Precursor Magnetosonic Solitons from Moving Charged Objects in the Ionosphere

A charged object moving faster than the characteristic sound speed of a plasma medium can give rise to nonlinear wave excitations ahead of it in addition to wake structures behind it. These novel fore-wake structures that often take the form of propagating solitons, can serve as useful precursors to forewarn the arrival of the object at a specific location ahead of time. This property can be usefully exploited for Space Situational Awareness (SSA) purposes, for example, as was suggested in [1] . The basic idea is that space debris objects orbiting the earth in the ionosphere and which acquire a lot of charge from the ambient plasma can give rise to multiple emissions of such solitons and thereby create a cloud of plasma irregularity that may be easily detectable from the earth and act as a tracking aid for the debris. Motivated by such considerations we have in the past carried out a series of investigations establishing the existence of such excitations using laboratory experiments [2] , fluid simulations [3] and molecular dynamic studies [4] . These investigations have been confined to electrostatic waves e.g ., dust acoustic waves in the experiment and MD simulations and ion acoustic waves for fluid simulations. However electrostatic structures can be quite short lived in the ionosphere of the Low Earth Orbit (LEO) region due to strong Landau damping. Electromagnetic nonlinear structures might provide a better alternative by creating longer life times and larger spatial extent irregularity clouds. In this talk we will present our theoretical findings on the fore-wake excitation of electromagnetic structures in the form of magneto-sonic solitons and compare and contrast their features with our earlier electrostatic results. We have used extensive fluid simulations as well as particle-in-cell simulations to investigate the nature and existence domain of such electromagnetic excitations. A comparison between the electrostatic and electromagnetic precursors reveals some interesting differences. The electrostatic wake fields are quite weak for low Mach numbers in comparison to those for the electromagnetic runs. Also, while electrostatic precursors in the form of ion acoustic solitons occur only when the source speed is supersonic (larger than the ion acoustic speed) the (electromagnetic) magneto-sonic solitons can appear even when the object speed is less than the characteristic linear magneto-sonic speed [5] . The difference can be traced to the basic mechanism for the creation of the solitons. The ion acoustic solitons arise from the pile up of the density in front of the charged source due to a balance between the nonlinear steepening and thermal broadening effects. Once formed they detach from the source and move away as their speed is faster than the source. The electromagnetic solitons on the other hand are not dependent on the density pile up and are created from the large amplitude wake fields that arise in the downstream region. If the source is moving at a sub-magneto-sonic speed then the solitons can easily overtake the source and move ahead of it. The wake field excitations get larger as the source speed approaches or slightly exceeds the magneto-sonic speed and the resultant solitons then acquire a higher speed and again move ahead of the source. The physical implications of these characteristic properties of electromagnetic fore-wakes will be discussed in the context of SSA applications and other potential space plasma scenarios.