The crossword solution for the term 'Sector Boundary' is ARC, which stands for 'Annular Ring of Coronagraphs'. This is a type of instrument used to observe the sun's corona, which is the outermost layer of its atmosphere. The instrument consists of an array of coronagraphs arranged in a ring-like pattern, allowing it to capture images of the sun's corona from different angles. The sector boundary is the line that separates the different coronagraphs, and thus the different angles of observation.