Spatial and Temporal Ionospheric Monitoring Using Broadband Sferic Measurements

Abstract

The D region of the ionosphere (60–90 km altitude) is highly variable on timescales from fractions of a second to many hours, and on spatial scales up to many hundreds of kilometers. Very low frequency (VLF) and low‐frequency (LF) (3–30 kHz and 30–300 kHz) radio waves are guided to global distances by reflections from the ground and the D region. Therefore, information about its current state is encoded in received VLF/LF signals. VLF transmitters have been used in the past for D region studies, with ionospheric disturbances manifesting as perturbations in amplitude and/or phase. The return stroke of lightning is an impulsive VLF radiator, but unlike VLF transmitters, lightning events are distributed broadly in space allowing for much greater spatial coverage of the D region compared to VLF transmitter‐based remote sensing in addition to the broadband spectral advantage over the narrowband transmitters. The challenge is that individual lightning‐generated waveforms, or “sferics,” vary due to the lightning current parameters and uncertainty in the time/location information, in addition to D region ionospheric variability. These factors make it difficult to utilize the VLF/LF emissions from lightning in a straightforward manner. We describe a technique to recover the time domain and amplitude/phase spectra for both Bϕ and Br with high fidelity and consider the utility of our technique with ambient and varied ionospheric conditions. We demonstrate a technique to simulate sferics and infer a parameterized ionosphere with the Wait and Spies parameters (h ′ and β) offering all of the tools needed for a global measurement.

Three‐Dimensional Mapping of Lightning‐Produced Ionospheric Reflections

“…An example of ionograms made using this process can be found in Figure , which shows the o‐mode reflection of several correlated events over a 200‐ms‐long capture. This technique is similar in nature to previous work using very low frequency radio signals, or 1‐ to 160‐kHz frequency signals, to probe the D region ionosphere (Lay et al, ; McCormick et al, ).…”

Broadband longwave radio remote sensing instrumentation

“…The example representative sferics are processed according the technique discussed in Ref. 150. The example storm systems took place over Panama throughout the afternoon which allowed for long-term probing of the ionosphere.…”

“…A more complete description of this process is given in Ref. 150. Figure 15 shows high-sensitivity detection of the WWVB 60 kHz timing beacon operated by the National Institute of Standards and Technology (NIST) near Fort Collins, Colorado (40.67N, 105.04W), at 60 kHz.…”

Measuring the Electron Density Roughness of the D‐Region Ionosphere

“…Since the D‐region (and the ground) reflects lower frequency waves efficiently, the region between the Earth and the D‐region is often referred to as the “Earth‐ionosphere waveguide”. An effective and widespread method to study the D‐region is through the use of very low frequency (VLF, 3–30 kHz) and low‐frequency (LF, 30–300 kHz) radio waves from man‐made transmitters (e.g., Füllekrug et al, 2019), or natural sources (e.g., McCormick et al, 2018), due to the efficient reflection of waves that allow propagation to global distances. As the frequency of the wave increases, the attenuation of the reflected signal increases as well (Bickel, 1957), as does the reflection height.…”