Experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds

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 energyCorrespondence to: M. Füllekrug (eesmf@bath.ac.uk) 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.

Relativistic electron beams above thunderclouds

“…However, these processes remained sub-visual in the optical observations reported in this work, and it is not clear why such a hypothetical process would emit low frequency electromagnetic radiation while the sprite itself does not. It is interesting to note that the low frequency electromagnetic radiation reported here can be observed in space as a result of its transionospheric propagation (Füllekrug et al, 2011(Füllekrug et al, , 2009) and it may be associated with high frequency electromagnetic radiation (Parrot et al, 2008).…”

Electron acceleration above thunderclouds

“…It is interesting to note that a recent detailed comparison of ground based optical sprite observations in southern France with electric field recordings on board the DEMETER satellite on 17 November 2006 (Parrot et al 2013), revealed low frequency radio signals up to ∼130 kHz associated with the sprite and/or the causative lightning discharge which have never been observed before in association with ordinary lightning discharges ( figure 4). Given that the ionosphere attenuates ∼100 kHz radio signals by ∼2 orders of magnitude (Füllekrug et al 2011a), the signal intensity of the lightning and/or sprite was undoubtedly exceptionally large. This observation shows that powerful low frequency radio signals associated with sprite producing lightning, as reported here, can be observed in space with unprecedented temporal and spectral resolution which is the aim of the French TARANIS satellite due to be launched in 2015 (Blanc et al 2007).…”

Map of low‐frequency electromagnetic noise in the sky

“…The locations of these electromagnetic noise sources are closely monitored and mapped around the world by the use of sensitive electromagnetic recordings on board of satellites. For example, the electromagnetic radiation from transmitters for submarine communication [ Sauvaud et al , ] and marine navigation [ Füllekrug et al , , ] escapes from the Earth ionosphere waveguide into near‐Earth space in the Whistler mode [e.g., Lefeuvre et al , ; Helliwell , ]. These low‐frequency radio communication transmitters are widely used for geophysical exploration, for example, to study particle precipitation from the radiation belts into the Earth's atmosphere and the impact of lightning discharges on the upper atmosphere [e.g., Denton et al , ; Haldoupis et al , , ; Sauvaud et al , ; Inan et al , ; Horne et al , , and references therein].…”