Locating Radio Noise from Sprites

Abstract:

Sprites are composed of individual streamer discharges (e.g., Pasko, 2010) which split into exponentially growing streamer tips (McHarg et al., 2010). The acceleration of the electrons to a few eV results in the radiation of a small amount of electromagnetic energy. The incoherent superposition of many streamers causes the low frequency radio noise from sprites near ~40 km height (Qin et al., 2012). The presence of this theoretically predicted radiation was recently confirmed by low frequency radio noise measurements during dancing sprites with a very sensitive radio receiver (Fullekrug et al., 2013). To locate the radio noise from sprites in the sky, an interferometric network of low frequency radio receivers was developed (Mezentsev and Fullekrug, JGR, 2013). The key parameter for the interferometric signal processing is the frequency dependent wave propagation velocity of the radio waves within the Earth's atmosphere. This wave propagation velocity is determined by the wave number vector which needs to be inferred from the measurements. Here we adapt and subsequently apply array analyses which have been developed for seismic and infrasound arrays to determine the horizontal wave number vectors of ~20-24 kHz radio waves measured with an array of ten radio receivers distributed over an area of ~1 km × 1 km. It is found that the horizontal slowness of ~20-24 kHz radio waves ranges from ~2.7 ns/m to ~4.1 ns/m depending on the arrival azimuth of the radio wave. For comparison, an electromagnetic wave in vacuum has a slowness of ~3.34 ns/m. A larger slowness indicates an apparent velocity which is smaller than the speed of light and a smaller slowness indicates that the radio wave arrives at the array from an elevation angle. The observed variability of the observed slowness almost certainly results from the distance dependent superposition of the transverse electric and magnetic TEn and TMn radio wave propagation modes.