Various geomagnetic indices (Kp, AE, Dst, SYM-H, PC) and averaged solar wind/IMF parameters quantify the energy supply in certain regions of the solar wind-magnetosphere-ionosphere system, and are used as primary tools in statistical studies of solar-terrestrial relationships. However, these indices characterize the steady-state level of the electrodynamics of the near-Earth environment. The turbulent character of solar wind drivers and the existence of natural MHD waveguides and resonators in the magnetospheric plasma in the ULF frequency range (~2-10 mHz) ensures a quasi-periodic magnetic field response to forcing at the boundary layers. Therefore, much of the turbulent nature of plasma processes of solar wind-magnetosphere-ionosphere interactions can be monitored with ground-based or space observations in the ULF frequency range. We have constructed a new index, coined a “ULF wave index,” characterizing the turbulent character of the energy transfer from the solar wind into the upper atmosphere and the short-scale variability of near-Earth electromagnetic processes. We suppose that a wide range of space physics studies will benefit from the introduction of this new index.