Multi-instrument investigation of gravity wave seeding for the onset of Equatorial Plasma Bubbles (EPBs)
Mani SivakandanAmit PatraS. SripathiRamkumar ThokuluwaIgo PaulinoK. NiranjanPriyanka GhoshA. TaoriAnderson Vestena Bilibio
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Abstract Plasma density depletions in the equatorial ionosphere, or so‐called equatorial plasma bubbles (EPBs), are generated in the postsunset period and tend to have a very complex spatial structure. Especially, the east‐west asymmetry of EPBs has been reported by various observations. Using a high‐resolution bubble (HIRB) model, which is a newly developed three‐dimensional numerical model for the equatorial ionosphere, small‐scale structuring at the west wall of large‐scale F layer upwelling is clearly reproduced for the first time. It is not an eastward neutral wind but a vertical shear of zonal plasma drift velocity at the bottomside of the F region that plays an important role in accelerating the instability growth at the west wall and generating the east‐west asymmetry of EPBs.
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Abstract 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.
Middle latitudes
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Companion papers in this series present (1) the role of equatorial E region postsunset ionosphere, (2) the origin of horizontal plasma shear flow in the postsunset equatorial ionosphere (this paper), (3) the Coloured Bubbles experiments results, and (4) computer simulations of artificial initiation of plasma density depletions (bubbles) in the equatorial ionosphere. Within this paper, equations describing the time evolution of the equatorial ionosphere are developed using flux tube integrated and flux tube weighted quantities which model the chemistry, dynamics, and electrodynamics of the equatorial ionosphere. The resulting two-dimensional set of equations can be used to investigate equatorial electric fields neglecting small-scale phenomena (λ < 1 km). An immediate result derived from the integrated current equations is an equation describing the physics of the shear in the horizontal flow of the equatorial plasma during the evening hours. The profile of the horizontal flow has three important contributing terms relating to the neutral wind dynamo, Hall conduction, and the equatorial electrojet current divergence. Using a one-dimensional model of the velocity shear equation and the integrated ionospheric transport equations, a time history of the development of the shear feature during the postsunset hours is presented. The one-dimensional model results are compared to the velocity shear measurements from the Coloured Bubbles experiments.
Electrojet
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В работе определяются и изучаются два параметра процесса развития экваториальных плазменных пузырей (ЭПП): максимальная скорость внутри ЭПП и время развития ЭПП. Исследования проводятся для случаев, когда ЭПП возникают из одной, двух или трех зон повышенной концентрации, или начальных плазменных облаков. Механизмом развития ЭПП является неустойчивость Релея–Тэйлора (НРТ). Ранее было выяснено, что время начальной стадии развития ЭПП должно уложиться в интервал времени, благоприятный для формирования ЭПП (в этом случае линейный инкремент нарастания больше нуля). Этот интервал укладывается для экваториальной ионосферы Земли в промежуток от 3000 с до 7000 с.
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