B737-700 FLC RECEIVES RA DURING CRUISE.

Narrative

I WAS THE PF. DURING CRUISE FLT AT FL280; WE WERE APPROX 10 MI FROM BIKKR INTXN WHEN WE RECEIVED A TCASII ALERT SHOWING A TARGET AT 11:30 O'CLOCK POS AND 20 MI. THE TARGET PRESENTATION HAD GONE FROM WHITE/SMALL TO YELLOW/LARGE. THE TARGET WAS SHOWING LEVEL AT FL288. THE SCALE ON THE MOVING MAP DISPLAY WAS ON 40 MI; SO I CHANGED IT TO 20 MI TO INCREASE THE ACCURACY OF THE TARGET'S LOCATION. THE ACFT REMAINED AT FL288 AND THERE WAS NO CLB/DSCNT ARROW. WITH BOTH CREW MEMBERS LOOKING FOR THE TFC; THERE WAS NO TIME TO CONTACT ATC; NOR DID THEY CONTACT US. I SELECTED THE 'FLT LEVEL' MODE FOR THE TCASII DISPLAY; AND FURTHER REDUCED THE SCALE TO 10 MI AS THE TARGET ACFT WAS WITHIN 9 MI OF OUR POS. AT THIS JUNCTURE; THE TARGET ACFT WAS STILL LEVEL AT FL288 AND NEITHER MYSELF NOR THE FO HAD VISUAL CONTACT WITH THE TARGET ACFT. AS THE INTRUDER ACFT NEARED THE 5 MI ARC; I FURTHER REDUCED THE TCASII SCALE TO 5 MI AND WE RECEIVED AN RA COMMANDING US TO DSND AT A RATE OF 2000 FPM. I DISCONNECTED THE AUTOPLT FROM THE ALT HOLD/RNAV MODE TO FOLLOW THE COMMAND AS THE FO MADE VISUAL CONTACT WITH THE NEARING ACFT. I HESITANTLY STARTED THE DSCNT; FOR I NOTICED ON THE TCASII DISPLAY A 'DOWN ARROW' AND THAT THE ACFT'S ALT WAS DSNDING. SOMETHING DID NOT LOOK RIGHT. I SHALLOWED OUT THE DSCNT AT FL277 WHILE THE FO POINTED OUT THE ACFT AT 10 O'CLOCK POS; 1-2 MI OFFSET TO OUR L. IT WAS A SMALL JET DSNDING. HAVING JUST STOPPED THE DSCNT; THE TCASII NOW COMMANDED A CLB TO BE EXECUTED AT A RATE OF 1500 FPM. WE CLBED BACK TO OUR CRUISING ALT OF FL280. AS THE ACFT PASSED OUR L WING; ATC CONTACTED US ABOUT THE TFC. WE INFORMED THEM THAT THE ACFT WAS IN SIGHT AND THAT WE HAD AN RA. ATC THOUGHT THAT THE ACFT WAS LEVEL; AND WE INFORMED THEM THAT THE OTHER ACFT WAS NOT LEVEL AND IN FACT WAS DSNDING. A FACTOR CONTRIBUTING TO THE CONFUSION WAS THAT THE INTRUDER ACFT WAS IN ZLA'S AIRSPACE WHILE WE WERE IN ZOA'S AIRSPACE. THE CTLRS WERE UNSURE IF THE TARGET ACFT HAS FOLLOWED A TCASII COMMAND. HAD WE AGGRESSIVELY FOLLOWED THE TCASII COMMAND TO DSND AT 2000 FPM; WE WOULD HAVE BEEN AT THE SAME ALT AS THE SMALL JET AND HAD A DIFFICULT TIME TRYING TO RECOVER AND FOLLOW THE TCASII COMMAND. THE CTLR ASKED US TO CALL THEM UPON ARR. I CONTACTED DISPATCH; ADVISED THEM OF THE EVENT AND ASKED IF THEY WANTED ANY RECORDER CIRCUIT BREAKERS PULLED. AFTER CONFERRING WITH DUTY CTL; DISPATCH JUST ASKED FOR US TO CALL THEM UPON ARR.