Abstract
Abstract. Relativistic electron beams above thunderclouds emit 100 kHz radio waves which illuminate the Earth's atmosphere and near-Earth space. This contribution aims to clarify the physical processes which are relevant for the spatial spreading of the radio wave energy below and above the ionosphere and thereby enables an experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds. The simulation uses the DEMETER satellite which observes 100 kHz radio waves from fifty terrestrial Long Range Aid to Navigation (LORAN) transmitters. Their mean luminosity patch in the plasmasphere is a circular area with a radius of 300 km and a power density of 22 μW/Hz as observed at 660 km height above the ground. The luminosity patches exhibit a southward displacement of 450 km with respect to the locations of the LORAN transmitters. The displacement is reduced to 150 km when an upward propagation of the radio waves along the geomagnetic field line is assumed. This residual displacement indicates that the radio waves undergo 150 km sub-ionospheric propagation prior to entering a magnetospheric duct and escaping into near-Earth space. The residual displacement at low (L < 2.14) and high (L > 2.14) geomagnetic latitudes ranges from 100 km to 200 km which suggests that the smaller inclination of the geomagnetic field lines at low latitudes helps to trap the radio waves and to keep them in the magnetospheric duct. Diffuse luminosity areas are observed northward of the magnetic conjugate locations of LORAN transmitters at extremely low geomagnetic latitudes (L < 1.36) in Southeast Asia. This result suggests that the propagation along the geomagnetic field lines results in a spatial spreading of the radio wave energy over distances of 1 Mm. The summative assessment of the electric field intensities measured in space show that nadir observations of terrestrial 100 kHz radio waves, e.g., from relativistic electron beams above thunderclouds, are attenuated by at least 50 dB when taking into account a transionospheric attenuation of 40 dB.
Highlights
Electron beams above thunderclouds are initiated by cosmic ray showers which coincide with intense lightning discharges (Fullekrug et al, 2010; Roussel-Dupreand Gurevich, 1996)
It is possible to observe terrestrial relativistic electrons in space with particle detectors on satellites (Carlson et al, 2009; Dwyer et al, 2008; Feldman et al, 1996, 1995; Burke et al, 1992), but the direct association with individual intense lightning discharges remains to be made. Another possibility to detect electron beams above thunderclouds is to observe their broadband electromagnetic radiation (Fullekrug et al, 2010), but the physical processes which are relevant for the spatial spreading of radio wave energy from the middle atmosphere into near-Earth space are Published by Copernicus Publications on behalf of the European Geosciences Union
The 100 kHz radio wave intensities observed on the DEMETER satellite show that more powerful Long Range Aid to Navigation (LORAN) transmitters generally produce larger electric field intensities, but the electric field intensities exhibit a natural variability with a factor ∼2–3 which corresponds to ∼3–5 dB (Fig. 5)
Summary
Electron beams above thunderclouds are initiated by cosmic ray showers which coincide with intense lightning discharges (Fullekrug et al, 2010; Roussel-Dupreand Gurevich, 1996). It is possible to observe terrestrial relativistic electrons in space with particle detectors on satellites (Carlson et al, 2009; Dwyer et al, 2008; Feldman et al, 1996, 1995; Burke et al, 1992), but the direct association with individual intense lightning discharges remains to be made Another possibility to detect electron beams above thunderclouds is to observe their broadband electromagnetic radiation (Fullekrug et al, 2010), but the physical processes which are relevant for the spatial spreading of radio wave energy from the middle atmosphere into near-Earth space are Published by Copernicus Publications on behalf of the European Geosciences Union. The determination of the total transionospheric attenuation allows for an experimental simulation of space based observations of radio waves emitted by relativistic electron beams above thunderclouds
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