Variation of the first cut-off frequency of the Earth-ionosphere waveguide observed by DEMETER

摘要: [1] More than four years of VLF electric field data recorded by DEMETER have been analyzed, in order to monitor the first cut-off frequency (QTM1) Earth-ionosphere waveguide, at around 1.6–1.8 kHz. Since losses a waveguide are maximized right frequency, (∼700 km orbit) can detect minimum energy leaking fields coming from waveguide. This measurement permits draw global map its value (f1), which is directly related effective height ionosphere (h) relation f1 = c/2h (c speed light). It enables remote sensing D region, one less known layers ionosphere, because it too low for satellites orbit inside and high balloons reach it. The depends mainly on electron density (Ne) neutral (Nn) profiles, determine plasma mobility. shifts downward 5–10 km southern warm season South Pacific Ocean. Another effect observed Indian Atlantic Oceans; decreases twice year, area roughly ±15° geomagnetic equator. main causes changes reflection solar radiation thunderstorm activity. However, more prominent over oceans, possible explanation this difference could be attributed i) polluted conditions above oceans (aerosols change atmospheric conductivity then circuit), ii) current associated thunderclouds bottom thunderstorms much numerous land, or iii) ionization elves their occurrence larger oceans.