Spread F is probably the most well-known phenomenon in the ionospheric community. The broad spectrum of density irregularities associated with the causal Rayleigh-Taylor instability generates a wide range of radio signals spanning the HF through UHF frequency bands. Longitudinal differences in seasonal activity patterns, largely explained by the relationship between the local magnetic field geometry and the solar terminator orientation, have been well established through a combination of ground- and space-based observations over the last three decades. More recently, an analysis of daily spread F occurrence as determined by low latitude VHF and GNSS scintillation activity shows results that are statistically consistent with previously established climatologies, but the level of day-to-day variability in the occurrence data show marked variations with longitude and solar flux. Specifically, the American sector shows very low in-season variability for F10.7 values exceeding a relatively moderate value of about 130 sfu, while the African and Asian sectors exhibit true day-to-day variability regardless of seasonal variations or solar flux levels within the limits of the observations. These intriguing results are not fully understood yet, but the implications are clear: at least in limited longitude windows, seasonal factors control the occurrence of spread F irregularities with little variability; gravity waves or other “seeds” appear to play no significant role. Moreover, the physical parameters believed to control the Rayleigh-Taylor instability-field-line integrated $E$ & $F$ region conductivities, electric field, thermospheric winds, collision rate and vertical electron density gradient-must not all be relevant or at least truly variable; that is, some parameters are playing a more important role or are robust with respect to daily variability. Using a physics-based low latitude spread F model, we assess the primary drivers controlling occurrence as a function of longitude. The results have significant implications for space weather as they suggest that meaningful long-term forecasts of equatorial scintillation may be possible within specific longitude boundaries. And importantly, although day-to-day variability is higher at other longitudes, it is reasonable to believe that some parameters are more critical than others in determining instability occurrence even in those regions. These result were facilitated by beacon satellite measurements and would not have been possible with space-based in situ observations alone.

Abstract This paper presents the validation of modeled total electron content (TEC) from 14 ionospheric models, including empirical, physics‐based, and data assimilation (DA) models, hosted by the NASA/NSF Community Coordinated Modeling Center (CCMC), NOAA Space Weather Prediction Center, and NASA Jet Propulsion Laboratory (JPL). This study aims to assess the current progress and capability of the CCMC‐hosted ionospheric models in capturing the storm time ionosphere during the low and moderate solar flux years. We focus on the low‐latitude ionosphere (i.e., ±40° in magnetic latitude) and compare the modeled TEC with the Madrigal TEC during the 2013 March and 2021 November storms. Multiple metrics are used to quantitatively assess the models' accuracy, precision, association, bias, and capability in capturing the TEC changes in response to the storms. The skill score based on the metric scores is further proposed to evaluate the overall performance of ionospheric models against the reference model (International Reference Ionosphere 2016; IRI‐2016). The results indicate that the DA model GLObal Total Electron Content and JPL Global Ionospheric Map models show good performance in modeling the TEC and reasonably reflect the storm time TEC changes spatially and temporally. The empirical models IRI‐2016 and 2020 show relatively good performance compared with the physics‐based models regarding the model‐data comparison; however, it is difficult to characterize the TEC changes caused by storms. The physics‐based models can simulate the storm effect in spatial and temporal TEC variations better than the empirical model. The performance of ionospheric models in capturing the storm time TEC anomaly is presented and discussed.

We present the first observational determination of statistical limits on the rise of equatorial plasma bubbles as a function of solar flux. We analyzed in situ electron density data collected onboard the Communications/Navigations Outage Forecasting System (C/NOFS) satellite to characterize the distribution of peak altitudes of equatorial ionospheric plasma bubbles. We first describe our algorithm for detecting ionospheric irregularities within the observations and then use a series of statistical simulations to identify and compensate for the sampling biases inherent in observations from a single satellite in a low‐inclination elliptical orbit. The simulations also confirmed that space‐based orbital platforms such as the C/NOFS satellite undersample the existing irregularities in the ionosphere and provide a measure of the satellite’s inefficiency in observing those naturally occurring irregularities. In deducing the variation of the peak‐height distributions of the irregularities with solar activity, we find that the median maximum height of the bubbles increases linearly from about 490 km at the solar minimum (2008) to 740 km during the (2014) solar maximum in the longitude sector 80°W–10°E. The results will be valuable for the development of improved scintillation mapping models for both real‐time and postprocessing applications. We also confirm our observational findings with modeling results from a physics‐based model, allowing us to identify field‐line‐integrated Pedersen conductance as the key determinant of terminal bubble altitude: a bubble will cease to rise further when the conductance inside the bubble is equal to that of the background ionosphere.

The low-latitude ionosphere is characterized by large-scale instabilities in the post-sunset hours due to the distinct geometry of the earth’s magnetic field lines at the equator. The magnetic field lines are horizontal at the equator contributing to the high vertical drift velocity of the plasma bubbles growing from the bottomside of the ionospheric F-region. The phenomenon, commonly known as equatorial spread F, is an important problem in aeronomy as it can cause radio wave scintillation effects representing the most critical impacts of space weather on man-made technologies, such as satellite communications and global navigation satellite systems (GNSS). Here, we report results on the dependence of the peak heights of the irregularities at the magnetic equator, also called as apex-altitude, on solar flux by analyzing in-situ observations made on-board the Communications/Navigations Outage Forecasting System (C/NOFS) satellite mission. Our analysis indicates the median of the peak-height distributions of the irregularities increases linearly from about 491 km at solar minimum to 737 km during solar maximum. The Physics-Based Model (PBMOD) has been used to confirm the space-based observational results and we find the field-line integrated conductivity is the key parameter which controls the peak-heights of the irregularities. In this investigation, we also seek to understand the possible dependence of the irregularity parameter characterizing the equatorial ionospheric irregularities on the background ionospheric density.

An unseasonal equatorial plasma bubble (EPB) event over South‐East Asia was observed on July 22, 2014 that has not been studied before. An investigation into this event is presented with the 26th July, 2014 as a comparison, non‐bubble day. The 22nd July EPB event occurred in the late post‐sunset sector and was associated with a small upward plasma drift. This event was highlighted using a new filter on the SCINDA S4 data. Ionosonde data show that sporadic E was present during the growth period for the EPB event. Modeling results from Thermosphere‐Ionosphere Electrodynamics (TEC) Global Circulation Model were used to conduct a numerical experiment investigating the direct effect of sporadic E on the linear R‐T growth rate. It was shown that sporadic E located in the correct latitude and local time can increase the linear growth rate. The seeding conditions were investigated using TEC data from Patumwan, Thailand. Wave‐like structures were observed for both days of interest, with larger amplitudes on 22nd July compared with the 26th July. Finally, simulations using the high‐resolution model PBMOD showed that for forcing from above conditions similar to the days of interest, EPBs would form in the presence of large seed perturbations. Therefore, it is likely that this unseasonal event was caused by large seed perturbations in TEC.

We apply a wave-optics technique to model the propagation of high-frequency (HF) waves through simulated traveling ionospheric disturbances (TIDs) and developing plasma bubbles at low latitudes. Wave-optics is derived from a forward-propagation approximation of the Helmholtz equation governing the electric field in the frequency domain. The technique is implemented using the split-step Fourier approach commonly referred to as the multiple phase screen method (MPS). At an intermediate step in the computation, the electric field along each phase screen is expressed explicitly in terms of the angular spectrum of plane waves intersecting the screen. We use this approach to produce angle-of-arrival "maps," which depict the spectrum of angle-of-arrival (AOA) at all locations on the ground. These AOA maps identify all radio propagation modes reaching the receiver along with their individual amplitudes. With the wave-optics approach there is no need to 'home' rays to identify the propagation modes that are present. A full-wave technique, wave-optics accurately represents the interaction (via diffraction) between the different propagation modes, which can result in fading of the received HF signal. Ray-tracing techniques neglect diffraction and therefore cannot represent these interactions nor the signal fading they produce.

Abstract Predicting the daily variability of Equatorial Plasma Bubbles (EPBs) is an ongoing scientific challenge. Various methods for predicting EPBs have been developed, however, the research community is yet to scrutinize the methods for evaluating and comparing these prediction models/techniques. In this study, 12 months of co‐located GPS and UHF scintillation observations spanning South America, Atlantic/Western Africa, Southeast Asia, and Pacific sectors are used to evaluate the Generalized Rayleigh‐Taylor (R‐T) growth rates calculated from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). Various assessment metrics are explored, including the use of significance testing on skill scores for threshold selection. The sensitivity of these skill scores to data set type (i.e., GPS versus UHF) and data set size (30, 50, 60, and 90 days/events) is also investigated. It is shown that between 50 and 90 days is required to achieve a statistically significant skill score. Methods for conducting model‐model comparisons are also explored, including the use of model “sufficiency.” However, it is shown that the results of model‐model comparisons must be carefully interpreted and can be heavily dependent on the data set used. It is also demonstrated that the observation data set must exhibit an appropriate level of daily EPB variability in order to assess the true strength of a given model/technique. Other limitations and considerations on assessment metrics and future challenges for EPB prediction studies are also discussed.

The Earth's ionosphere/thermosphere (I/T) system exhibits complicated weather variability that can have adverse effects on human operations and systems, and consequently, there is a need for both accurate and reliable specifications and forecasts for this region. As part of the international effort to evaluate and assess the predictive capabilities of space weather models, four working groups for the I/T system have been created with the goal to devise a concerted model validation effort for the I/T environment. This paper presents an overview of the team efforts and reports on the progress made. As a first step, the working teams have selected to limit the validation efforts to critical I/T parameters that include total electron content, the peak density and height of the ionospheric F region, ionospheric scintillations, and thermospheric neutral densities. As part of this effort, initial lists of participating models and events have been constructed and validation data sets have been identified. In the future appropriate metrics will be selected for the various user and scientific needs.

In the post-sunset equatorial ionosphere the Generalised Rayleigh-Taylor (R-T) instability causes small-scale plasma irregularities to increase in size, generating large scale plasma depletions called Equatorial Plasma Bubbles (EPB). Diffractive scattering caused by these EPBs can cause scintillation of Global Navigation Satellite System (GNSS) signals. Space weather agencies around the world recognise the need for accurate forecasting of EPBs. However, there are currently no global scintillation forecasts freely available for GNSS users. climatology of EPBs is relatively understood and been shown to correlate well with the strength of the Pre-Reversal Enhancement (PRE) of the zonal electric field. Longitudinal gradients in the electron density across the day-night terminator control the strength of the PRE. These gradients are reduced when the ionospheric plasma can flow from the sunlit ionosphere into the post-sunset ionosphere, which requires a large angle between the magnetic field and the day-night terminator. The longitudinal and seasonal variations in the EPBs are relatively well explained in terms of this angle. However, the ability to provide accurate requires an understanding of the day-to-day variability of EPB occurrence. of the of the initial perturbations, the RT growth of e-folding the RT growth EPB the and space weather conditions that provide this daily work of a number of unseasonable EPB events over South-East Asia that occurred in July 2014. This event was observed in the COSMIC S4 index and as Spread F in ionograms obtained from Sanya, China. Ionosonde data from Bac Lieu and Cebu show the upward plasma drift was small compared with typical values for EPB days. Solar wind data were obtained and used to categorise the geomagnetic environment during the event and showed no clear indication that EPBs were likely. The presence of low latitude sporadic E is observed using the ionograms from Sanya and its potential effects on EPB growth and plasma destabilisation are analysed.

An unseasonal development of Equatorial Plasma Bubble (EPB)/Equatorial Spread-F (ESF) activity in a wide longitudinal sector over India and Southeast Asia during the post-sunset hours of 28 July 2014 has been investigated in detail using the multiple independent observations in this paper and a companion paper by (Prog Earth Planet Sci 5:10, 2018). This post-sunset ESF/EPB event was preceded by a substantial elevation of the equatorial F-layer due to strong pre-reversal enhancement (PRE) during the season which is climatologically unfavorable for strong PRE and post-sunset EPBs. In this paper, it is found that a strong equatorward meridional neutral wind at off-equatorial E-region and enhanced longitudinal gradient of equatorial electrojet appear to be the responsible candidates for the enhanced PRE and EPB development on 28 July 2014. These changes in the local wind system and PRE are found to have an apparent connection with the planetary wave forces of lower atmospheric origin. Both the post-sunset height rise (PSSR) and lower thermospheric meridional neutral wind consistently exhibit quasi-2-day planetary wave-like oscillations during 20–31 July 2014 with maximum amplitudes on 28 July 2014. This study emphasizes the role of planetary wave forces from the lower atmosphere that can alter the local neutral wind system and E-region conductivities that may lead to the development of strong PRE and EPB activity which were otherwise climatologically unfavorable for post-sunset EPB/ESF development.

Clouds of vaporized samarium (Sm) were released during sounding rocket flights from the Reagan Test Site, Kwajalein Atoll in May 2013 as part of the Metal Oxide Space Cloud (MOSC) experiment. A network of ground-based sensors observed the resulting clouds from five locations in the Republic of the Marshall Islands. Of primary interest was an examination of the extent to which a tailored radio frequency (RF) propagation environment could be generated through artificial ionospheric modification. The MOSC experiment consisted of launches near dusk on two separate evenings each releasing similar to 6 kg of Sm vapor at altitudes near 170 km and 180 km. Localized plasma clouds were generated through a combination of photoionization and chemi-ionization (Sm + O. Sm+O -> SmO(+)e(-)) processes producing signatures visible in optical sensors, incoherent scatter radar, and in high-frequency (HF) diagnostics. Here we present an overview of the experiment payloads, document the flight characteristics, and describe the experimental measurements conducted throughout the 2 week launch window. Multi-instrument analysis including incoherent scatter observations, HF soundings, RF beacon measurements, and optical data provided the opportunity for a comprehensive characterization of the physical, spectral, and plasma density composition of the artificial plasma clouds as a function of space and time. A series of companion papers submitted along with this experimental overview provide more detail on the individual elements for interested readers.

The effects on the plasma/electrodynamic environment in the low-latitude ionosphere produced by the artificial plasma clouds created in the Metal Oxide Space Cloud (MOSC) experiment are studied via simulations. The electric fields and plasma flow in the vicinity of the cloud are calculated using its estimated field-line-integrated conductance; it is found that the “comma-like” flow around the cloud seen in the ALTAIR (Advanced Research Project Agency [ARPA] Long-range Tracking and Identification Radar) observations can be explained by the perturbations to the electric field produced by the conductance gradients around the cloud. Next, the conductance is introduced into a simulation of the development of the Rayleigh-Taylor instability. The simulations suggest that a moderately denser cloud than the MOSC cloud, closer to the bottom edge of the F layer, could indeed suppress the development of the low-density plumes and the shorter-wavelength irregularities associated with radio scintillation that form with the Rayleigh-Taylor instability in the low-latitude ionosphere.

An analysis of the occurrence of equatorial plasma bubbles (EPBs) around the world during the 2015 St. Patrick's Day geomagnetic storm is presented. A network of 12 Global Positioning System receivers spanning from South America to Southeast Asia was used, in addition to colocated VHF receivers at three stations and four nearby ionosondes. The suppression of postsunset EPBs was observed across most longitudes over 2 days. The EPB observations were compared to calculations of the linear Rayleigh‐Taylor growth rate using coupled thermosphere‐ionosphere modeling, which successfully modeled the transition of favorable EPB growth from postsunset to postmidnight hours during the storm. The mechanisms behind the growth of postmidnight EPBs during this storm were investigated. While the latter stages of postmidnight EPB growth were found to be dominated by disturbance dynamo effects, the initial stages of postmidnight EPB growth close to local midnight were found to be controlled by the higher altitudes of the plasma (i.e., the gravity term). Modeling and observations revealed that during the storm the ionospheric plasma was redistributed to higher altitudes in the low‐latitude region, which made the plasma more susceptible to Rayleigh‐Taylor growth prior to the dominance of the disturbance dynamo in the eventual generation of postmidnight EPBs.