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

In order to understand the evolution of equatorial spread F structures and their dynamics an investigation was carried out with the perturbation consi sting of a single wavelength mode and superposition of two modes. The inves tigation revealed that the depleted region always moves upward with a singl e wavelength mode, while the superposition of two modes gives rise to low-l evel plasma depletion which moves downward even when the the ambient plasma motion is upward, in addition to well-developed upward moving plasma bubbl es and downdrafting enhancements with varying degrees of development. Furth er, the evolution of well-developed plasma bubbles with scale size correspo nding 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 t o form a lower envelope over which multiple plumes with varying degree of d epletions ride, with the separation between the plumes decided by the short -wavelength perturbation subject to the amplitude of the former being large r in comparison with the latter. The rising multiple plumes and the descend ing structure along with a downward moving streak ws observed by the Indian Mesosphere-Stratosphere-Troposphere (VHF) radar can qualitatively be under stood on the basis of the interaction between the two modes which gives ris e to depletions with varying growth rates and a rapidly descending enhancem ent followed by a downward moving weakly depleted region depending upon the wave parameters of the two modes.