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1. The Lower Ionospheric VLF/LF Response to the 2017 Great American Solar Eclipse Observed Across the Continent

   https://doi.org/10.1002/2018GL077351  

   Cohen, M. B.; Gross, N. C.; Higginson-Rollins, M. A.; Marshall, R. A.; Gołkowski, M.; Liles, W.; Rodriguez, D.; Rockway, J. (April 2018, Geophysical Research Letters)
2. Using Ionosondes for Lower‐Ionosphere Remote Sensing

   https://doi.org/10.1029/2024GL113123  

   Strong, M; Cohen, M; Galkin, I (June 2025, Geophysical Research Letters)

   Abstract Ionosondes are primarily used to measure the electron densities of the ionosphere's E and F‐region via frequency‐range analysis of the probing signal returns. The amplitude of the returning signal has often been ignored, however, and may allow estimates of other propagation effects such as D and E‐region absorption. We introduce a methodology to extract this information from amplitude data and view results in ensemble with Very Low Frequency‐derived, D‐Region absorption estimates. This comparison allows us to infer what portion of High Frequency (HF) attenuation is due to D‐region versus E‐region absorption. The attenuation observed by both methodologies are congruent with each other in the diurnal cycle across HF frequencies between 2.5 and 4.5 MHz. This technique may extend the utility of ionosondes beyond their traditional applications. 

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3. Citizen Scientists Conduct Distributed Doppler Measurement for Ionospheric Remote Sensing

   https://doi.org/10.1109/LGRS.2021.3063361  

   Collins, Kristina; Montare, Aidan; Frissell, Nathaniel; Kazdan, David (January 2022, IEEE Geoscience and Remote Sensing Letters)
4. Ionospheric D Region Remote Sensing Using ELF Sferic Group Velocity

   https://doi.org/10.1029/2018GL080108  

   Gołkowski, M.; Sarker, S. R.; Renick, C.; Moore, R. C.; Cohen, M. B.; Kułak, A.; Młynarczyk, J.; Kubisz, J. (December 2018, Geophysical Research Letters)

   Abstract The lowest region of the ionosphere, theDregion, plays an important role in magnetosphere‐ionosphere coupling but is challenging to directly observe. The group velocity of the extremely low frequency (ELF; 3–300 Hz) portion of lightning induced electromagnetic radiation can be used to diagnose theDregion electron density profile. Day‐night conditions can be assessed using ELF receivers and lightning detection networks. Analytical formulations and the Long Wave Propagation Capability software package show that ELF group velocity has particular sensitivity to the sharpness of the exponential electron density profile. Applying the technique to sudden ionospheric disturbances shows that the group velocity increases in response to incidence of solar X‐ray flux . A small number of ELF receivers can provide a large‐scale diagnostic of theDregion. ELF remote sensing using lightning is complementary to very low frequency remote sensing and can be used to assess the Earth‐ionosphere propagation channel for very low frequency transmitters. 

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5. New O Partial Photoionization Cross Sections Resolve Ionospheric EUV Remote Sensing Issues

   https://doi.org/10.1029/2023JA031533  

   Meier, R_R; Soto, Emmaris; Evans, J_Scott; Stephan, Andrew_W; Tashiro, Motomichi (October 2023, Journal of Geophysical Research: Space Physics)

   Abstract The ionospheric O⁺number density can be measured remotely during the day by observing its optically thick 83.4 nm radiance. Some ambiguity is present in the process of retrieving the density due to uncertainties in the initial excitation rate. This can be removed by observing a companion optically thin emission at 61.7 nm originating from the O⁺(3s²P) state, providing that the ratio of the initial excitation rates is known. Analyses of ICON EUV data using an 83.4/61.7 emission ratio of order 10 result in O⁺densities lower by ∼2 than other measurements. Key to relating the two emissions is accurate knowledge of the partial photoionization cross sections and the spectroscopy of O⁺—the topic of this paper. Up to now, no independent evaluation of the ratio of the 83.4/61.6 emission ratio exists. The recent availability of state‐of‐the‐art calculations of O partial photoionization cross sections into a variety of O⁺states presents an opportunity to evaluate the O⁺(2p⁴⁴P)/O⁺(3s²P) ionization rate ratio. We calculate excitation of these parent states of the emissions including both direct and cascade excitation from higher lying O⁺energy states. The resulting theoretical prediction gives ratios that range from 13.5 to 12 from solar minimum to maximum, larger than the value of 10 used by the ICON 83.4 and 61.7 nm algorithm. The higher theoretical values for the ratio reconcile the ∼2 discrepancy between simultaneous ICON and other electron density measurements. 

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