Observations by DEMETER of Man‐Made MF Waves Escaping From the Ionosphere

Abstract

Man-made medium frequency (MF) waves are in a frequency range between 300 kHz and 3 MHz. They are typically used by broadcasting, radio-navigation (maritime and aeronautical), and maritime mobile communications. They propagate in the Earth-ionosphere waveguide (Milsom, 2003). Normally they are reflected and cannot cross the ionosphere because their frequencies are lower than the plasma frequency in the F layer. But there is a possibility for these waves to cross the ionosphere if they follow a magnetic field line. Individual observations of broadcast transmitters above the F layer have been already done (Hashimoto et al., 2018; LaBelle et al., 1989; Kaiser et al., 1996; Kelley et al., 1997). Concerning the low altitude satellite DEMETER, if the recorded MF spectra are averaged over a large time interval, the transmitter locations appear on a terrestrial map of wave activity as hot spots which are a little bit shifted equatorwards due to their propagation along a magnetic field line (

Ionospheric Plasma Fluctuations Induced by the NWC Very Low Frequency Signal Transmitter

“…The climatology of the NWC-associated plasma fluctuations shows that the fluctuations are strongest during magnetic midnight, and partially during dawn, when the ambient plasma density is low. This harmonizes with findings that VLF spectral broadening over NWC favors conditions with low ambient plasma density (Xia et al, 2020), and with recent reports that a low ambient plasma density creates conditions favorable to the transmission of man-made electromagnetic waves (Parrot, 2021).…”

Propagation Mechanism of Medium Wave Broadcasting Waves Observed by the Arase Satellite: Hectometric Line Spectra

“…They proposed the possible scenario based on the mode conversion between the L-O mode (Stix, 1962) and trapped Z-mode waves at density irregularities like equatorial plasma bubbles. Man-made MF waves observed by DEMETER at the altitude of about 700 km have recently been reported by Parrot (2021).…”