In this paper, we report measurements in Colorado and Utah of the disturbed very-low-frequency (VLF) signals from the NML Navy transmitter in North Dakota during the 2017 solar eclipse. Using an occultation mask of solar fluxes together with detailed chemistry and VLF propagation simulations, we quantify the D-region response to the eclipse, in terms of electron density variation, as well as the expected signatures of VLF transmitter signals. The VLF measurements, including an anomalous amplitude enhancement recorded in UT, can be quantitatively explained using the Wait and Spies ionospheric profile with a sharpness parameter of $\beta = 0.3$ km−1 above ~55 km and an increase in the D-region ionosphere height of $\Delta h' \simeq 8$ km. This sharpness parameter is consistent with previously reported rocket measurements and first-principles calculations. The best-fit results suggest a reduction of D-region electron density by ~90% during the eclipse in the D-region, implying an occultation of Lyman- $\alpha $ by nearly 99%. This finding agrees with detailed calculations of time-dependent obscuration factors utilizing the He 30.4-nm images from Solar Dynamics Observatory as a proxy for the distribution of Lyman- $\alpha $ across the solar disk and limb. Moreover, the present results show that subionospheric VLF propagation is sensitive to the sharpness parameter of the electron density profile in the D-region. Previously reported first-principles simulations have shown that the sharpness parameter is mostly controlled by the background concentration of minor neutral species. Thus, the VLF technique can be likely used to remotely sense these neutral species at and below the effective reflection altitudes of VLF waves.

VLF Measurements and Modeling of the D-Region Response to the 2017 Total Solar Eclipse

We present numerical simulations and analysis of atmospheric effects of a beam of 1~MeV electrons precipitating in the upper atmosphere from above. Beam parameters of 100~J or 1~kJ injected in 100~ms or 1~second were chosen to reflect the current design requirements for a realistic mission. We calculate ionization signatures and optical emissions in the atmosphere, and estimate the detectability of optical signatures using photometers and cameras on the ground. Results show that both instruments should be able to detect the beam spot. Chemical simulations show that the production of odd nitrogen and odd hydrogen are minimal. We use electrostatic field simulations to show that the beam-induced electron density column can enhance thunderstorm electric fields at high altitudes enough to facilitate sprite triggering. Finally, we calculate signatures that would be observed by incoherent scatter radar (ISR) and subionospheric VLF remote sensing techniques, although the latter is hindered by the limitations of 2D simulations.

/pdf/atmospheric-effects-of-a-relativistic-electron-beam-injected-4tf2rne40s.pdf

Atmospheric Effects of a Relativistic Electron Beam Injected From Above: Chemistry, Electrodynamics, and Radio Scattering

Large and easily distinguishable perturbations of the VLF transmitter signals due to interactions with thundercloud-driven ionospheric modifications have been observed and studied for about three decades. These events are called “early/fast VLF” or “early VLF” events due to their immediate detection (∼20 ms) after the causative lightning flash on the ground and the fast rise time of the perturbed signal. Despite many years of study, the physical mechanisms responsible for these perturbations are still under investigation. Modifications of the sustained heating level of the ionosphere due to a lightning flash has been previously proposed as the causative mechanism of early/fast VLF events. The perturbations predicted by this mechanism, however, have been much smaller than experimental observations of 0.2–1 dB or higher. In this study, by using an improved 3D thundercloud electrostatic upward-coupling model which uses a realistic geomagnetic field, we find that the sustained heating model can predict perturbations that are consistent with reported experimental observations. Modifications in the quiescent heating of the lower ionosphere by thundercloud fields by individual lightning flashes may thus account for some observations of early/fast VLF events.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JD026528

Early/fast VLF Events Produced by the Quiescent Heating of the Lower Ionosphere by Thunderstorms

Precipitation in the upper atmosphere of energetic radiation belt electrons leads to a number of secondary effects, including the production of new ionization, emission of X-rays via bremsstrahlung, optical emissions, and chemical changes. In this chapter, we describe these effects and the physics behind them. We then discuss modeling techniques to calculate these effects, including scaling and Monte Carlo techniques. We next describe the diagnostic techniques used to detect atmospheric effects of precipitation; these include space-based and balloon-based particle and X-ray measurements, ground-based optical detection, and ground-based radar, riometer, and subionospheric very-low-frequency (VLF) remote sensing of the D-region electron density disturbance. Finally, we describe future experiments and observations that will help to improve quantification of energetic precipitation fluxes and spectra.

Atmospheric effects and signatures of high-energy electron precipitation

An Electron Density Model of the D- and E-Region Ionosphere for Transionospheric VLF Propagation

Simulations of Very-low-frequency (VLF) transmitter signals are conducted using three models: the long-wave propagation capability, a finite-difference (FD) time-domain model, and an FD frequency-domain model. The FD models are corrected using Richardson extrapolation to minimize the numerical dispersion inherent in these models. Using identical ionosphere and ground parameters, the three models are shown to agree very well in their simulated VLF signal amplitude and phase, to within 1 dB of amplitude and a few degrees of phase, for a number of different simulation paths and transmitter frequencies. Furthermore, the three models are shown to produce comparable phase changes for the same ionosphere perturbations, again to within a few degrees. Finally, we show that the models reproduce the phase data of existing VLF transmitter–receiver pairs reasonably well, although the nighttime variation in the measured phase data is not captured by the simplified characterization of the ionosphere.

Michael Turbe

Papers

Finite-Difference Modeling of Very-Low-Frequency Propagation in the Earth-Ionosphere Waveguide