Quick perusal of http://www.nikkei.co.jp/ suggests that it means (literally) sun circle of longitude. A possible freer translation might be circle of Japanese (literally, sun origin, the first character in Nihon is the character for sun) influence. Other, more educated opinions are welcome.

I found it interesting that you refer to FTSE, which many purists would not consider an acronym due to its unpronounceability, as an acronym but refer to the easily pronounceable NASDQ and DAQ as would-be acronyms.