Stimulated Fore-wake Excitations from Moving Charged Objects in the Ionosphere

Abstract

When a charged object moves in a plasma it can create a wake structure behind it consisting of electrostatic or electromagnetic waves in a manner analogous to wakes created by boats or marine animals moving in water. However, when the boat speed exceeds a critical velocity, like the phase speed of the surface water waves, it can excite nonlinear structures in the form of solitons or shocks that move away from it in the forward direction. Such fore-wake excitations have been predicted and widely studied in hydrodynamics [1], [2]; precursor solitons have also been observed in laboratory experiments of models ships being towed in shallow water channels [3]. In principle, such a phenomenon should also take place in a plasma when the speed of the charged object exceeds the phase speed of a typical collective mode of the plasma such as the ion acoustic speed or the magneto-sonic speed. Fore-wake excitations have received very little attention in plasma physics although the conditions for such excitations exist naturally in many space plasma situations, e.g. the interaction of the supersonic solar wind component with the earth or moon, the fast streaming of space craft or charged debris objects interacting with the plasma in the ionosphere etc. Recently a proof-of-principle laboratory experiment observed the excitation of upstreaming dust acoustic solitons when a dusty plasma was made to flow supersonically over a stationary electrostatic potential hill [4]. The experimental results were well validated by model calculations based on a forced Korteweg-de Vries (fKdV) equation [4] as well as fluid and molecular dynamic simulations [5]. These precursor structures can be usefully exploited for Space Situational Awareness (SSA) purposes as was suggested in [6] and served as a major motivation for the investigations in [4], [5]. The basic idea is that the multiple emissions of solitons (ion acoustic ones in this case) can create a cloud of plasma irregularity that may be easily detectable from the earth and act as a tracking aid for the debris. However these electrostatic structures can be quite short lived due to strong Landau damping in the Low Earth Orbit (LEO) region and electromagnetic nonlinear structures might provide a better alternative by creating longer life time and larger spatial extent irregularity clouds. In this presentation, after a brief review of past work on electrostatic excitations, we will discuss the stimulated excitation of electromagnetic waves. Treating the moving charged object as a current source we have developed an appropriate theoretical framework to study the excitation of electromagnetic waves like magneto-sonic waves, shear Alfven waves etc. A nonlinear formulation, based on the reductive perturbation technique, provides an equation analogous to the fKdV for the study of electromagnetic emissions. We use analytic and numerical solutions of this equation to discuss the features of magneto-sonic solitons and magneto-sonic shocks in the LEO region and delineate the parametric regimes and conditions for their excitation. The results are further extended using a full set of two fluid equations for ionospheric conditions and obtaining numerical solutions under various conditions. We also discuss the feasibility of exciting and detecting such structures in a laboratory setup.