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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. Remote Sensing of Mid‐Latitude Ionospheric Magnetic Field Fluctuations Using Cosmic Radio Sources

   https://doi.org/10.1029/2021RS007372  

   Helmboldt, J. F. ; Clarke, T. E. ; Kassim, N. E. ( April 2022 , Radio Science)

   This paper describes a new method for remote sensing of magnetic field fluctuations at ionospheric altitudes using a relatively long‐baseline interferometer and exceptionally bright cosmic radio sources at 35 MHz. The technique uses sensitive measurements of the difference in phase between two phased array telescopes separated by about 75 km and between the right and left circular polarizations to measure the amount of differential Faraday rotation. Combined with estimates of the background magnetic field and total electron content, these can be converted to measurements of fluctuations in the differential magnetic field parallel to the line of sight, ΔB_‖. The temporal gradient in ΔB_‖roughly follows the diurnal pattern expected for B_‖due to the vertical gradient in the background electric field, but at roughly 25% the magnitude and offset by ∼50 nT hr⁻¹. This suggests that the diurnal variation in the electric fields observed by the two telescopes are similar but slightly different (|ΔE| ≲ 0.1 mV m⁻¹). Fluctuations in ΔB_‖were typically ∼10–30 nT with wavelike fluctuations often apparent. These typically have oscillation periods of about 10–30 min, similar to traveling ionospheric disturbances (TIDs). Simultaneous observations toward two sources separated by 25.4° on the sky (∼140 km in the F‐region) show a few detections of wavelike disturbances with lags of ±10–30 min between them. These imply speeds on the order of 100–200 m s⁻¹, also similar to TIDs. We estimate that gravity waves with amplitudes within the dynamo region of ∼10 m s⁻¹could generate the observed fluctuations in ΔB_‖.

    

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3. 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)

   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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4. 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)

   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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5. 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)