%0 Journal Article %T Variation of the first cut-off frequency of the Earth-ionosphere waveguide observed by DEMETER %+ Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E) %+ Department of Electromagnetism and Matter Physics [Granada] (EFM) %A Toledo-Redondo, S %A Parrot, Michel %A Salinas, A %Z Centre National d’Etudes Spatiales, the Ministerio de Ciencia e Innovación of Spain and Consejería de Innovación, Ciencia y Empresa of Andalusian Government under projects with references FIS2010-15170 and PO7-FQM-03280 co-financed with FEDER funds of the European Union %< avec comité de lecture %@ 2169-9380 %J Journal of Geophysical Research Space Physics %I American Geophysical Union/Wiley %V 117 %P A04321 (11 pages) %8 2012 %D 2012 %R 10.1029/2011JA017400 %Z Sciences of the Universe [physics]Journal articles %X [1] More than four years of VLF electric field data recorded by DEMETER have been analyzed, in order to monitor the first cutoff 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 cutoff 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/2h (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 AE15 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. %G English %2 https://insu.hal.science/insu-01179859/document %2 https://insu.hal.science/insu-01179859/file/Toledo_Redondo_et_al-2012-Journal_of_Geophysical_Research__Space_Physics_%281978_2012%29.pdf %L insu-01179859 %U https://insu.hal.science/insu-01179859 %~ OBSPM %~ INSU %~ CNRS %~ UNIV-ORLEANS %~ CNES %~ OSUC %~ LPC2E %~ PSL %~ OBSPM-PSL