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

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