Fractal characteristics of the natural ULF emissions in relation to geomagnetic activity

Fractal analysis of the ULF emissions registered at low-latitude observatory Guam (Φ_m=9°N, Λ_m=225°) has been performed. Dynamics of scaling characteristics (spectral exponents β and fractal dimensions D0) has been analyzed depending on the level of geomagnetic activity and in relation to the nearby earthquake of August 8, 1993. It is found that the spectrum of ULF emissions exhibits power law behavior S(f)∞f-β which is typical for fractal (self-affine) time series. It is also revealed that spectral exponent β varies in the range β=0.7-2.5 in correlation with Kp-index of geomagnetic activity, and manifests a downward tendency when approaching the earthquake date. The results obtained are discussed on the basis of the SOC (self-organized criticality) approach to study the ULF emission source dynamics.