Drift wave turbulence and anomalous plasma diffusion in lower hybrid cavities observed in the ionosphere

Abstract

In plasma of the Earth’s upper ionosphere, regions with a depleted plasma density and an increased level of oscillations with a lower hybrid frequency compared to the environment were found. It was established that such plasma density cavities have cylindrical symmetry and are elongated along the geomagnetic field, so that the longitudinal dimensions significantly exceed the transverse ones. Such structures, called lower hybrid cavities, are quite stable, so that during the passage through them spacecraft do not observe significant changes in the parameters of the cavities. Thus, the nature of the change in cavities over time remains unclear. In this paper, we theoretically investigate the temporal evolution of a cavity in plasma of ionosphere. Since depletion of the plasma density is a cylindrically symmetric region, it creates a radial inhomogeneity in the plasma. In turn, the inhomogeneity of plasma leads to the development of low-frequency drift instability and a turbulent state of plasma. The anomalous plasma diffusion across the geomagnetic field resulting from the development of turbulence of drift waves of inhomogeneous plasma is considered as a mechanism for changing the cavity. In this paper the equation of plasma diffusion in cavity is solved, where the initial radial distribution of plasma density is the upside-down Gaussian. Plasma diffusion occurs radially towards the center, since the plasma density increases with increasing radial coordinate. Obtained solution of the diffusion equation gives the rate of decrease in the depth of cavity. In addition to reducing the depth of the cavity, its expansion also occurs, however, the expansion of the cavity is slower than the decrease in depth. The paper gives plots of the plasma density distribution over the radius for several time values, which show the temporal evolution of the cavity. These dependences show that in a time of the order of 1 second the cavity changes significantly, but does not completely disappear.