Determihation of gradient-drift instability parameters under conditions of multiple plasma bubbles

N. M. Kashchenko ; S. A. Ishanov ; E. V. Zubkov ; V. N. Khudenko

Matematičeskoe modelirovanie, Tome 34 (2022) no. 12, pp. 116-128

Résumé

The results of numerical calculations of the spatial distribution of increments of gradientdrift instability in the region of developed multiple equatorial ionospheric bubbles are presented. The results of numerical modeling of the spatial structure of equatorial ionospheric bubbles, as well as 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 from 10$^{-4}$ m$^{-1}$ to 10$^{-3}$ m$^{-1}$ and high drift transfer velocities exceeding 1000 m/sec. 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 investigated. With this type of instability, small-scale electron density inhomogeneities can be generated, the spatiotemporal scales of which are characteristic of equatorial Fscattering. Due to developed plasma bubbles, on the fronts of which gradient-drift instabilities develop, the study was 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 numerical simulation of the instability using the two-dimensional approximation of the Rayleigh-Taylor instability model as the background electron concentration. Unlike previous works of the authors, here the 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 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 work, 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 has been found that electron density gradients at the fronts of developed ionospheric bubbles can be an effective mechanism for the development of gradient-drift instability; the increment of gradient-drift instability at the fronts of plasma bubbles reaches 0.01 sec$^{-1}$, and for multiple bubbles these values are slightly less than for a single bubble. Such values, taking into account the time of existence of regions with such increments of more than 1000 sec, create conditions for the development of small-scale inhomogeneities on the "legs" of plasma bubbles.

Détail

Keywords: gradient-drift instability, growth rate, F-scatter, Rayleigh-Taylor instability, ionospheric bubbles, geomagnetic field.