Determination of Gradient-Drift Instability Parameters under Conditions of Multiple Plasma Bubbles

The results of numerical calculations of the spatial distribution of increments of gradient-drift instability in the region of developed multiple equatorial ionospheric bubbles are presented. The results of the numerical modeling of the spatial structure of equatorial ionospheric bubbles and the measurement data in the areas of developed plasma bubbles, both satellite and terrestrial, show the presence in these areas of large electron density gradients reaching values ranging from 10⁻⁴ to 10⁻³ m⁻¹ and high drift transfer velocities exceeding 1000 m/s. With such plasma parameters, small-scale inhomogeneities can develop in the region of plasma bubbles, which increase due to various types of instabilities at positive growth rates. The spatial distribution of the gradient-drift instability increment is studied. With this type of instability, small-scale electron density inhomogeneities can be generated, whose spatiotemporal scales are characteristic of equatorial F-scattering. Due to the developed plasma bubbles on the fronts of which gradient-drift instabilities develop, the study is carried out in the approximation of a strong elongation of plasma bubbles along the geomagnetic field lines. This makes it possible to use the results of the numerical simulation of the instability using the two-dimensional approximation of the Rayleigh–Taylor instability model as the background electron concentration. Unlike the previous works of the authors, this study is aimed at obtaining the spatial distribution of the growth rate both for different configurations of plasma bubbles and for different ratios of wave numbers. When studying the features of the distribution of the increment of the gradient-drift instability in the region of the development of plasma bubbles, it is necessary to take into account the significant inhomogeneity of the background values of the electron density and drift velocities. In this study, this is achieved by comparing the wavelengths with the parameters of the zones of development of these waves; in particular, the spatial and temporal extent of the region of large positive values of the increase increment is estimated. These estimates determine the choice of the range of wave numbers. It is found that electron density gradients at the fronts of the developed ionospheric bubbles can be an effective mechanism for the development of a gradient-drift instability; the increment of gradient-drift instability at the fronts of plasma bubbles reaches 0.01 s^–1, and for multiple bubbles these values are slightly less than for a single bubble. Taking into account the time of the existence of regions with such increments of more than 1000 s, such values create the conditions for the development of small-scale inhomogeneities on the “legs” of the plasma bubbles.