Final answer:
The brevity code word for 'C2 sensors are unavailable due to degradation' is 'Midnight,' which is the opposite of 'Sunrise' in NATO military communications codes.
Step-by-step explanation:
The brevity code word that indicates 'C2 (Command and Control) sensors are unavailable due to degradation' and is the opposite of 'Sunrise' is 'Midnight'. In military communications, specifically within the context of NATO standardized military brevity codes, 'Sunrise' means the reestablishment of C2 (Command and Control) systems which had been previously degraded, whereas 'Midnight' conveys the onset of a degradation in these systems, making them unavailable for use.