Rodolfo de Jesus

This study examines the equatorial ionospheric response over the South American sector during the onset of the extreme 2024 Mother's Day geomagnetic storm, which began at ∼17 UT on 10 May and reached the G5 level. Pronounced disturbances in ionospheric electrodynamics were observed under both daytime and nighttime conditions. Variations in the F2‐layer critical frequency (foF2) and peak height (hmF2) were analyzed using ionosonde observations from equatorial stations at Belém and São Luís (SL) (Brazil) and Jicamarca (Peru), together with simulations performed with the SUPIM‐INPE model. At the onset of the storm's main phase, a simultaneous and abrupt uplift of the F2 layer was observed at all three stations. Model results indicate that this enhancement was primarily driven by eastward undershielding electric fields, with meridional winds contributing to hmF2 variations over the Brazilian sector. Around 19:00 UT, a complex descent of the ionosphere was detected, which is attributed to the combined effects of westward overshielding, disturbance dynamo electric fields, meridional winds, F3‐layer disruption, and large‐scale traveling ionospheric disturbances (LSTIDs). Although the model reproduced the formation of the F3 layer over Belém and SL, its duration was overestimated compared to observations. On the other hand, its results showed a consistent trend. Near 20:00 UT, a pronounced uplift of the F layer, followed by an anomalous decrease in foF2, was observed over Brazil. In contrast, different features were observed at Jicamarca, suggesting distinct combined roles of LSTIDs and the eastward electric field in the Brazilian sector, as qualitatively confirmed by the modeling.

This study presents the first systematic catalog and long‐term statistical analysis of post‐sunset Equatorial Plasma Bubble (EPB) suppression events over northeastern Brazil, based on a 12‐year observational database spanning 2013–2024. The EPB activity was identified using GNSS‐derived Rate of Total Electron Content Index measurements over the São Luís longitudinal sector (40°–50°W). The results demonstrate that EPB suppression occurs under a broad spectrum of geomagnetic conditions, including intense, moderate, and weak geomagnetic storms, as well as during geomagnetically quiet periods (SYM‐H > −30 nT and Kp 3). A clear semiannual variation is observed, with maxima around the equinoxes, coinciding with periods of enhanced geomagnetic storm occurrence. Most suppression events occurred during the descending and ascending phases of the solar cycle. In some cases, suppression effects were observed more than 300 hr after storm onset, suggesting the influence of disturbance dynamo electric fields, modifications in the integrated E‐region Pedersen conductivity, and/or overshielding Prompt Penetration Electric Fields during the evening sector.

Ground-based resonance LiDAR has been used to study the morphology of the mesospheric potassium (K) and sodium (Na) layers from 2016 to 2024 at São José dos Campos (SJC; 23.1°S, 45.9°W). The seasonal variation of Na densities around 85–100 km altitude shows a pronounced minimum in summer and a flat maximum in equinox and winter. Otherwise, the K layer shows semiannual and annual behavior with a single maximum in winter. Spectral analysis of K and Na densities revealed oscillations with periods ranging from ∼5 to 380 days in the MLT. The annual (AO) and semiannual (SAO) oscillation amplitudes for the K and Na densities were larger than those for the 90-day period. The AO and SAO amplitudes increase with altitude up to about 90 km, then gradually decrease with altitude up to 99 km. The vertical phase profiles of AO in the K and Na densities show a descending phase, probably implying an upward energy propagation. On the other hand, the phase profiles of the SAO in the K density exhibit a likely ascending mode and downward energy propagation.

The responses of the ionospheric F‐region, as measured by Global Navigation Satellite System and ionosonde, in the equatorial and low‐latitude regions over Brazil during two geomagnetic storms, one very weak (Dst _min = −33 nT) and another intense (Dst _min = −138 nT), occurred on 15–17 April 2025, are investigated. Furthermore, the nighttime OI 135.6 nm emissions measured by Global‐scale Observation of the Limb and Disk (GOLD) are used to visualize equatorial plasma bubbles (EPBs) to investigate their pattern change. Several prompt penetration electric fields (PPEFs) occurred during both very weak and intense storms. The westward PPEF during the intense storm suppressed the pre‐reversal enhancement (PRE), leading to the suppression of EPBs. The storms induced equatorward thermospheric winds and eastward PPEF, which caused unusual uplifts of the ionosphere. These effects resulted in strong positive ionospheric storms during both events. The very weak geomagnetic storm caused an atypical intensification of the fountain effect that enhanced the equatorial ionization anomaly. Negative ionospheric storms occurred over the Brazilian low‐latitude stations, probably related to changes in the thermospheric N ₂ /O ratio. Therefore, these findings also highlight the significant role of thermospheric composition changes in driving negative ionospheric storms at low latitudes.

The behavior of the Brazilian equatorial ionosphere during the solar minimum periods, 1996 and 2009, which cover the solar cycles 22/23 and 23/24, respectively, is investigated. For this, the F₂ layer critical frequency (foF₂) and peak height (hmF₂) registered by a Digisonde operated at São Luis (2.33° S; 44° W) are carefully analyzed. The results show that the seasonal mean values of the foF₂ and the hmF₂ in the equinoxes and winter during 2009 were lower than in 1996. In the summer, an anomalous response to solar variability was observed. In this case, the hmF₂ in 2009 is higher than in 1996 during a specific daytime interval. Besides that, it was verified that the prereversal enhancement of the zonal electric field (PRE) during the equinoxes in 2009 occurred a few minutes earlier than in 1996. Additionally, a Fast Fourier Transform (FFT) analysis was used to investigate the impacts of solar atmospheric tides (amplitude, diurnal, semidiurnal, and terdiurnal modes) on foF₂ and hmF₂ parameters with respect to its seasonality. Significant differences were observed between their values during the two minima, mainly in the amplitude of hmF₂, which was higher in 1996 than in 2009 for all days analyzed. Moreover, the seasonality in the diurnal and semidiurnal modes for both periods presented an annual variability, while the terdiurnal mode exhibited annual and semiannual components. The results are compared with the International Reference Ionosphere (IRI) model, and the main differences between the observation and the model results are discussed in this work.

In this work, we report the ionospheric F‐layer responses over the Brazilian equatorial sector to a counter electrojet event that occurred during the solar minimum period of June 2009. The data collected by the Digisonde over São Luis (2.33°S; 44°W; I: −4751°; Dip. Lat. 238°S) showed a strong modification in the ionospheric F₂ layer trace, which in this case appeared to be “broken in half.” In this process, the first part of the F₂ layer (lower frequency) was thrown down whilst the upper part remained at higher altitudes. Such characteristics occurred simultaneously with an abrupt decrease in the strength of equatorial electrojet and with intensification in the auroral activity. The origin of this phenomenon seems to have a local nature and seems not to be connected to any magnetic disturbance since similar responses were not observed in other longitudinal and latitudinal sectors. Excluding this possibility, we assume that the strong changes observed in the F layer over São Luis had been caused probably by the gravity wave (GW) propagation, as seen in the downward phase propagation of the altitude contours with time over São Luis and Fortaleza and the remarkable signatures in ionograms over both regions, such as the forking traces that are typically caused by GWs.

The main objective of this study is to investigate the ionospheric irregularities observed by Global Positioning System‐total electron content (GPS‐TEC) receivers during the high solar activity years of 2013 and 2014 at different stations in the equatorial and low‐latitude regions in the South American sector. The ionospheric parameters used in this investigation are the TEC, the rate of change of the TEC index (ROTI), and the amplitude scintillation index (S4). In the South American sector, the ROTI and S4 indices showed that the ionospheric irregularities have an annual variation with maximum occurrence from September to April, between 20:00 LT and 02:00 LT, and no occurrence from May to August. Also, strong phase fluctuations (ROTI >1) are observed over South America at 19 LT in October and November. Morlet wavelet analysis of ROTI and S4 showed that planetary wave‐scale periods ranging from 2 to 8 days are predominant during September–March at 20–02 LT in South America. In addition, using a keogram it was possible to evaluate the distance between adjacent ionospheric plasma depletions, and this result is presented and discussed. The longitudinal distances between adjacent bubbles vary around ~600–1000 km, which is larger than values reported in most previous studies.

Traveling ionospheric disturbances (TIDs) are wave-like disturbances in ionospheric plasma density. They are often observed during both quiet (medium-scale TID) and geomagnetically disturbed (large-scale TID) conditions. Their amplitudes can reach double-digit percentages of the background plasma density, and their existence presents a challenge for accurate ionosphere specification. In this study, we examine TID properties using observations obtained during two geomagnetically disturbed periods using multiple ground and space-borne instruments, such as magnetometers, Global Navigation Satellite System (GNSS) receivers, and the SWARM satellite. Reference quiet time observations are also provided for both storms. We use a thermosphere–ionosphere–electrodynamics general circulation model (TIEGCM) results to properly interpret TID features and their drivers. This combination of observations and modeling allows the investigation of variations of TID generation mechanisms and subsequent wave propagation, particularly as a function of different plasma background densities during various geophysical conditions. The trans-equatorial coupling of TIDs in the northern and southern hemispheres is also investigated with respect to attenuation and propagation characteristics. We show that TID properties during trans-equatorial events may be substantially affected by storm time background neutral wind perturbation.

The present study investigates the ionospheric F region response in the Brazilian sector due to sudden stratospheric warming (SSW) events of 2014. The data used for this work are obtained from GPS receivers and magnetometer measurements during day of year (DOY) 01 to 120, 2014 at different stations in the equatorial and low‐latitude regions in the Brazilian sector. In addition, the data obtained from Communication/Navigation Outage Forecasting System satellites during DOY 01 to 75 of 2014 are used. The main novelty of this research is that, during the 2014 SSW events, daytime vertical total electron content (VTEC) shows a strong increase on the order of about 23% and 11% over the equatorial and low‐latitude regions, respectively. We also observed that the nighttime VTEC (SSW days) is increased by 8% and 33% over equatorial and low‐latitude regions, respectively. The magnetometer measurements show a strong counterelectrojet during the SSW days. The results show an amplification of the 0.5 day and ~2–16 day periods in the VTEC and equatorial electrojet during the SSWs. The occurrences of ionospheric irregularities during the SSW events are around 84% and 53% in the equatorial and low‐latitude regions, respectively, which is less frequent when compared with those during the pre‐SSW periods.