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A multi‐instrument study is conducted at the dayside polar ionosphere to investigate the spatio‐temporal evolution of scintillation in Global Navigation Satellite System (GNSS) signals during non‐storm conditions. Bursts of intense amplitude and phase scintillation started to occur at ∼ ${\sim} $ 9 MLT and persisted for more than 1 hour implying the simultaneous existence of Fresnel and large‐scale sized irregularities of significant strength in the pre‐noon sector. Measurements from the EISCAT radar in Svalbard (ESR) revealed the presence of dense plasma structures with significant gradients in regions of strong Joule heating/fast flows and soft precipitation when scintillation was enhanced. Plasma structuring down to Fresnel scales were observed both in the auroral oval as well as inside the polar cap with the associated amplitude scintillation exhibiting similar strengths regardless of whether the density structures were in regions of active auroral dynamics or not. The observations are placed within the context of different sources of free energy, providing insights into the important mechanisms that generate irregularities capable of perturbing GNSS signal properties in the dayside ionosphere. Furthermore, a strong negative excursion in the interplanetary magnetic field (IMF) By ${B}_{y}$ component during the northward turning of Bz ${B}_{z}$ led to the transport of a depleted region of plasma density into the post‐noon sector that significantly weakened both amplitude and phase scintillation. Plain Language Summary: Ionospheric scintillation is a disturbance imposed by the ionosphere on radio signals of satellites in the form of rapid fluctuations in amplitude and phase. They are caused by electron density structures of varying sizes as they move across the path between a satellite signal and a ground based receiver. During strong scintillation, tracking signals become difficult thereby affecting technologies dependent on positioning, navigation and timing (PNT) services. It is therefore important to understand dynamics of the ionosphere that would result in enhanced scintillation in order to develop prediction or mitigation strategies for products using PNT information. In this study, we present evidence for the occurrence of intense amplitude and phase scintillation during quite geomagnetic conditions in the high latitude ionosphere. Our analysis revealed that large density structures propagating from lower latitudes can be modulated by different ionospheric processes such as fast flows and/or soft precipitation generating irregularities of varying scale‐sizes which are associated with scintillation. Additionally, we also observed the depletion of irregularities in relation to the variation of interplanetary magnetic field components which, from a practical viewpoint, are beneficial for the users of satellites technologies and services due to the reduced impact of scintillation during such periods. Key Points: Intense amplitude and phase scintillation in the pre‐noon sector during non‐storm conditionsStructuring down to Fresnel scales in both the auroral oval and inside the polar capDepletion of Fresnel structures due to IMF Bz and By reversal in the post‐noon sector [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of ∼ ${\sim} $30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration. Plain Language Summary: On 8 April 2024, a total solar eclipse traversed across North America with a dense network of observational equipment in place, providing a great opportunity for analyzing ionospheric effects during the eclipse. This paper presents a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density response to this solar eclipse, utilizing Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm satellite in situ measurements, and a new TEC‐based ionospheric data assimilation system (TIDAS) over continental US and adjacent regions with the SAMI3 as the background model. The observations and SAMI3‐TIDAS data assimilation reveals the time‐evolving 3‐D spatial distribution of the ionospheric electron density during the eclipse, highlighting key features of altitude‐dependent ionospheric variation with significant discrepancies and time delays between the bottomside and topside ionosphere. Key Points: The altitude‐resolved Ne response to the solar eclipse in the 3‐D domain was effectively reconstructed by TIDAS‐SAMI3 data assimilationThe eclipse led to a substantial ionospheric Ne reduction of 20%–50%, with the maximum depletion occurring in the F region of 200–250 kmThe Ne showed a time‐delayed variation with increasing altitude, from 5 to 10 min in the bottomside to 1–2.5 hr in the topside ionosphere [ABSTRACT FROM AUTHOR]

Millions of ionograms are acquired annually to monitor the ionosphere. The accumulated data contain untapped information from a range of locations, multiple solar cycles, and various geomagnetic conditions. In this study, we propose the application of deep convolutional neural networks to automatically classify and scale high-latitude ionograms. A supervised approach is implemented and the networks are trained and tested using manually analyzed oblique ionograms acquired at a receiver station located in Skibotn, Norway. The classification routine categorizes the observations based on the presence or absence of E- and F-region traces, while the scaling procedure automatically defines the E- and F-region virtual distances and maximum plasma frequencies. Overall, we conclude that deep convolutional neural networks are suitable for automatic processing of ionograms, even under auroral conditions. The networks achieve an average classification accuracy of 93% ± 4% for the E-region and 86% ± 7% for the F-region. In addition, the networks obtain scientifically useful scaling parameters with median absolute deviation values of 118 kHz ± 27 kHz for the E-region maximum frequency and 105 kHz ± 37 kHz for the Fregion maximum O-mode frequency. Predictions of the virtual distance for the E- and F-region yield median distance deviation values of 6.1 km ± 1.7 km and 8.3 km ± 2.3 km, respectively. The developed networks may facilitate EISCAT 3D and other instruments in Fennoscandia by automatic cataloging and scaling of salient ionospheric features. This data can be used to study both long-term ionospheric trends and more transient ionospheric features, such as traveling ionospheric disturbances. [ABSTRACT FROM AUTHOR]

The 233 MHz EISCAT_3D radar system currently under construction in northern Fennoscandia will be able to resolve ionospheric structures smaller than the transmit beam dimensions through the use of interferometric imaging. This capability is made possible by the modular design and digitisation of the 119 91-antenna panels located at the main Skibotn site. The main array consists of a cluster of 109 panels, with 10 outlier panels producing unique interferometry baselines. In the present study synthesized incoherent scatter radar signal measurements are used for interferometric imaging analysis with the EISCAT_3D system. The Geospace Environment Model of Ion-Neutral Interactions (GEMINI) model is used to simulate a Kelvin-Helmholtz instability in the cusp region at 50 m resolution to obtain plasma parameters which are then used to generate the synthetic data. The ionospheric data is forward propagated to the EISCAT_3D array, noise is added to the synthetic data, and then an inversion of the data is performed to reconstruct the incoherent scatter spectra at relatively fine scales. By using Singular Value Decomposition (SVD) with Tikhonov regularization it is possible to pre-calculate the inversion matrix for a given range and look direction, with the regularization value scaled based on the SNR. The pre-calculation of the inversion matrix can reduce computational overhead in the imaging solution. This study provides a framework for data processing of ion-line incoherent scatter radar spectra to be imaged on fine-scales. Furthermore, with more development it can be used to test experimental set-ups and to design experiments for EISCAT_3D by investigating the needed integration time for various signal-to-noise ratios, beam patterns and ionospheric conditions. [ABSTRACT FROM AUTHOR]

We present two methods for improved calibration of multi-point electron density measurements from incoherent scatter radars (ISR). They are based on the well-established Flatfield correction method used in imaging and photography, where we exploit the analogy between independent measurements in separate pixels in one image sensor and multi-beam radar measurements. Applying these correction methods adds to the current efforts of estimating the magic constant or system constant made for the calibration of multi-point radars, increasing data quality and usability by correcting for variable, unaccounted, and unpredictable variations in system gain. This second-level calibration is especially valuable for studies of plasma patches, irregularities, turbulence, and other research where inter-beam changes and fluctuations of electron density are of interest. The methods are strictly based on electron density data measured by the individual radar and require no external input. This is of particular interest when independent measurements of electron densities for calibration are available only in one pointing direction or not at all. A correction factor is estimated in both methods, which is subsequently used to scale the electron density measurements of a multi-beam ISR experiment run on a phased array radar such as RISR-N, RISR-C, PFISR, or the future EISCAT3D radar. This procedure could improve overall data quality if used as part of the data-processing chain for multi-beam ISRs, both for existing data and for future experiments on new multi-beam radars. [ABSTRACT FROM AUTHOR]

On 15 January 2022, the Hunga volcano produced a massive explosion that generated perturbations in the entire atmosphere. Nonetheless, signatures in the mesosphere and lower thermosphere (MLT) have been challenging to identify. We report MLT horizontal wind perturbations using three multistatic specular meteor radars on the west side of South America (spanning more than 3,000 km). The most notorious signal is an exceptional solitary wave with a large vertical wavelength observed around 18 UT at all three sites, with an amplitude of ∼50 m/s mainly in the westward direction. Using a customized analysis, the wave is characterized as traveling at ∼200 m/s, with a period of ∼2 hr and a horizontal wavelength of ∼1,440 km in the longitudinal direction, away from the source. The perturbation is consistent with an L1 Lamb wave mode. The signal's timing coincides with the arrival time of the tsunami triggered by the eruption. Plain Language Summary: The eruption of the Hunga volcano in January 2022 had a widespread impact on the atmosphere, affecting various layers. We describe a perturbation in horizontal winds caused by the event, which was observed over the west coast of South America by three different meteor radar systems separated by more than 3,000 km between them. The perturbation behaved similarly in the altitude range of 80–100 km, and the wave parameters observed were consistent with high‐order Lamb wave solutions from simulations carried out using the Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension. This finding complements other studies that have explored the impacts of the eruption on different atmospheric levels. Overall, this study provides valuable insights into the complex and far‐reaching effects of volcanic eruptions on the atmosphere. Key Points: Hunga eruption generated extreme horizontal wind perturbations at 80–100 km of altitude over South AmericaThe signal was detected almost simultaneously by three multistatic meteor radar systems spanning more than 3,000 kmThe perturbation had a period of ∼2 hr, a horizontal phase velocity of ∼200 m/s, and a horizontal wavelength of ∼1,440 km [ABSTRACT FROM AUTHOR]