OCTAVES (Observation of CondiTion of ionized Atmosphere by VLF Experiment) is a VLF/LF ionosphere observation network that measures modification of electron density in the lower ionosphere. Low frequency (LF) and very low frequency (VLF) radio waves propagate between earth’s surface and lower edge of ionosphere (70-90km, approx.) at long distance.

As modification of the lower ionosphere causes changes in received signal amplitude and phase, LF/VLF radio observation is a useful probe to detect variation of electron density in the lower ionosphere.

Scientific targets of this network includes (1) Energetic particle precipitation from radiation belts, (2) Lightning effects on upper and middle atmospheres, (3) Solar X-ray flare and proton event effect on upper and middle atmospheres, (4) Effects of solar eclipse, atmospheric gravity waves, and sound wave on the lower ionosphere, and (5) Detection of gamma ray burst.

More detailed information is found here.