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We present a parameterized X‐ray solar flare effects model relating the physics of radiation transport to the observable impact of solar flares on low‐altitude ionospheric absorption of High Frequency (HF) signals. Tunable parameters of time‐varying flare spectral energy density and characteristic flare temperature provide a novel capability to simulate HF experiments over a wide range of X‐ray solar flare behavior. Results from our model are consistent with HF propagation data collected over a period of heightened solar flare activity during 5–7 September 2017, including M and X class solar flares. Our predictions and measurements are compared with results from DRegion Absorption Prediction (Akmaev et al., 2010, https://www.ngdc.noaa.gov/stp/drap/DRAP-V-Report1.pdf) and the Wait Very Low Frequency (VLF)‐driven model (Wait & Spies, [Wait, J., 1964]). We present a physics‐based model for high‐frequency (HF) signal absorption resulting from the effects of X‐ray solar flares on the low‐altitude ionosphere. Our model calculates the extent of ionization enhancement in the Dregion due to an X‐ray flare and predicts the altitude‐dependent change of electron density and consequent increase in HF signal absorption. We demonstrate that results from our model are consistent with ionosonde data collected during three solar flare events. This model provides a novel approach to analyzing the impact of X‐ray solar flares on HF propagation and is complementary to the widely used empirical DRegion Absorption Prediction and Wait models. We developed a physics‐based model of HF absorption due to X‐ray solar flare impact on the low‐altitude ionosphereModel results are consistent with ionosonde data collected during flare events on a 3‐day experimental campaignThe model is a novel approach to X‐ray flare impact on HF propagation, complementing existing empirical models

Ionospheric heating is known to induce density and temperature irregularities in the ionospheric plasma, which have been observed by various in‐situ and radar techniques. Analysis of incoherent scatter radar (ISR) plasma line spectra offers high resolution reconstruction of ionospheric features, but the plasma line profile is frequently difficult to reconstruct at altitudes where the heater is generating Langmuir turbulence and thus creating additional signals in the ISR spectrum. This work presents a novel, automated method based on Gabor filter image analysis to reconstruct plasma line spectra recorded with the coded long‐pulse technique. The method permits the unsupervised reconstruction of the Langmuir frequency with resolution in frequency (sub‐kHz) and altitude (∼300 m) typical of the coded long‐pulse technique even in regions directly coupling to the heater. The technique is applied to ISR data from the June 2019 Arecibo Observatory heating experiment, demonstrating the formation of field‐aligned irregularities of magnitude ∼0.5% in the Langmuir frequency during daytime conditions and spatial scale perpendicular to the magnetic field (fluctuation periodicity of several kilometers, corresponding to k≈ 1 km−1) consistent with the patches of depletions of free electrons previously observed by in‐situ methods. Additionally, the method allows observation of the formation and evolution of the bunches over the course of several hours. Additional F‐layer irregularities were found in the ambient ionosphere in regions away from the heater coupling altitude. The automated nature of this technique offers the potential for precision analysis of Arecibo's ISR data archives for a variety of ionospheric physics applications. A novel technique for extracting incoherent scatter radar plasma line profiles is presentedThe technique illuminates magnetic field aligned plasma irregularities and their temporal evolutionIonospheric heating‐induced plasma irregularities were observed to induce scintillation in high‐frequency oblique sounding signals A novel technique for extracting incoherent scatter radar plasma line profiles is presented The technique illuminates magnetic field aligned plasma irregularities and their temporal evolution Ionospheric heating‐induced plasma irregularities were observed to induce scintillation in high‐frequency oblique sounding signals

The formation of an anomalous, high‐altitude plasma cavity was observed during the 11–15 June 2019 ionospheric heating campaign at Arecibo Observatory. The cavity was induced by interactions of the transmitted 5.125‐MHz heater wave with the ionospheric plasma. Simultaneous ion line and plasma line measurements were taken by the Arecibo Incoherent Scatter Radar (ISR). We used a novel experimental technique, enabling rapid pulsing of the heater, to collect ISR spectral data of the heated ionosphere without high frequency (HF)‐induced enhanced ion line (HFIL), simplifying the analysis. ISR data were analyzed using Tikhonov regularization, revealing that the cavity was confined to a very narrow altitude range around 350 km and constituted a deep depletion of the electron density, along with strong electron temperature enhancement, and ion temperature enhancement. We report on the details of these observations, and the possibility for unique combinations of physical interactions to explain the underlying mechanisms. Formation of a strong plasma cavity occurred during a recent ionospheric heating experimentCavity formation was simultaneous with disappearance of incoherent scatter radar enhanced ion and plasma linesIncoherent scatter radar ion line spectra indicate strong enhancement of ion temperaturein the plasma cavity