ACR MLG ALTDEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

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FLT WAS CLRED TO CROSS CETUS INTXN NW OF CLE AT 12000 FT. APCH CLRED US DOWN TO 11000 FT TURNED US TO A 320 DEG HDG AND INFORMED US WE'D SEE XING TFC (W OF E) DEPARTING DTW CLBING TO 10000 FT. CLOUD DECK BEGAN AT APPROX 7500 FT +/- (SOLID). FO WAS FLYING WITH AUTOPLT ENGAGED. TCASII SHOWED PROXIMATE TFC BELOW OUR ALT. VISUALLY WE COULD SEE THE LIGHTS OF MANY (?) OTHER ACFT AHEAD; TO OUR L AND DISTANT R. I OBSERVED TFC BELOW PASSING L TO R (W TO E) AND TCASII SHOWED THAT TFC WITH A 'TAG' INDICATING IT WAS BELOW US APPROX 1000 FT. ON APCH BTWN 4500 FT AND 4000 FT. SUDDENLY TCASII AURALLY COMMANDED AN RA 'DSND; DSND; NOW! -- DSND; DSND; NOW.! BOTH OF US LOOKED OUTSIDE MOMENTARILY; I SAW NO TFC AHEAD WITHIN 60 DEGS EITHER SIDE OF CENTER. I TOLD THE FO TO DSND WHILE DROPPING MY FOCUS TO THE TCASII DISPLAY AND NOTED NOTHING OTHER THAN A RED ARC EXTENDING FROM +6000 FT DOWN TO 1500 FT WHERE A GREEN ARC EXTENDED BTWN -1500 TO -2000 FPM. THE FO SIMULTANEOUSLY DISENGAGED THE AUTOPLT AND THE TCASII IVSI INDICATED AN INCREASING DSCNT FROM -800 FPM TO -1700 FPM. I INFORMED DTW APCH THAT WE WERE IN A TCASII COMMANDED DSCNT. THE TCASII AURAL COMMANDS HAD STOPPED BUT THE RED/GREEN ARCS WERE STILL DISPLAYED UNTIL APPROX 10500 FT MSL WHERE THE 'ARCS' DISAPPEARED. IT SHOULD BE NOTED THAT WHILE WE WERE IN THE DSCNT; AND AFTER I INFORMED APCH OF OUR DSCNT; DTW APCH WAS COMMANDING US TO CLB BACK TO OUR ASSIGNED ALT. WHEN THE TCASII 'DSND ARCS' DISAPPEARED THE FO ESTABLISHED A CLB BACK TO OUR ASSIGNED ALT OF 11000 FT. THE CLB BEGAN AT APPROX 10400 FT. AT THE ONSET OF OUR ALT RECOVERY I RECALL SEEING A SOLID AMBER HALF CIRCLE WITHIN A -03 (CANNOT RECALL ANY ARROW) INDICATING 300 FT ABOVE?/BELOW? OUR ACFT. THE AMBER CIRCLE WAS AT APPROX 4 O'CLOCK RELATIVE POS. IN THAT WE WERE CLBING AT THE TIME THE AMBER TARGET PROBABLY WAS IN LEVEL FLT (NOT DSNDING) BUT WE WERE INCREASING THE DISTANCE BTWN US BY VIRTUE OF OUR CLB; THE AMBER CIRCLE RAPIDLY DISAPPEARED. AT NO TIME DID I SEE A RED SQUARE (RA) PRIOR TO OUR DSCNT; OR DURING THE DSCNT. THE FO STATES HE DID SEE A RED SQUARE AT OUR 12-1 O'CLOCK POS. TCASII IN RA/TA MODE CONTINUED TO SHOW TFC (BLUE) DURING THE REMAINDER OF OUR VECTORING AND APCH.