![]() The PRE (vertical motion) and the zonal drifts present a strong day-to-day variability in the equatorial ionosphere (e.g., Liu, 2020 Aswathy and Manju, 2021, and references therein). In contrast, during the magnetic storms, this behavior can be totally disturbed i.e., the drifts can inclusively reverse to the west (e.g., Abdu et al., 2003 Li et al., 2009 Paulino et al., 2010 Santos et al., 2016). Clear seasonal and solar activity dependencies were also observed (e.g., Pimenta et al., 2001 Paulino et al., 2011 b). Although gravity waves, thermospheric wind, post-sunset vortex, large-scale waves, and magnetic disturbances have been pointed out as possible seedings for EPBs (e.g., Kudeki et al., 2007 Abdu et al., 2009 Abalde et al., 2009 Saito and Maruyama, 2009 Takahashi et al., 2009 Paulino et al., 2011 a Huang et al., 2013 Tsunoda et al., 2018), this topic continues to be under scientific investigation (e.g., Fritts et al., 2009, and references therein).ĮPBs move zonally eastward under quiet magnetic conditions during the nighttime, reaching high drift values during the evening hours (e.g., Pimenta et al., 2003 Paulino et al., 2011 b). ![]() Even so, the RTI theory requires a seeding process in order to initiate the instability. This scenario is very favorable to the Rayleigh–Taylor instability (RTI) development, which has been recognized as the main mechanism for generating EPBs ( Dungey, 1956 Haerendel et al., 1992). The PRE has well-defined temporal dependencies, being more intense during the summer and with high solar activity ( Fejer et al., 1991).īesides the PRE, after the sunset, there is a quick recombination in the ionospheric E region (e.g., Bates, 1988), producing a strong vertical gradient of plasma with high-density levels in the F region. However, after the sunset, the pre-reversal enhancement (PRE) can occur, which is a rapid upward movement of the F region before it reverses, i.e., before the motion becomes downward ( Farley et al., 1986). In general, during the daytime the plasma moves upward, while during the nighttime, the motion is downward. In the equatorial ionosphere, the zonal electric field controls the vertical movement of the F layer. Additionally, as the EPBs contain plasma irregularities, radio techniques have also been used to investigate them (e.g., Woodman and La Hoz, 1976 Abdu et al., 1985, 1998 Fejer et al., 1996 de Paula and Hysell, 2004 Chu et al., 2005). EPBs can be understood as a depletion of the plasma density compared to the background ionosphere (e.g., Sobral et al., 1980 b).Īirglow emissions from the thermosphere can be used to detect and study the morphology and dynamics of EPBs (e.g., Sobral et al., 1980 a Mendillo and Baumgardner, 1982 Fagundes et al., 1995 Takahashi et al., 2001). On the other hand, the very large M 2 amplitudes found during the high solar activity agree with previous observations of the lunar tide in the ionospheric E region.Įquatorial plasma bubbles (EPBs) appear during the nighttime near the magnetic equator and extend across the tropics along the magnetic field lines (e.g., Weber et al., 1978). The seasonality agrees with other observations of the M 2 in the ionospheric parameters such as vertical drifts and electron concentration, for instance. Regarding the seasons, during the Southern Hemisphere summer, the M 2 amplitude was twice as large (12 %) compared to the equinox ones. The amplitude of the M 2 was stronger during the high solar activity, reaching over 10 % of the EPB zonal drift average. The results showed that the M 2 amplitudes in the EPB zonal drifts were solar cycle and seasonally dependent. The M 2 presented an amplitude of 3.1 m s −1 in the EPB zonal drifts, which corresponds to 5.6 % of the average drifts. The present work investigated the contribution of the semidiurnal lunar tide M 2 for the EPB zonal drifts. A strong day-to-day variability was observed in the EPB zonal drifts, which is directly associated with the very complex dynamics of the nighttime thermosphere–ionosphere system near the Equator. Using OI6300 airglow images collected over São João do Cariri (7.4 ∘ S, 36.5 ∘ W) from 2000 to 2007, the equatorial plasma bubble (EPB) zonal drifts were calculated.
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