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Observations of coherent scatter from patchy sporadic E layers in the subauroral zone made with a 30‐MHz coherent scatter radar imager are presented. The quasiperiodic (QP) echoes are similar to what has been observed at middle latitudes but with some differences. The echoes arise from bands of scatterers aligned mainly northwest to southeast and propagating to the southwest. A notable difference from observations at middle latitudes is the appearance of secondary irregularities or braids oriented obliquely to the primary bands and propagating mainly northward along them. We present a spectral simulation of the patchy layers that describes neutral atmospheric dynamics with the incompressible Navier Stokes equations and plasma dynamics with an extended MHD model. The simulation is initialized with turning shears in the form of an Ekman spiral. Ekman‐type instability deforms the sporadic E layer through compressible and incompressible motion. The layer ultimately exhibits both the QP bands and the braids, consequences mainly of primary and secondary neutral dynamic instability. Vorticity due to dynamic instability is an important source of structuring in the sporadic E layer. Key Points: Radar observations and theory of subauroral sporadic E layer irregularities are presentedCoherent scatter from the layers presents as primary bands of irreguarities with secondary, oblique, braided structureSimulations of neutral dynamic instability associated with turning shears in Es layers consistent with observations [ABSTRACT FROM AUTHOR]

Numerical forecasts of plasma convective instability in the postsunset equatorial ionosphere are made based on data from the Ionospheric Connections Explorer satellite (ICON) following the method outlined in a previous study. Data are selected from pairs of successive orbits. Data from the first orbit in the pair are used to initialize and force a numerical forecast simulation, and data from the second orbit are used to validate the results 104 min later. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the forecast model. Thirteen (16) data set pairs from August (October), 2022, are considered. Forecasts produced one false negative in August and another false negative in October. Possible causes of forecast discrepancies are evaluated including the failure to initialize the numerical simulations with electron density profiles measured concurrently. Volume emission 135.6‐nm OI profiles from the Far Ultraviolet (FUV) instrument on ICON are considered in the evaluation. Plain Language Summary: Numerical forecasts of disruptions in the postsunset equatorial ionosphere are made from data from the Ionospheric Connections Explorer (ICON) satellite using the method outlined in a previous study. Data are selected from pairs of successive orbits–the first providing data for the numerical forecast model, and the second providing a means of validation. Specifically, measurements of vertical plasma drifts and horizontal wind profiles are used. Thirteen (16) orbital pairs from August (October) of 2022 are considered. The forecasts were accurate in the main but produced one false negative in August and another false negative in October. Reasons for the forecast discrepancies are analyzed including the failure to include electron density profile measurements in the forecast model initialization. Data from the Far Ultraviolet instrument on Icon are considered in the analysis. Key Points: ICON‐based numerical forecasts of plasma convective instability in the equatorial ionosphere extended through 2022Results mainly consistent with earlier ICON‐based forecastsICON FUV profiles used to investigate forecast discrepancies further [ABSTRACT FROM AUTHOR]

The numerical forecast methods used to predict ionospheric convective plasma instabilities associated with Equatorial Spread‐F (ESF) have limited accuracy and are often computationally expensive. We test whether it is possible to bypass first‐principle numeric simulations and forecast irregularities using machine learning models. The data are obtained from the incoherent scatter radar at the Jicamarca Radio Observatory located in Lima, Peru. Our models map vertical plasma drifts, time, and solar activity to the occurrence and location of clusters of echoes telltale of ionospheric irregularities. Our results show that these models are capable of identifying the predictive power of the tested inputs, obtaining accuracies around 75%. Plain Language Summary: Equatorial Spread‐F (ESF) is a phenomenon that happens in the ionosphere and it can create disturbances that affect communication and navigation systems, such as GPS or satellite phones. Scientists have been trying to predict when and where ESF will happen using numerical forecast methods, but these methods are not very accurate and can be expensive. In this study, we tested a new way to predict ESF using machine learning models. We used data from a radar in Lima, Peru, and looked at information such as how the plasma in the ionosphere moves vertically and how active the sun was. The models were able to predict where and when ESF would occur with about 75% accuracy, which is a promising development. Key Points: Clusters of radar echo bins associated with Equatorial Spread‐F can be identified using the DBSCAN algorithm from coherent scatter dataA random forest model showed that the day of year and vertical plasma drifts previous to sunset are good predictors of high SNR clustersA convolutional neural network was able to predict ionospheric irregularities with an accuracy of more than 70% despite scarce data [ABSTRACT FROM AUTHOR]

Aperture‐synthesis images of ionospheric irregularities in the equatorial electrojet are computed using multiple‐input multiple‐output (MIMO) radar methods at the Jicamarca Radio Observatory. MIMO methods increase the number of distinct interferometry baselines available for imaging (by a factor of essentially three in these experiments) as well as the overall size of the synthetic aperture. The particular method employed here involves time‐division multiplexing or time diversity to distinguish pulses transmitted from different quarters of the Jicamarca array. The method comes at the cost of a large increase in computation time and complexity and a reduced signal‐to‐noise ratio. We discuss the details involved in the signal processing and the trade space involved in image optimization. Key Points: Multiple‐input, multiple output (MIMO) radar imaging method implemented at Jicamarca to observe large‐scale waves in the equatorial electrojetSpecific method involves time division multiplexing or time diversityThe resolution of the images was increased but is limited by motion of scatterers which needs mitigation [ABSTRACT FROM AUTHOR]

Artificial periodic inhomogeneity or API experiments were conducted at the HAARP facility in Gakona, Alaska, in October 2022. The experiments concentrated on measuring ionospheric irregularities induced in the E‐region. The irregularities exhibited characteristics regarding their occurrence altitudes, rise and fall times, and Doppler shifts comparable to results from experiments conducted previously at HAARP and elsewhere. The irregularities also occurred in discrete altitude bands. Seeking to quantify these results, we constructed a simple, one‐dimensional fluid model which includes the effects of HF wave heating (direct and indirect) together with electron and ion cooling and thermal conduction, ion production, loss, and diffusion. Critically, the model includes a potential solver and can represent the ambipolar electric field. The model produced API irregularities in three distinct altitude bands which decayed according to the ambipolar diffusion rate. Key Points: A simple fluid model is able to reproduce the salient features of heater‐induced E‐region irregularitiesThese include irregularity generation by direct and indirect heating and dissipation by ambipolar diffusionA potential solver is an essential component of the model [ABSTRACT FROM AUTHOR]

Measurements from the Ionospheric Connections Explorer satellite (ICON) form the basis of direct numerical forecast simulations of plasma convective instability in the postsunset equatorial F region ionosphere. ICON data are selected and used to initialize and force the simulations and then to test the results one orbit later when the satellite revisits the same longitude. Data from the IVM plasma density and drifts instrument and the MIGHTI red‐line thermospheric winds instrument are used to force the simulation. Data from IVM are also used to test for irregularities (electrically polarized plasma depletions). Fourteen datasets from late March 2022, were examined. The simulations correctly predicted the occurrence or non‐occurrence of irregularities 12 times while producing one false positive and one false negative. This demonstrates that the important telltales of instability are present in the ICON state variables and that the important mechanisms for irregularity formation are captured by the simulation code. Possible refinements to the forecast strategy are discussed. Plain Language Summary: Measurements from the Ionospheric Connections Explorer satellite (ICON) are incorporated into numerical simulations of plasma instability known to inhabit the equatorial postsunset F‐region ionosphere. Instability produces ionospheric irregularities which can interfere with radio communication, navigation, and imaging systems, and the simulations are intended as part of a strategy for forecasting the associated space weather hazard. Plasma drift and zonal thermospheric wind measurements in particular are used to drive the simulations. Overall, the method was able to predict instability on 12 of 14 attempts with one false positive and one false negative. Key Points: ICON satellite measurements used to force forecast simulations of plasma convective instability in the equatorial ionosphereNumerical simulations based on data from March 2022, were mainly accurate although there were some failuresSignificant improvements can be made to simulation initialization [ABSTRACT FROM AUTHOR]

We investigate the mechanism underlying lower hybrid waves associated with high altitude echoes recently detected in the post‐sunset equatorial topside ionosphere and inner plasmasphere by the Jicamarca VHF radar. These waves are visible as prominent sidebands in the echo Doppler spectra. New experimental results and newly processed incoherent scatter radar (ISR) datasets are presented that provide clues as to the conditions in which the echoes and associated waves occur. Numerical simulations are presented which demonstrate the feasibility of an inverse energy cascade coupled with a short wavelength instability, that is, the lower hybrid drift instability, in explaining the waves. An inverse cascade is required for short wavelength lower hybrid waves to extend to the 3 m wavelengths measured by the Jicamarca radar. The simulations were able to reproduce some features of the measurements including the lower hybrid sidebands at 3 m wavelengths, asymmetry in the sidebands, and the damping effect of higher densities and lower altitudes. Key Points: Lower hybrid lines were observed in echoes from 1,500 to 2,100 km with stronger intensities at higher altitudes and lower background densitiesA plausible mechanism for lower hybrid waves in echoes involves a lower hybrid drift instability paired with an inverse energy cascadeSeveral features of the echoes are consistent with the assumption that the irregularities originate from remnants of post‐midnight ESF [ABSTRACT FROM AUTHOR]

This paper uses a regional simulation of plasma convective instability in the postsunset equatorial ionosphere together with a global atmosphere/ionosphere/plasmasphere GCM (WAM‐IPE) to forecast irregularities associated with equatorial spread F (ESF) for 1–2 hr after sunset. First, the regional simulation is initialized and forced using ionosphere state parameters derived from campaign data from the Jicamarca Radio Observatory and from empirical models. The irregularities produced by these simulations are found to be quantitatively similar to those observed. Next, the aforementioned state parameters are replaced with parameters from WAM‐IPE, and the resulting departures between the simulated and observed irregularities are noted. In one of five cases, the forecast failed to accurately predict ESF irregularities due to the late reversal of the zonal thermospheric winds. In four of five cases, significant differences between the observed and predicted prereversal enhancement (PRE) of the background vertical drifts resulted in degraded forecast accuracy. This highlights the need for improved PRE forecasting in the global‐scale model. Key Points: Equatorial ionospheric F‐region stability can be reproduced quantitatively using a regional data‐driven direct numerical simulation (DNS)DNS model runs become forecasts when initialized and driven with parameters from a comprehensive GCM such as Whole Atmosphere Model with Ionosphere, Plasmasphere, ElectrodynamicThe post‐sunset background vertical plasma drift and zonal thermospheric wind profile are the most critical forecast parameters [ABSTRACT FROM AUTHOR]

We present measurements of the equatorial topside ionosphere above Jicamarca made during extremely low solar flux conditions during the deep solar minimum of 2019–2020. Measurements were made in October, 2019, February, 2020, and September, 2020. The main features observed are a large and extended decrease in noontime temperatures unlike that seen in studies at moderate solar flux levels, predawn ionospheric heating as early as 0300 LT, large day‐to‐day variability in the O+/H+ transition height, and negligible helium ion concentration at all altitudes. Data from the Ion Velocity Meter (IVM) instrument onboard the Ionospheric Connection Explorer (ICON) and the Topside Ionospheric Plasma Monitor (SSIES) onboard the Defense Meteorological Satellite Program (DMSP) satellites are used to assess agreement with ISR data and assist with the analysis of the predawn heating phenomena. We also analyze the data in light of the SAMI2‐PE model which shows less agreement with the data than at higher solar flux. The main areas of discrepancy with the data are outlined, such as the absence of significant predawn heating, less pronounced decreases in noontime temperatures, and much higher O+ fractions at high altitudes, particularly in September. Finally, a sensitivity analysis of the model to various forcing agents such as neutral winds, plasma drifts, solar flux, and heat flow is performed. A discussion is presented on bridging the discrepancies in future model runs. Novel techniques of clutter removal and noise power bias correction are introduced and described in the appendices. Key Points: New Jicamarca ISR topside measurements of the 2019–2020 solar minimum are presentedISR comparison with DMSP and with ICON for first time at very low solar flux shows consistency and provides insight into predawn heatingComparisons between ISR and SAMI2‐PE model outputs reveals new insight into composition and energetics of topside [ABSTRACT FROM AUTHOR]

Data from a network of high‐frequency (HF) beacons deployed in Peru are used to estimate the regional ionospheric electron density in a volume. Pseudorange, accumulated carrier phase, and signal power measurements for each of the 36 ray paths provided by the network at a 1 min cadence are incorporated in the estimates. Additional data from the Jicamarca incoherent scatter radar, the Jicamarca sounder, and GPS receivers can also be incorporated. The electron density model is estimated as the solution to a global optimization problem that uses ray tracing in the forward model. The electron density is parametrized in terms of B‐splines in the horizontal direction and generalized Chapman functions or related functions in the vertical. Variational sensitivity analysis has been added to the method to allow for the utilization of the signal power observable which gives additional information about the morphology of the bottomside F region as well as absorption including absorption in the D and E regions. The goal of the effort is to provide contextual information for improving numerical forecasts of plasma interchange instabilities in the postsunset F region ionosphere associated with equatorial spread F (ESF). Data from two ESF campaigns are presented. In one experiment, the HF data revealed the presence of a large‐scale bottomside deformation that seems to have led to instability under otherwise inauspicious conditions. In another experiment, gradual variations in HF signal power were found to be related to the varying shape of the bottomside F layer. Key Points: A high‐frequency beacon network in Peru used for inferring ionospheric electron number densities regionally has been greatly expandedSensitivity analysis and absorption computation turn signal power into additional, useful observableRegional electron density data can be combined with incoherent scatter radar measurements for studying and forecasting space weather [ABSTRACT FROM AUTHOR]

Observations of backscatter from field‐aligned plasma density irregularities in sporadic E (Es) layers made with a 30‐MHz coherent scatter radar imager in Ithaca, New York are presented and analyzed. The volume probed by the radar lies at approximately 54° geomagnetic latitude, under the midlatitude trough and at the extreme northern edge of the zone where Es layers are prevalent. Nonetheless, the irregularities exhibit many of the characteristics of quasiperiodic echoes observed commonly at lower middle latitudes. These include a tendency to occur in elongated bands stretching from the northwest to southeast in the Northern hemisphere separated by tens of kilometers and propagating to the southwest. In addition, the irregularities were found to exhibit finer‐scale structures with secondary bands oriented nearly normally to the primary bands. We investigate the proposition that the primary bands are telltale of Es‐layer structuring caused by neutral Kelvin Helmholtz (KH) instability in the lower thermosphere and that the secondary bands signify secondary KH instability. Results from a 3D numerical simulation of KH support this proposition. Key Points: Quasiperiodic (QP) sporadic E layers observed under the midlatitude trough using coherent scatter radarRadar imagery shows secondary bands of irregularities normal to the primary QP bandsThe secondary bands are attributed to secondary Kelvin Helmholtz instability in the lower thermosphere [ABSTRACT FROM AUTHOR]

We implemented a hybrid continuous solver for fluid electrons and kinetic ions. Because the simulation is continuous, numerical noise is not an issue as it is for particle‐in‐cell approaches. Moreover, given that the ion kinetic equation is solved using a characteristic based method, no particle pushes have to be done. Our main goals are to reduce the computational cost of the simulations proposed by Kovalev (Kovalev et al., 2008, https://doi.org/10.5194/angeo2628532008) and reproduce the main experimental features of Farley‐Buneman instabilities measured by radars and rockets. The equations were derived from first principles using the approximations that are satisfied in the auroral E‐region. Various tests will be presented to assess numerical accuracy. With the proposed numerical framework, we are able to recover important nonlinear features associated with Farley‐Buneman instabilities: wave turning of dominant modes, and saturation of density irregularities at values consistent with experiments. Key Points: Farley‐Buneman simulations based on particle‐in‐cell (PIC) are remarkably successful, but new approaches are needed to explore nonlocal processesHybrid continuous solvers are now an alternative when plasma structures are comparable to PIC numerical noiseVarious aspects of Farley‐Buneman instabilities were reproduced, with magnitudes comparable to experiments [ABSTRACT FROM AUTHOR]

We use novel radar observations at the Jicamarca Radio Observatory to study the first observations of highly enhanced plasma temperatures in the evening equatorial ionosphere. These short-lived solstice evening events occurred following large upward plasma drifts and sharply reduced plasma densities. The electron temperatures increase during the August 5 and 6, 2011 moderate solar event reached peak values of about 1,000 K near 350 km. Smaller electron temperature increases were observed to altitudes up to about 500 km. There were also smaller concurrent ion temperature enhancements. During the very low solar flux January 20 and 21, 2020 event, a peak electron temperature increase of about 700 K occurred at an altitude of about 270 km. We also show that SAMI2-PE simulations using the measured vertical plasma drifts reproduce the main characteristics of the measured plasma densities and temperatures up to 300 km, but not at higher altitudes. These simulations indicate that the evening anomalous plasma heating below about 300 km is due to the decrease in the electron-ion cooling rate resulting from upward plasma drift driven decrease in the plasma density. Plasma transport and reduced cooling rate is a potential source of higher altitude heating. [ABSTRACT FROM AUTHOR]

We describe a new class of nonthermal plasma density irregularities observed in the postmidnight topside equatorial ionosphere under low solar flux conditions. They are distinct from irregularities associated with equatorial spread F (ESF) in terms of their morphology and because they exhibit strong spectral sidebands at the lower-hybrid frequency. The coherent echoes were observed in a series high-altitude radar experiments performed at Jicamarca utilizing long- and coded double-pulse modes and a dual-beam mode. The coded double-pulse mode was used to measure the low-frequency characteristics of the echoes with fine range resolution. Doppler shifts of the main backscatter line were observed to fall between ±150 m/s. The long-pulse mode was employed for high-frequency spectral analysis which revealed the presence of strong spectral sidelobes at the lower-hybrid frequency. A dual-beam mode was used to investigate the horizontal structure of the echoes. Zonal drift speeds of 50--70 m/s were inferred with this mode, and longitudinal dimensions of approximately 270 km were estimated. The study summarizes with a discussion of different mechanisms that may be responsible for the phenomenon and the lower-hybrid sidebands in particular. [ABSTRACT FROM AUTHOR]

We use novel radar observations at the Jicamarca Radio Observatory to study the first observations of highly enhanced plasma temperatures in the evening equatorial ionosphere. These short‐lived solstice evening events occurred following large upward plasma drifts and sharply reduced plasma densities. The electron temperatures increase during the August 5 and 6, 2011 moderate solar event reached peak values of about 1,000 K near 350 km. Smaller electron temperature increases were observed to altitudes up to about 500 km. There were also smaller concurrent ion temperature enhancements. During the very low solar flux January 20 and 21, 2020 event, a peak electron temperature increase of about 700 K occurred at an altitude of about 270 km. We also show that SAMI2‐PE simulations using the measured vertical plasma drifts reproduce the main characteristics of the measured plasma densities and temperatures up to 300 km, but not at higher altitudes. These simulations indicate that the evening anomalous plasma heating below about 300 km is due to the decrease in the electron‐ion cooling rate resulting from upward plasma drift driven decrease in the plasma density. Plasma transport and reduced cooling rate is a potential source of higher altitude heating.Key Points: First observations of large anomalous electron temperature increases in the evening equatorial ionosphereSAMI2‐PE simulations are consistent with increased electron temperatures below about 300 kmTemperature increase consistent with upward drift‐driven reduced electron density and electron‐ion cooling rate [ABSTRACT FROM AUTHOR]

We describe a new class of nonthermal plasma density irregularities observed in the postmidnight topside equatorial ionosphere under low solar flux conditions. They are distinct from irregularities associated with equatorial spread F (ESF) in terms of their morphology and because they exhibit strong spectral sidebands at the lower‐hybrid frequency. The coherent echoes were observed in a series high‐altitude radar experiments performed at Jicamarca utilizing long‐ and coded double‐pulse modes and a dual‐beam mode. The coded double‐pulse mode was used to measure the low‐frequency characteristics of the echoes with fine range resolution. Doppler shifts of the main backscatter line were observed to fall between ±150 m/s. The long‐pulse mode was employed for high‐frequency spectral analysis which revealed the presence of strong spectral sidelobes at the lower‐hybrid frequency. A dual‐beam mode was used to investigate the horizontal structure of the echoes. Zonal drift speeds of 50–70 m/s were inferred with this mode, and longitudinal dimensions of approximately 270 km were estimated. The study summarizes with a discussion of different mechanisms that may be responsible for the phenomenon and the lower‐hybrid sidebands in particular.Key Points: High altitude echoes observed in the topside ionosphere over Jicamarca at postmidnight times during solar minimumThe echoes inhabit altitudes between 1,000 and 2,200 kmHigh altitude echoes exhibit spectral sidebands at the lower‐hybrid frequency [ABSTRACT FROM AUTHOR]

Radar and sounding rocket observations of plasma irregularities in the F-region ionosphere acquired on 19 June 2019 during NASA experiment Too WINDY on Kwajalein Atoll are presented. The experiment was conducted near local midnight during a period of low solar flux and quiet geomagnetic conditions. Plasma density irregularities were seen by the rocket and also in the incoherent scatter radar data to emerge and persist mainly in the topside. Density irregularities in the bottomside remained very small by comparison throughout the observations. Zonal plasma drifts measured by the rocket were highly structured in the topside. Patches of coherent scatter entrained in the large-scale topside density irregularities appeared to propagate slowly westward in a narrow flow channel detected by the rocket. Broadband ELF emissions were also detected in the topside. Some of the characteristics of the topside irregularities are typical of postsunset equatorial F-region irregularities observed frequently by coherent scatter radars, and some of the common features in the coherent scatter database are reviewed. Two scenarios that have been proposed to account for postmidnight spread F are tested computationally. One involves unseasonably large background zonal electric fields, and the other involves forcing from below by neutral waves and turbulence. Neither scenario appears to be able to account for the Too WINDY observations and the preponderance of topside irregularities without bottomside precursors in particular. [ABSTRACT FROM AUTHOR]

Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR) data acquired during the NASA Waves and INstabilities from a Neutral DYnamo (WINDY) sounding rocket campaign on Kwajalein Atoll in late August and September 2017 are analyzed to study the development of postsunset F‐region ionospheric plasma density irregularities associated with equatorial spread F (ESF) conditions. Unstable conditions existed on nine of ten observing nights during the campaign. The first indication of instability onset each night was a patchy layer of coherent scatter in the bottomside/valley region. Patchy bottom‐type layers are telltales of plasma convective instability driven by vertical current in the F region associated with bottomside shear flow. The resulting flow evolves rapidly into fully developed instability through a bootstrapping process. Sometimes, the resulting topside depletions exhibited large‐scale structuring with a spatial scale too large to be associated with the preferred scale for plasma instability. We examine incoherent scatter observations of the background ionospheric morphology prior to the appearance of bottom‐type layers and find that a small degree of large‐scale bottomside structuring is sometimes present. Numerical simulations suggest that this structuring can act as a seed and provide the initial conditions necessary to account for the clustering of the topside plumes that emerge later. Key Points: Ionospheric interchange instability in the equatorial F region was observed with the ALTAIR radar throughout the NASA WINDY sounding rocket campaignThe radar dataset allows us to distinguish between the "seeding" and "bootstrapping" mechanisms contributing to instabilityVery little ionospheric preconditioning was observed from night to night; what was observed may explain depletion clustering, however [ABSTRACT FROM AUTHOR]

Ionospheric conditions on two adjacent nights in March 2019 were observed at the Jicamarca Radio Observatory using a combination of incoherent scatter, coherent scatter, and high‐frequency (HF) radio modes. The HF data came from a network of beacons consisting of three transmitters and six receivers operating at two frequencies and deployed regionally. The HF beacons employ pseudorandom noise coding and can be used to measure group delay (pseudorange) and Doppler shift, and the time derivative of optical path length. A method for inferring volumetric estimates of electron density regionally from the HF data is described. The radar and HF data are interpreted in light of a direct numerical simulation of the ionospheric interchange instability to elucidate why convective plumes and equatorial spread F conditions occurred on one night but not the other. The numerical simulation accurately predicted whether convective plumes would develop on a given night, utilizing initial conditions and forcings derived from the incoherent scatter data. The HF data were consistent with the incoherent scatter observations and remained intelligible throughout the equatorial spread F event. Crests in the bottomside electron density associated with convective plumes at higher altitudes could be seen propagating through the region in the HF data. It should be possible to incorporate HF data in assimilative simulations of interchange instabilities in order to predict where and when individual convective plumes emerge. Key Points: An HF beacon network has been deployed in Peru for regional electron density specificationElectron density estimates recovered from the beacon network are consistent with incoherent scatter results but have regional coverageForecast models incorporating assimilation of beacon data could be used to predict where convective plumes form during ESF events [ABSTRACT FROM AUTHOR]

We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements. Key Points: A radar capable of probing a wide swath of geospace could be assembled from a HPLA transmitter and a number of radio‐array receiversApplications for such a geospace radar include MST, meteor, ionospheric, plasmaspheric, planetary, and solar researchA geospace radar would promote discovery research and support space weather applications [ABSTRACT FROM AUTHOR]

Inverse methods involving compressive sensing are tested in the application of two‐dimensional aperture‐synthesis imaging of radar backscatter from field‐aligned plasma density irregularities in the ionosphere. We consider basis pursuit denoising, implemented with the fast iterative shrinkage thresholding algorithm, and orthogonal matching pursuit (OMP) with a wavelet basis in the evaluation. These methods are compared with two more conventional optimization methods rooted in entropy maximization (MaxENT) and adaptive beamforming (linearly constrained minimum variance or often "Capon's Method.") Synthetic data corresponding to an extended ionospheric radar target are considered. We find that MaxENT outperforms the other methods in terms of its ability to recover imagery of an extended target with broad dynamic range. Fast iterative shrinkage thresholding algorithm performs reasonably well but does not reproduce the full dynamic range of the target. It is also the most computationally expensive of the methods tested. OMP is very fast computationally but prone to a high degree of clutter in this application. We also point out that the formulation of MaxENT used here is very similar to OMP in some respects, the difference being that the former reconstructs the logarithm of the image rather than the image itself from basis vectors extracted from the observation matrix. MaxENT could in that regard be considered a form of compressive sensing. Key Points: Compressive sensing inverse methods have been applied to aperture synthesis radar imaging of ionospheric plasma density irregularitiesPerformance of basis pursuit denoisint (BPDN) and orthogonal matching pursuit (OMP) are generally inferior to that of the maximum‐entropy method (MaxENT)Computational speed of OMP is attractive and prompts research into more suitable function library [ABSTRACT FROM AUTHOR]

Ionospheric modification experiments have been performed at the High‐Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, using a Very High Frequency (VHF) coherent scatter radar in Homer, Alaska, for experimental diagnostics. The experiments were intended to determine the threshold pump electric field required to initiate thermal parametric instability in the E region. The pump power level was ramped systematically to determine the threshold, and the experiment was repeated at four closely spaced pump frequencies. This provided threshold estimates at four E region altitudes. The theory for thermal parametric instability based on the work of Dysthe et al. (1983, https://doi.org/10.1063/1.863993) has been modified for application in the E region. The theory considers magneto‐ionic effects on the pump mode, linear mode conversion theory for upper hybrid wave generation, wave heating, and the effects of transport and dissipation based on fluid theory. The theory amounts to an eigenvalue problem where the eigenvalue is the threshold pump electric field for instability. The theory shows how the threshold depends on ionospheric transport coefficients and on the fractional cooling rate for inelastic electron‐neutral collisions. The theoretical predictions for threshold are roughly consistent with experimental values although the latter are probably affected by excess ionospheric absorption. Key Points: The threshold electric field for thermal parametric instability in the E region has been estimated experimentallyA modified theory for thermal parametric instability in the E region has been formulated for comparisonComparisons between theory and measurements can be used to constrain transport, dissipation, and absorption in the ionosphere with remote sensing [ABSTRACT FROM AUTHOR]

Coherent radar backscatter spectra from auroral E region plasma irregularities can be measured with great precision and accuracy. The Doppler spectra obtained from those measurements can be used to estimate auroral convection patterns using an empirical model. These estimates are consistent with rocket, optical, and incoherent scatter radar data, but direct comparison between the different data sets is not straightforward. In order to establish the quality of the empirical model, alternative approaches are then needed. In this work, we use the electrostatic nature of the convection electric field to assess the physical consistency of the empirical model: The model will be satisfactory to the extent that the convection patterns derived from them are incompressible. The incompressibility criteria were assessed for the estimated convection patterns obtained from the substorm of 20 December 2015. Two different approaches were used for this assessment. First, an electrostatic potential was fitted to the convection pattern produced by the empirical model. A good fit implies that there exists an incompressible convection pattern consistent with the Doppler spectra measurements. Second, the uncertainties on the Doppler spectral estimates where propagated through the empirical model to calculate the expected uncertainties on the divergence of the convection field obtained directly from the empirical model. The convection field will be incompressible if its divergence lies within the uncertainty estimates. Both approaches indicate that the evaluated convection patterns are consistently incompressible. Key Points: An empirical relation between coherent backscatter spectra and convection flows can be constructed based on experiments and simulationsThe estimates from the empirical model were assessed by evaluating whether they produced incompressible flowsFlows estimates associated with the 20 December 2015 substorm were found to be incompressible within experimental errors [ABSTRACT FROM AUTHOR]

A network of high‐frequency (HF) transmitters and receivers used for ionospheric specification is being installed in Peru. The HF transmitters employ multiple frequencies and binary phase coding with pseudorandom noise, and the observables provided by the receivers include group delay, Doppler shift, amplitude, bearing (from interferometry), and polarization. Statistical inverse methods are used to estimate F region density in a volume from the data regionally. The method incorporates raytracing based on the principles of Hamiltonian optics in the forward model and involves an ionospheric parametrization in terms of Chapman functions in the vertical and bicubic B‐spline interpolation in the horizontal. Regularization is employed to minimize the global curvature of the reconstructions. HF beacon data for two nights in January 2018 are presented. We use the reconstructions to investigate why plasma irregularities associated with equatorial spread F formed on one occasion and not the other. The data indicate that the background ionospheric flow is not simply frozen in, that is, that longitude and local time variations cannot be equated, even at regional scales. This has ramifications for equatorial spread F forecasting strategies that assume equivalence. Key Points: An expanding network of HF beacon transmitters and receivers is being deployed in Peru for purposes of ionospheric specificationPseudorange and Doppler shift data from network observed at two frequencies are inverted to give a regional specification of F regionIn a campaign in January 2018 the beacon data help explain why ESF occurred on one evening but not the next [ABSTRACT FROM AUTHOR]

Abstract: Artificial E region field‐aligned plasma density irregularities (AFAIs) have been generated using the High‐frequency Active Auroral Research Program (HAARP) ionospheric modification facility in Gakona and observed with a 30 MHz coherent scatter radar imager in Homer, Alaska. The AFAIs were generated using a distinctive, twisted‐beam antenna pattern that illuminated a particularly broad volume overhead. The broad beam facilitates studies of natural sporadic E layer patches when they are present. The center of the pattern was pointed at different angles between zenith and magnetic zenith to examine the effects on the AFAI morphology. Radar images of AFAIs generally resemble the radiation pattern of the high‐frequency source, but the irregularities are strongest within a narrow range of zenith angles bounded approximately by the Spitze angle. A number of factors which might influence AFAI generation and detection are examined. The most important is most likely the requirement for the pump mode to have a standing‐wave component for thermal parametric instability to operate. [ABSTRACT FROM AUTHOR]

Plasma irregularities in the postsunset equatorial valley region ionosphere are investigated experimentally and through numerical simulation. Coherent radar backscatter observed at the Jicamarca Radio Observatory shows two classes of irregularities in different altitude bands-one mainly below about 125 km and the other mainly above. Irregularities in both bands are organized into wavefronts with wavelengths of a few kilometers. However, only the irregularities in the high-altitude band exhibit consistent propagation speeds and directions. Some previous observations of irregularities in the nighttime electrojet suggest that gravity waves may sometimes influence their morphology. The possibility that the valley region irregularities are also related to gravity waves (GWs) is therefore investigated numerically. A model of a GW packet propagating through a tidal wind field is used to drive another model which predicts the resulting ionospheric electrodynamics. The combined simulation shows that GWs can induce field-aligned currents and excite resistive drift waves which could be responsible for the valley region irregularities in the high-altitude band. The GWs also induce irregularities in the upper E region directly through simple dynamo action which subsequently deform under the influence of shear flow. This may explain the irregularities in the low-altitude band. [ABSTRACT FROM AUTHOR]