No - not John Lennon's wife (not even the same spelling)!
The name of a Japanese satellite, launched in 1991, which gave detailed images and spectra of X-rays from the Sun. The name means 'sunbeam' in Japanese. One of Yohkoh's instruments was built in the UK.
Unfortunately, after many years of successful operation, Yohkoh stopped working in December 2001. Even so the data it produced are still being used in many solar physics research projects.
Find out more about YOHKOH