A ULF wave index, characterizing the level of the geomagnetic field variability in the frequency range 2--7~mHz, has been suggested to the space physics and geophysical community. This global wave index is produced from all available arrays of magnetometers and isolated stations in the Northern hemisphere. A similar ULF wave index is calculated using magnetometer data from geostationary (GOES) and interplanetary (Wind, ACE) satellites. In this review we demonstrate that a wide range of space physics studies, such as the solar wind-ionosphere coupling, wave energy transport, substorm physics, relativistic electron energization, ring current formation, electrodynamics of the ionosphere and magnetosphere, search for electromagnetic precursors of earthquakes, etc., has benefited from the introduction of the provisional ULF wave index. Possible ways of the ULF index advancement and development are discussed. The permanently updating ULF-index database is freely available via the website \href{http://ulf.gcras.ru/}{ulf.gcras.ru} for all interested researchers for further validation and statistical studies.
ULF waves, space weather, substorms, magnetic storms, earthquake precursors, geomagnetically induced currents, discrete mathematical analysis
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