We report results of a systematic analysis of a large number of observations of equatorial noise between the local proton cyclotron frequency and the local lower hybrid frequency. The analysis is based on the data collected by the STAFF-SA instruments on board the four Cluster spacecraft. The data set covers their first two years of measurement in the equatorial magnetosphere at radial distances between 3.9 and 5 Earth radii. Inspection of 781 perigee passages shows that the occurrence rate of equatorial noise is approximately 60%. We identify equatorial noise by selecting data with nearly linearly polarized magnetic field fluctuations. These waves are found within 10° of the geomagnetic equator, consistent with the published past observations. Our results show that equatorial noise has the most intense magnetic field fluctuations among all the natural emissions in the given interval of frequencies and latitudes. Electric field fluctuations of equatorial noise are also more intense compared to the average of all detected waves. Equatorial noise thus can play a non-negligible role in the dynamics of the internal magnetosphere.

Results of a statistical study of intensity of VLF electromagnetic waves observed in the vicinity of earthquakes are presented. A unique set of data obtained by the micro‐satellite DEMETER (altitude of about 700 km, nearly Sun‐synchronous orbit) and a robust two‐step data processing has been used. In the first step, all the measured data are used to construct a map of electromagnetic emissions containing a statistical description of wave intensity at a given point of the satellite orbit under given conditions. In the second step, the intensity measured close to earthquakes is analyzed using the statistical distribution of background intensity obtained in the first step. The changes of wave intensity caused by seismic activity are investigated and their statistical significance is evaluated. Altogether, more than 2.5 years of satellite data have been analyzed and about 9000 earthquakes with magnitudes larger than or equal to 4.8 that occurred all over the world during the analyzed period have been included in the study. It is shown that, during the night, there is a statistically significant decrease by 4 – 6 dB of the measured wave intensity shortly (0–4 hours) before an intense surface (depth less than or equal to 40 km) earthquake.

We present results of a systematic study of intensity of VLF electromagnetic waves observed by the DEMETER spacecraft in the upper ionosphere (altitude 700 km). We focus on the detailed analysis of the previously reported decrease of wave intensity shortly before the main shock during the nighttime. Using a larger set of data (more than 3.5 years of measurements) and a newly developed data processing method, we confirm the existence of a very small but statistically significant decrease of wave intensity 0–4 hours before the time of the main shock at frequencies of about 1.7 kHz. It is shown that the decrease does not occur directly above the earthquake epicenter but is shifted about 2° in the westward direction. Moreover, it is demonstrated that the decrease occurs more often close to shallower earthquakes and close to earthquakes with larger magnitudes, as it is “intuitively” expected, representing an additional proof of validity of the obtained results. Finally, no dependence has been found on the occurrence of the earthquake below the ocean or below the continents.