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

Abstract

[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 (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/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.Citation: Toledo-Redondo, S., M. Parrot, and A. Salinas (2012), Variation of the first cut-off frequency of the Earth-ionosphere waveguide observed by DEMETER,

Terrestrial VLF transmitter injection into the magnetosphere

“…Furthermore, the ionospheric absorption of VLF transmitter signals does not appear to strongly vary with geomagnetic activity, at least as measured by k p . Recent analysis of DEMETER data have shown variations in ELF/ VLF signals from lightning over the lifetime of the mission, which may be a result of the solar cycle activity impacting the ionosphere [Toledo-Redondo et al, 2012]. We do not find a similar trend in VLF transmitter amplitudes, even though the lightning activity analyzed by Toledo-Redondo et al [2012] is dominated by the same mid latitude region (L < 2.6) as the set of transmitters discussed here.…”

Short‐Fractional Hop Whistler Rate Observed by the Low‐Altitude Satellite DEMETER at the End of the Solar Cycle 23

“…For example, it has been shown by Thomson et al () that during daytime, the height of the D region (the lowest edge of the Earth's ionosphere) decreases when the sunspot number increases. The same effect was observed by Toledo‐Redondo et al () but for nighttime. One could also understand that this study is certainly biased by the global warming which increases the lightning activity year by year (see section ).…”

Longitudinal Dependence of Whistler Mode Electromagnetic Waves in the Earth's Inner Magnetosphere

“…Additionally, these peaks appear to extend—with somewhat lower intensity—also to higher frequencies. One can also identify a reduced median intensity at frequencies just below about 1.7 kHz, which corresponds to the cutoff frequency of the Earth‐ionosphere waveguide (Toledo‐Redondo et al, ). Another decrease of median intensities is observed at frequencies just below the proton gyrofrequency (about 500 Hz), related to the L = 0 cutoff frequency (Santolík, Němec, et al, ).…”