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Horizontals drifts of equatorial Spread F (ESF) at post-sunset and post-midnight are investigated by analyzing six ESF events observed during the period of November 2022–March 2023. Horizontal drift velocities of ESFs are calculated from the time lags between signals recorded by different transmitter–receiver pairs of a new Continuous Doppler Sounding (CDS) system operating at 6.80 MHz in a low latitude station, Tucumán, Argentina (26° 49’ S, 65° 13' W, mag. latitude ~ 13°) and by the older CDS system working at 4.63 MHz. A new method of time lags determination for spread structures is presented. In addition, the occurrence of airglow depletions associated with ESF events is verified using images of airglow emissions of atomic O red line, 630 nm. We found that the typical speeds of the ESF drift in the post-sunset hours (around 130 m/s) are about two times greater than the speeds of ESF occurring around midnight or in post-midnight hours (around 80 m/s). The drift speeds obtained using 4.63 and 6.80 MHz systems were practically the same with the exception of one event, which might have been due to wind shear. Azimuths obtained by 4.63 and 6.80 MHz systems are almost similar. No systematic dependence of the azimuth on the local time and sounding frequency was found. All ESF events drift roughly eastward with an average azimuth of about 105 ∘ with respect to the geographic north. Graphical Abstract

We present results of a study of post-midnight equatorial spread F (ESF) events over the Jicamarca Radio Observatory (JRO) that examined unambiguous radar measurements of event origin in the American sector. Our analysis considers variations in post-midnight ESF generation due to changing seasonal, solar, and geomagnetic conditions. We analyzed 396 nights of observations made with the 14-panel version of the Advanced Modular Incoherent Scatter Radar (AMISR-14) between July 2021 and August 2023. We leveraged the 10-beam AMISR-14 mode, which effectively measures ~ 400 km zonally of the equatorial F-region ionosphere, to identify and classify post-midnight ESF as either local (i.e., generated within the instrument field of view) or non-local (i.e., generated outside the instrument field of view). Our results for the occurrence rates of post-midnight ESF exhibit a strong seasonal dependence, with maximum values in June solstice and minimum values for equinoxes. The results also show the post-midnight ESF occurrence rates are anticorrelated to the solar flux conditions. As for geomagnetic activity, the results indicate that occurrence rates decrease considerably under geomagnetically quiet conditions. The combination of these seasonal, solar flux, and geomagnetic activity influences suggests the weakened downward plasma drifts late at night during June solstice conditions can be reversed to upward drifts by contributions from disturbance drifts. In the case of upward drifts caused by geomagnetic disturbances, the reversed upward post-midnight drifts may then contribute to conditions favoring ESF development provided that a prompt penetration or disturbance dynamo electric field with appropriate polarity, even from modest geomagnetic activity, is present. In support of this proposed post-midnight ESF generation mechanism, we also present and discuss simultaneous AMISR-14 and collocated incoherent scatter radar measurements of a June solstice 2023 event. Perhaps most importantly, our results show the occurrence rates of local and non-local post-midnight ESF as observed with AMISR-14 are nearly identical. That is, local events were observed effectively as often as non-local events, and vice versa, under all seasonal, solar, and geomagnetic conditions. Therefore, data-driven forecasting approaches relying exclusively on local (i.e., “overhead”) measurements of ionospheric/thermospheric conditions may not always be well-suited to reproducing the observed ESF phenomenology. Graphical Abstract Key Points We analyzed ~2 years of two-dimensional radar measurements to determine the climatology of post-midnight ESF generated locally (i.e., within the radar field of view) and non-locally. We found nearly the same (50/50) occurrence rates for post-midnight ESF events that developed locally and non-locally, independent of season and solar flux conditions. Collocated ESF and drift observations show the unequivocal case of a post-midnight ESF event generated under conditions of abnormal vertical plasma drifts. The observations also show that even moderate geomagnetic activity can contribute to the generation of post-midnight ESF during June solstice. The observations support the hypothesis that post-midnight ESF is more likely to occur under certain conditions of weak post-midnight drifts with contributions from disturbance electric fields.

The occurrence of equatorial spread F (ESF) has the potential to detrimentally impact space-based technological systems. This study investigates the utility of ionosondes in forecasting the incidence of post-sunset ESF in the zonal direction, utilizing observational data obtained from four ionosondes located near the magnetic Equator in Southeast Asia. Data were collected during the equinox seasons (March–April and September–October) between 2003 and 2020. To establish a relationship between the probability of post-sunset ESF occurrence and the evening vertical plasma drift (v), a logistic regression model was employed. Post-sunset ESF occurrence is defined as the presence of ESF during the time window between 19:00 and 21:00 LT, while v is derived from the average time derivative of virtual heights during the interval from 18:30 to 19:00 LT. Results indicate that the probability of post-sunset ESF occurrence approaches zero, signifying that ESF is unlikely to develop when v is negative. Conversely, when v exceeds 30 m/s, the probability of post-sunset ESF occurrence surpasses 0.87, indicating that ESF occurs almost invariably. The likelihood of post-sunset ESF occurrence reaches 1 when v equals or exceeds 40 m/s. Utilizing this model, the study determined that a single ionosonde positioned at the Equator can effectively forecast the incidence of post-sunset ESF up to a longitudinal distance of 30° from its location. The accuracy of ionosondes in predicting post-sunset ESF occurrence above their respective locations is approximately 0.80, with a 10% decrease in accuracy when forecasting ESF occurrence at longitudinal distances of 30°. In conclusion, this study enhances our understanding of the link between the evening vertical plasma drift and the manifestation of post-sunset ESF by leveraging ionosonde data. Furthermore, it provides valuable insights into the recommended coverage range of ionosondes for predicting post-sunset ESF occurrence in the zonal direction, which can be employed to fortify regional space weather services.

The predictability of the nighttime equatorial spread-F (ESF) occurrences is essential to the ionospheric disturbance warning system. In this work, we propose ESF forecasting models using two deep learning techniques: artificial neural network (ANN) and long short-term memory (LSTM). The ANN and LSTM models are trained with the ionogram data from equinoctial months in 2008 to 2018 at Chumphon station (CPN), Thailand near the magnetic equator, where the ESF onset typically occurs, and they are tested with the ionogram data from 2019. These models are trained especially with new local input parameters such as vertical drift velocity of the F-layer height (Vd) and atmospheric gravity waves (AGW) collected at CPN station together with global parameters of solar and geomagnetic activity. We analyze the ESF forecasting models in terms of monthly probability, daily probability and occurrence, and diurnal predictions. The proposed LSTM model can achieve the 85.4% accuracy when the local parameters: Vd and AGW are utilized. The LSTM model outperforms the ANN, particularly in February, March, April, and October. The results show that the AGW parameter plays a significant role in improvements of the LSTM model during post-midnight. When compared to the IRI-2016 model, the proposed LSTM model can provide lower discrepancies from observational data.

The nighttime equatorial spread‐F (ESF) occurrence is essential to cause disturbances of the trans‐ionospheric radio wave signals. Previous research has successfully modeled the probability or growth rate of ESF, but remains deficient in distinguishing their typology and duration, which are also critical for predicting their influences on radio wave propagation. In this paper, an ensemble learning approach is proposed to predict the occurrence and typology of ESF, based on over one solar cycle observations of coherent scattering radar (CSR) located at Jicamarca. The training data set cover from 2000 to 2016, and the observations in 2017 are chosen as testing data set. The model inputs include local time (LT), day of year (DoY), height, solar flux index (P10.7), geomagnetic activity related indices (Kp, SYMH, AE), the true height of the ionospheric F layer (HF), the vertical plasma drift velocity of the ionospheric F layer, and the occurrence rate of ESF from the previous 3 hr. By using the Shapley Additive exPlanations (SHAP) tool analysis, it shows that the LT is the most influential parameter, and the P10.7 contributes more than the geomagnetic indices, to predict the occurrence of ESF. An important advantage of our model is that it can present different types of ESF by reconstructing their duration versus height distributions. Comparison results between prediction and observation suggest that the model predicts all bins with an accuracy reaching 81.2%, and it is able to predict the daily occurrence of ESF with accuracy of about 90.1%.

Electric and magnetic field data from the Swarm constellation mission are used to report on the Poynting flux associated with postsunset topside equatorial spread F. A three-dimensional numerical simulation of plasma density irregularities in the F region ionosphere leading to spread F is used to interpret and support the satellite observations. Here, we focus on quasi-static magnetic and electric fields nearby equatorial plasma depletions (EPDs). The observations show a correlation of the Poynting flux with the plasma number density when background densities are larger than 105cm-3—typical of pre-midnight hours. In other words, the Poynting flux increases as EPDs get more depleted. As time passes, both plasma density and Poynting flux decay. For the latter, however, this temporal dependence is evident in the pre-midnight sector only. Concerning spatial variations, the Poynting flux is observed to enhance inside EPDs as a function of magnetic latitude mainly due to the strengthening of field-aligned currents as they flow away from the dip equator. The Poynting flux follows the dynamo theory, wherein the winds in the F region act as the generator at night and the E region conductivity on shared magnetic field lines as the load. That said, the Poynting flux is generally expected to flow along the field lines away from a dynamo source at the dip equator. Nevertheless, observations show unidirectional flows from one magnetic hemisphere to another, suggesting a generator below the satellites’ altitude. The numerical simulations confirm these observations and show that such latitudinal shifts of the generator are due almost entirely to the winds.

Equatorial spread F (ESF) was discovered almost a century ago using the first radio wave instrument designed to study the upper atmosphere: the ionosonde. The name came from the appearance of reflections from the normally smooth ionosphere, which were spread over the altitude frequency coordinates used by the instrument. Attempts to understand this phenomenon in any depth activated such tools as radars and in situ probes such as rockets and satellites in the 1960s. Over the next 15 years, these tools expanded our experimental understanding enormously, and new nonlinear theoretical methods developed in the late 1970s, which led to proposing a name revision from ESF to convective ionospheric storms. Interest in these phenomena continues, but a new, practical aspect has developed from the associated turbulence effects on communications (transionosphere) and navigation (GPS). The first satellite to specifically investigate this problem and the associated goal of predicting occurrences is under the umbrella of the Communications/Navigation Outage Forecast System (C/NOFS). In contemplating the successful first years of the C/NOFS program, reviewing the state of the art in our knowledge of convective ionospheric storms seems appropriate. We also present some initial results of this satellite program. A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals. Although ambitious, predictive capability is a noble and important goal in the current technological age and is potentially within our reach during the coming decade.

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%.

The occurrence probability of equatorial plasma bubbles and the associated spread F (ESF) irregularities have been derived from ground-based and space-borne measurements. In general, ESF occurrence depends on season and longitude and is high in equinoctial months and low around June solstice. In the West Pacific sector, previous statistical results show that the ESF occurrence probability increases gradually and continuously from March to August. In this study, we use trans-ionospheric VHF data received at Kwajalein Atoll in 2012 to derive the occurrence characteristics of scintillation. It is found that the occurrence probability of strong scintillation had two maxima in June and September and a minimum in July in the evening and midnight sector but only one maximum in June in the post-midnight sector. The monthly variations of scintillation occurrence at Kwajalein are different from almost all previous studies on ESF and scintillation at or near this longitude. To identify the cause for the June peak and the July minimum of scintillation, the ion density and velocity data measured by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite in 2011–2012 are used to derive the ESF occurrence and the post-sunset vertical ion drift near Kwajalein. The ESF occurrence probability and the ion drift measured by the C/NOFS satellite showed two maxima in May/June and August/September and a minimum in July, verifying that the June peak and the July minimum of the VHF scintillation are realistic and caused by the similar variations in the ionospheric ion drift and density.

The Naval Research Laboratory three‐dimensional simulation code SAMI3/ESF is used to study the response of the post‐sunset ionosphere to electrified mesoscale traveling ionospheric disturbances (MSTIDs). An MSTID is modeled as an externally‐imposed traveling‐wave E field with wavelength 250 km and period 1 h that drives vertical E × B drifts of up to ±50 m/s. We find that the coupling between the MSTID at low‐ to mid‐latitudes and the equatorial F layer leads to growth of equatorial plasma bubbles (EPBs). This coupling is strongest when the wave vector is perpendicular to the geomagnetic field. Model results reproduce key features of observed nighttime MSTIDs and associated EPBs. Key Points Electrified MSTIDs “map“ along field lines to affect the conjugate ionosphere A midlatitude MSTID can couple to the equatorial F layer, producing ESF bubbles Coupling is strong if the wave vector is perpendicular to the geomagnetic field