NONLINEAR EVOLUTION OF WAVES AND THEIR ASSOCIATED EFFECTS IN SPACE AND LABORATORY PLASMAS

Abstract

Plasma turbulence is a phenomenon that occurs in a variety of astrophysical, space plasmas and fusion energy. It is generated by several factors including shearing flows, density gradients, currents, and temperature, and is essential for particle heating and energy dissipation. A broad variety of waves (like plasma waves, lower hybrid wave, upper hybrid wave, magnetosonic wave, whistler wave, alfven wave etc) can be supported in plasma by the collective motion of charged particles. These waves are very important for the dynamics and behaviour of plasma systems. These wave modes are responsible for the various astrophysical phenomena like turbulence, magnetic reconnection, cascading heating, and acceleration of plasma particles. The objective of this proposed thesis is to investigate the amplification of beam driven whistler waves from background noise levels, resulting from the energy of the beam. This amplification is expected to reach a significant amplitude, leading to the emergence of nonlinear effects caused by the ponderomotive force. Consequently, these nonlinear effects are anticipated to induce the localization of whistler waves, ultimately leading to the development of a turbulent state. On the account of this nonlinear ponderomotive force, the whistler wave gets localized and density cavities and humps are formed. For understanding the nonlinear stage of the wave growth and the saturation, we consider the nonlinear interaction of a high-frequency whistler wave with a low-frequency wave such as ion acoustic waves (IAWs), magnetosonic waves (MSWs) are considering the ponderomotive nonlinearity due to the whistler wave. Using the two-fluid approach, nonlinear dynamical equations have been derived. Additionally, to solve the model equations, numerical simulation is used, with the pseudo-spectral technique for spatial integration and the finite difference method for temporal integration to explain the localization, turbulence, spectral break, and spectral indices in power spectrum. Also, fluctuations in whistler’s electric field shows that it is turbulent in nature. Further, for a better understanding of the physics behind whistler wave localization, a semi analytical model has been developed. Also, this simplified model has been investigated for the whistler’s convergent and divergent behaviour. Numerical outcomes of the study show the whistler turbulence in the magnetic reconnection sites created by the electron beam and show localized structures and whistler fluctuations, which are to the observations of Zhao et al. [1]. Abstract vii Jyoti, Delhi Technological University, Delhi, India It is thought that Whistler waves, which are generated at electron scales, have a significant impact on the microphysics of magnetic reconnection in the electron diffusion area. So, additionally, the thesis also intends to study the generation of whistler coherent structures formation and later whistler turbulence generation at magnetic reconnection site due to the energetic electron beam (as observed by Magnetospheric Multiscale Mission (MMS)) along with the influence of magnetic island. Nonlinear processes, such as ponderomotive force, density change, and the existence of magnetic islands, might be considered the cause of whistler turbulence. To determine the transverse scale sizes of coherent structures, a semi-analytical model has also been devised. Transverse scale of localized structures in the presence of pre existing magnetic islands are modified by power of whistler wave. We also analyze the contour plot of magnetic field lines, which serves as a spatial representation of the breaking and reconnection of magnetic field lines, leading to the release of trapped magnetic energy. Simulation outcomes shows the nonlinear evolution of multiple X-O points in the presence of magnetic islands shows chaotic structures at later time and may be responsible to the generation of turbulence. We have also shown the evolution of current sheet, their scale size of the order of electron skin depth. The corresponding power spectrum is also evaluated and discussed its relevance with Biskamp observations. Regarding the mechanics and energetics of the reconnection process, the separatix's characteristics are a useful source of knowledge. Along with this, we have also studied the formation of thermal tail of energetic electrons, which may be responsible for the heating and acceleration of plasma particles.