Statistical investigation of VLF quasiperiodic emissions measured by the DEMETER spacecraft

We present a survey of quasiperiodic (QP) ELF/VLF emissions detected onboard the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite (altitude of about 700km, nearly Sun-synchronous orbit at 10:30/22:30 LT). Six years of data have been visually inspected for the presence of QP emissions with modulation periods higher than 10s and with frequency bandwidths higher than 200Hz. It is found that these QP events occur in about 5% of daytime half orbits, while they are basically absent during the night. The events occur predominantly during quiet geomagnetic conditions following the periods of enhanced geomagnetic activity. Their occurrence and properties are systematically analyzed. QP emissions occur most often at frequencies from about 750Hz to 2kHz, but they may be observed at frequencies as low as 500Hz and as high as 8kHz. Modulation periods of QP events may range from about 10 to 100s, with typical values of 20s. Frequency drifts of the identified events are generally positive, but they are lower for events with larger modulation periods. The events are usually limited to higher L values (L>2). The upper L shell boundary of their occurrence could not be identified using the DEMETER data, but they are found to extend up to at least L similar to 6. The occurrence rate of the events is significantly lower at the longitudes of the South Atlantic anomaly (by a factor of more than 2).