very low frequency wave; VLF wave
Technical term in radio engineering; refers to electromagnetic waves in the 3–30 kHz range.
超長波は潜水艦との通信に利用される。
VLF waves are used for communication with submarines.
Compound of 超 (chō, 'super-') + 長波 (chōha, 'long wave'), literally 'super long wave'.