Modulational instability of electron-acoustic waves in the auroral region

Abstract

Interaction of solar wind with the Earth’s magnetic field leads to the formation of the magnetosphere which has several boundary layers like magnetopause, plasmas sheet boundary layer and auroral zone etc. These boundary layers play important role in the distribution of energy extracted from solar wind to various regions of the magnetosphere. The free energy available in the boundary layers in the form of gradients and currents can drive several plasma waves at different scale lengths. Thus, the boundary layer offers an opportunity to study the physics of the micro and macro scale plasma processes. Broadband plasma waves, having wide range of frequencies from lower hybrid to electron plasma frequency and above, have been observed by many satellites on the auroral and cusp field lines. The amplitude of the electric field of these waves can be from a few tens to hundreds of mV/m. These large amplitude waves can decay into kinetic Alfvén waves (KAWs) through parametric processes. Three wave interaction process involving a high frequency pump wave (electron-acoustic wave (EAW)), a kinetic Alfvén wave and another electron acoustic wave as the daughter wave is studied in the auroral region by using multi-fluid approach. Auroral plasma is considered to consist of cold electrons, hot electrons and ions. A nonlinear dispersion relation for the three-wave interaction process is obtained. It is found that the observed amplitudes of the EAWs exceed the threshold required to excite the KAWs. This mechanism may be a plausible candidate for the generation of KAWs in the auroral region. The kinetic Alfvén waves in turn can lead to the local acceleration/heating of the auroral plasma.