In this study, we analyze the atypical ionospheric irregularity patterns observed during the geomagnetic storm of October 10-13, 2024. This analysis is based on high-resolution Rate of Total Electron Content Index (ROTI) maps derived from data collected by approximately 1,600 Global Navigation Satellite System (GNSS) stations distributed across the American continent and Antarctica, complemented by local ionospheric observations. The morphological characteristics of equatorial and polar ionospheric irregularities during this extreme event are investigated. Our findings reveal a reversed-C-shaped depletion band extending from the magnetic equator to auroral latitudes, driven by the intensification of both vertical drift velocities at the equator and zonal drift velocities near the auroral region. Thus, the polar irregularity region expanded to midlatitudes, converging with stretched Equatorial Plasma Bubble (EPB) structures near ~45{degree sign} MLAT, highlighting a dynamic coupling mechanism between these regions. Additionally, distinct longitudinal asymmetries in EPB behavior were observed, as they extended into midlatitudes and tied up with the polar irregularities along the western coast of South America (~0{degree sign} MLON) but remained confined to the equatorial region along the eastern coast (~20{degree sign} MLON). These asymmetries are attributed to variations in electric field penetration efficiency due to seasonally enhanced ionospheric conductivity. Finally, disturbance dynamo electric fields sustained F-region height, leading to an atypical occurrence of post-sunrise EPBs.

In this study, we present the results of an analysis of the morphological features of Equatorial Plasma Bubbles (EPBs) over South America. In this context, we analyzed data from the Disturbance Ionosphere indeX (DIX) maps calculated using around 450 Global Navigation Satellite System (GNSS) stations. To mitigate the influence of magnetic disturbances on bubble development, only data from geomagnetically quiet days were utilized. This study covered the period from the post-peak of solar cycle 24 (2015) to the pre-peak of solar cycle 25 (2023), totaling 1321 nights with EPB occurrences, representing the largest dataset of EPBs ever compiled for South America. Our analysis unveiled several key findings regarding EPBs and their behavior over the South American region. Firstly, we observed that the amplitude of plasma depletions and the EPB latitudinal development follow an approximately 11-year cycle driven by solar radiation levels. Furthermore, our analysis highlights the significant influence of factors such as vertical plasma drift velocity during the pre-reversal enhancement (PRE), longitudinal variations associated with magnetic declination, as well as the saturation behavior of EPB development with extreme solar flux. Finally, we outline an empirical model to calculate the maximum latitudinal extent of EPBs based on solar flux and magnetic declination as an attempt to provide insights for anticipating EPB behavior across different solar cycle stages and in different longitude sectors.

The ROTI index based on the variation of the TEC is used to detect and characterize the ionospheric irregularities. In the present work, we present a comparative study of five different methodologies to ROTI calculation in order to evaluate the most suitable for the Brazilian region. This was performed over three GNSS stations at different latitudes: São Luís (SALU, 2°31′ S, 44°16′ W; dip: -6.60°) that is located near the dip equator; Cachoeira Paulista (CHPI, 22°40’ S, 44°59’ W; dip: -35.99°) which set close to the southern crest of the EIA at low latitude); and Santa Maria (SMAR, 29° 41′ S, 53° 48′ W, dip: -43.51°) a low-to-mid latitude station close to center of the SAMA region. The period of analysis covered January and December 2015. Our results show that only one out of the five techniques proposed seems to be appropriated for ROTI construction in the Brazilian sector. Our results are supported by comparison of the ROTI with TEC maps obtained over Brazil, ionograms acquired at Fortaleza (FZA0M), SALU, and CHPI ionosonde stations, and All-Sky imagers collected at the São João do Cariri, and CHPI. In addition, we were able to observe the typical irregularities of the Brazilian ionosphere by using the ROTI which we have classified as EPB.

Nighttime airglow images observed at the low-latitude site of São João do Cariri (7.4S, 36.5W) showed the presence of a medium-scale atmospheric gravity wave (AGW) associated with the 21 August 2017 total solar eclipse. The AGW had a horizontal wavelength of ~1,618 km, observed period of ~152 min and propagation direction of ~200 clockwise from the north. The spectral characteristics of this wave are in good agreement with theoretical predictions for waves generated by eclipses. Additionally, the wave was reverse ray-traced and the results show its path crossing the Moon’s shadow of the total solar eclipse in the tropical North Atlantic ocean at stratospheric altitudes. Investigation about potential driving sources for this wave indicate that the total solar eclipse as the most likely candidate. The optical measurements were part of an observational campaign carried out to detect the impact of the August 21 eclipse in the atmosphere at low latitudes.

Key Points: • Gravity waves impacts in the anomalous responses of the F layer over Brazil during a counter electrojet event; • Some signatures in ionograms suggest the gravity wave as responsible for the CEJ occurrence in this day. Abstract In this work, we report the ionospheric F-layer responses over the Brazilian equatorial sector to a counter electrojet (CEJ) 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; dip in 2009:-5.7°) showed a strong modification in the ionospheric F 2 layer trace, that in this case appeared to be “broken in half”. In this process, the first part of the F 2 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 (GWs) 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 formation of strong sporadic E-layers (Es) is frequently observed during the recovery phase of the magnetic storms over Boa Vista (BV, 2.8°N, 60.7°W), a low latitude region over the Brazilian sector. To provide some explanation for this behavior, we investigated in details the ionospheric response to the disturbed electric fields in these atypical Es layers appearance during the magnetic storm of 21-22 January 2016. The analysis was based on F region and Es layers ionospheric parameters obtained from digisonde, as well as on the Total Electron Content (TEC) obtained from Global Navigation SatelliteSystem (GNSS). Furthermore, a theoretical model for the E region named MIRE is used to simulate the Es layers development. Such simulation takes into account the E region winds and electric fields. The results show that the storm time electric field is enough to drive the strong Es layers development. Moreover, it is seen that the intensification of the Es layers is related to the inhibition of the F-region pre-reversal enhancement of the vertical drift due to a westward electric field during the disturbance dynamo effect. Finally, the combined results from the model and observational data seemed to contribute significantly to advance our understanding of the role of the electric fields in the Es layer formation.