Observation by the Intercosmos-24 satellite of ELF-VLF electromagnetic emissions associated with earthquakes

The measurements of emissions in the frequency range 8 Hz−20 kHz during 180 orbits of the Intercosmos-24 satellite from November 16, 1989 to December 31, 1989, are analyzed. Twenty-eight rather strong earthquakes (5.2<Ms<6.1) took place during this period. The following findings have emerged from the analysis of these events: 1) emissions in the two frequency bands with spectrum maxima at ULF-ELF (f<1000 Hz) and at VLF (f=10−15 kHz) are typically observed as bursts in the region nearly above the earthquake epicenter; 2) ULF-ELF emission spectrum intensity decreases with increasing frequency; 3) only VLF emissions are observed far from the epicenter but near the appropriate L shell; 4) emissions occurrence probability is a maximum at 12-14h before the main shock. Finally, the possible theoretical concepts are discussed in connection with the experimental results, and we suggest a plausible mechanism of precursory ELF and VLF emissions of earthquakes.