Interaction of two long-wavelength modes in the nonlinear numerical simulation model of equatorial spread F

Abstract

In order to understand the evolution of equatorial spread F structures and their dynamics an investigation was carried out with the perturbation consisting of a single wavelength mode and superposition of two modes. The investigation revealed that the depleted region always moves upward with a single wavelength mode, while the superposition of two modes gives rise to low-level plasma depletion which moves downward even when the the ambient plasma motion is upward, in addition to well-developed upward moving plasma bubbles and downdrafting enhancements with varying degrees of development. Further, the evolution of well-developed plasma bubbles with scale size corresponding to a shorter-wavelength mode is possible even with very small (0.5%) amplitude of perturbation when it rides over a long-wavelength mode with large (5%) amplitude of perturbation. The longer-wavelength mode develops to form a lower envelope over which multiple plumes with varying degree of depletions ride, with the separation between the plumes decided by the short-wavelength perturbation subject to the amplitude of the former being larger in comparison with the latter. The rising multiple plumes and the descending structure along with a downward moving streak as observed by the Indian Mesosphere-Stratosphere-Troposphere (VHF) radar can qualitatively be understood on the basis of the interaction between the two modes which gives rise to depletions with varying growth rates and a rapidly descending enhancement followed by a downward moving weakly depleted region depending upon the wave parameters of the two modes.