The September 6-10, 2017 two-step magnetic storm was caused by an X9 solar flare followed by a CME. The SSC that occurred at 23:43 UT on day 06 when Sym-H reached about 50 nT, was due to a sudden increase in solar wind. The first step of the storm was caused by a B z southward incursion on day 07. The magnetic index K p reached 08, and the Sym-H magnetic index reached a minimum value of − 146 nT on day 08 at 01:08 UT, ending the main phase. On day 07, the solar wind intensified once again and the auroral index AE reached 2500 nT. During the recovery phase of this first storm, there was another B z southward incursion on day 08 at 13:56 UT when Sym-H reached − 115 nT, and K p reached a value of 08.33, marking the second step of the storm. In this work, the ionospheric irregularity over São Luís (02.5°S, 44.3°W, dip lat − 04.67°) was studied using data from the VHF, Digisonde and GPS receivers. Electron density data from the satellite SWARM-A were also analyzed for those orbits close to São Luís, and they presented large fluctuations during the storm night of 07/08. To analyze the latitudinal effects of the storm on the plasma irregularities, GPS data from 6 Novatel receivers were used. The vertical plasma drifts during daytime hours were determined using magnetometer data and during the evening using ionogram data. Compared to the 'quiet' days of September 2017, the VHF and GPS S4 amplitude scintillation indices increased substantially during the night of 07/08 when there was a strong intensification in the vertical plasma drift due to a prompt penetration under shielding magnetospheric electric field of eastward polarity. On the other hand, on the night of 08/09 the ionospheric scintillation was completely inhibited due to the disturbance dynamo electric field of westward polarity associated with the first and second storms. The irregularity zonal drifts measured by a VHF receiver around 24 UT (21 LT) were eastward on the nights of 05/06 and 06/07; however, during the night of 07/08, it reversed to westward.

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The terrestrial ionosphere over low‐latitude regions presents the unique phenomena of the equatorial ionization anomaly (characterized by global maximum in plasma concentration) and plasma‐depleted regions known as equatorial plasma bubbles and associated smaller‐scale plasma irregularities. Transionospheric radio signals such as those from Global Navigation Satellite Systems constellations, traveling across this ambient, may suffer severe scintillation in amplitude and phase due to these plasma structures. Presently, three civilian signals available for GPS users, at L1 (1575.42 MHz), L2C (1227.60 MHz), and L5 (1176.45 MHz) are used to investigate the propagation effects due to these irregularities. The purpose of the present work is to evaluate statistically the distribution of severe fade events for each of these carrier frequencies based on the nonlinear ionospheric propagation effects as represented by the fading coefficients of α‐μ distribution. The results from the analyses of data sets recorded by stations at different geomagnetic latitude locations in Brazil show that regions closer to the equatorial ionization anomaly crest present higher probability of severe fade events. Additionally, the L5 signals, dedicated for safety‐of‐life applications, revealed more unfavorable results when compared to the L1 and L2C frequencies. The results further showed that for 0.8 ≤ S4 ≤ 1.0 the probabilities of fades deeper than −10 dB were between 8.0% and 6.5% depending on the station position. Considering the case of fades deeper than −20 dB, the results reach values near 1%, which is quite concerning. These results show empirically the fading environment that users of the new civilian signals may experience in low‐latitude region. Additionally, the fading coefficients may help in the comprehension of the distribution of amplitude scintillation and its relation with the frequency used, aiding in the future the development of signal processing algorithms capable to mitigate errors for navigation users.

Numerical studies concerning equatorial plasma bubble (EPB) seeding source mechanisms have been performed for the last decades. Density perturbations, gravity waves, shear instabilities among others were used in the literature. In all the cases the prereversal vertical drift has a central role, being a necessary condition in addition to all these seeding mechanisms. Notwithstanding, since the prereversal vertical drift presents an asymmetric distribution, this longitudinal variation was hypothesized to act as a seeding source mechanism. This mechanism is capable of providing both contributions: elevate the ionosphere and seed the collisional interchange instability. Two-dimensional investigation was already done, showing the validity of this hypothesis in a bidimensional scenario. Nonetheless, the inclusion of the parallel dynamics could alter the outcome of such analysis, once the component of the conductivity along the geomagnetic field direction causes a load effect reducing the growth rate and shifting its altitude of maximization, thus a 3-D model was developed, named MATPLAB_3D (Mathematical Plasma Bubble Model 3D), and the hypothesis was tested. A progressive approach was performed starting with an oversimplified configuration and concluding with a more realistic profile of prereversal vertical drift obtained using the SAMI2 model. The numerical simulations revealed the existence of a quasi-gaussian seeding source. Also, in case of a prereversal vertical drift varying within 20-60 m/s, the seeded instability evolves into an EPB structure with a longitudinal extension of~2°within~22 min. This EPB maps to low latitude regions and presents secondary structures in its west side. This result further suggests that even though a decrease in the EPB growth due to the inclusion of the parallel conductivity occurred, the hypothesis remains valid.