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 D Region 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, 1964). Plain Language SummaryWe 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 D region 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 D Region Absorption Prediction and Wait models. Key Points:• We developed a physics-based model of HF absorption due to X-ray solar flare impact on the low-altitude ionosphere • Model results are consistent with ionosonde data collected during flare events on a 3-day experimental campaign • The model is a novel approach to X-ray flare impact on HF propagation, complementing existing empirical modelsCorrespondence to:

Ionospheric data assimilation is a technique to evaluate the 3‐D time varying distribution of electron density using a combination of a physics‐based model and observations. A new ionospheric data assimilation method is introduced that has the capability to resolve traveling ionospheric disturbances (TIDs). TIDs are important because they cause strong delay and refraction to radio signals that are detrimental to the accuracy of high‐frequency (HF) geolocation systems. The capability to accurately specify the ionosphere through data assimilation can correct systems for the error caused by the unknown ionospheric refraction. The new data assimilation method introduced here uses ionospheric models in combination with observations of HF signals from known transmitters. The assimilation methodology was tested by the ability to predict the incoming angles of HF signals from transmitters at a set of nonassimilated test locations. The technique is demonstrated and validated using observations collected during 2 days of a dedicated campaign of ionospheric measurements at White Sands Missile Range in New Mexico in January 2014. This is the first time that full HF ionospheric data assimilation using an ensemble run of a physics‐based model of ionospheric TIDs has been demonstrated. The results show a significant improvement over HF angle‐of‐arrival prediction using an empirical model and also over the classic method of single‐site location using an ionosonde close to the midpoint of the path. The assimilative approach is extendable to include other types of ionospheric measurements.

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