Thunder Force AC is a horizontal space shooter from Sega/Technosoft. Due to the success of Technosoft's Thunder Force III for the Mega Drive (Sega Genesis), it was decided that the game would be brought to the arcade scene in the form of Thunder Force AC. Ported to Sega's System C-2 hardware, Thunder Force AC is almost graphically identical to its Mega Drive counterpart, with only very minor and often unnoticeable differences.
Thunder Force AC has been described as a retooling of Thunder Force III because it borrows enemies and stages from the earlier Thunder Force II, as well as adding some original content. The original Thunder Force AC Jamma board is now rare, but the game was ported to the Super Nintendo (SNES) in 1992 as Thunder Spirits. A Sega Saturn port was made and included on the 1996 release Thunder Force Gold Pack 2.
Source: Wikipedia, "Thunder Force AC", available under the CC-BY-SA License.