F100 FLT CREW HAS TCASII RA DURING INITIAL DSCNT INTO DAY.

Narrative

LOCATION APPROX 37 NM W OF DAYTON ARPT WHILE DSNDING TO 10000 FT; HEADING ROUGHLY ENE TOWARDS THE DAYTON ARPT; AND IN RADAR AND RADIO CONTACT BEING VECTORED BY DAY APCH CTL; IN VERY HAZY CONDITIONS. WE RECEIVED A SUDDEN TCASII RA. THE FO; PF; IMMEDIATELY PUNCHED OFF THE AUTOPLT AND SMOOTHLY FOLLOWED THE TCASII RA COMMANDS TO CLB. WE CLBED UP TO ABOUT 11100 FT; AND THEN GOT A CLR OF CONFLICT -- RETURNING RECOMMENCING OUR DSCNT TO 10000 FT. I TRIED TO SPOT THE OFFENDING ACFT; BUT NEVER GOT A VISUAL ON HIM; AND HE WAS APPARENTLY A VFR LIGHT ACFT FLYING S WESTERLY AT 10000 FT LEVEL AT AN IFR ALT. HE NEVER CHANGED HIS COURSE NOR ALT; NOR WAS IN RADIO CONTACT; BUT APPARENTLY HAD A GOOD TCASII CAPABLE XPONDER. HIS CLOSEST POINT OF APCH WAS 500 FT BELOW US ACCORDING TO THE TCASII DISPLAY. AS HE WAS FLYING AT AN IFR ALT; AN ATTEMPT TO FIND THIS PLT SHOULD BE MADE FOR CORRECTIVE ACTION. ALSO; THE APCH CTLR GAVE US NO WARNING OF THIS ACFT THAT SHOULD HAVE BEEN CLRLY DISPLAYED ON HIS SCOPE. THIS WAS A GENUINE TCASII SAVE; AND I AM GLAD THAT WE HAVE THEM. I WOULD LIKE TO KNOW THE OUTCOME OF THE SEARCH FOR THIS ACFT AND PLT THAT DOES NOT FOLLOW THE APPROPRIATE RULES FOR VFR FLT CRUISING ALT. THIS PLT PRESENTS A SERIOUS HAZARD TO OTHER AIRSPACE USERS.