A following an AO and selection on the U.S. side), the Yohkoh experience may not play much of a role. In comparison the Solar-B payload is extremely complex, and the science that it addresses equally so. How do two quite different science communities plan such a mission together? The basic mechanism regulating the growth of science knowledge, the open literature, moves so ponderously and with such apparent confusion and misunderstanding, that it cannot serve as a good basis for decision making involving a broad community. Thus we have science working groups to define future missions. In the case of Solar-A the next step was pretty clear scientifically; for Solar-B or any other major mission, other forces come into play. Thus the main legacy of Yohkoh may simply be the goodwill of the groups participating in it and the enhanced communications resulting from working within the same program so closely.