Abstract
Abstract. We analyze data on radio-reflection from the D-region of the lower ionosphere, retrieving the energy-reflection coefficient in the frequency range ~5–95 kHz. The data are the same as developed for a recent study of ionospheric-reflection height, and are based on recordings of powerful (multi-Gigawatt) radio emissions from a type of narrow (~10 μs) lightning discharge known as "Narrow Bipolar Events". The sequential appearance of first the groundwave signal, and then the ionospheric single-hop reflection signal, permits us to construct the energy-reflection ratio. We infer the energy reflection's statistical variation with solar zenith angle, angle-of-incidence, frequency, and propagation azimuth. There is also a marginally-significant response of the energy reflectivity to solar X-ray flux density. Finally, we review the relationship of our results to previous published reports.
Highlights
Introduction and backgroundThe reflection of long radio waves from the lower ionosphere (D-region) has been studied since the 1940s, and from that perspective one might infer that anything worth doing has already been done
More recent work on lower-ionosphere disturbances associated with lightning (Cho and Rycroft, 1998: Inan et al, 1996a, b; Lev-Tov et al, 1995) and with lightning-induced energetic-particle precipitation (Bortniket al., 2006a, b; Rodger et al, 2005) motivate further improvements in radio reflectometry from the D-region
In this paper we build upon recently published results (Jacobson et al, 2007), (“J7”), on improved retrieval of ionospheric reflection height from the delayed echoes seen in recordings of VLF/LF lightning sferics of Narrow Bipolar Event (NBE) discharges (Le Vine, 1980; Smith et al, 2004; Smith et al, 1999; Willett et al, 1989)
Summary
The reflection of long radio waves from the lower ionosphere (D-region) has been studied since the 1940s, and from that perspective one might infer that anything worth doing has already been done. The D-layer’s high electron-neutral collision rate causes high signal loss in D-layer radio sounding, further worsening the signal-to-noise problem already implicit in the low-gain antennas Together, in practice, these two factors prevented the ionosonde approach from being extended downward in altitude below 95 km. Thomson’s approach has been to compare careful recordings of phase and amplitude at long range to predictions of a numerical model of VLF propagation in the Earth-ionosphere waveguide developed under the auspices of the United States Navy (Pappert and Ferguson, 1986). Cummer’s approach exploited the immense peak radiated VLF power (multi-GW) of lightning strokes, which elegantly bypassed the obstacles to an engineered D-region pulsed sounder This wideband VLF approach was later applied to the study of ionospheric disturbances associated with lightning itself (Cheng and Cummer, 2005) and with particle precipitation (Cheng et al, 2006). The remainder of this article will describe our technical approach (Sect. 2), present our statistical results (Sects. 3– 5), and discuss these results in the context of prior work (Sect. 6)
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