Whistler waves are a key mode in the magnetosphere, particularly within the radiation belts, where they regulate energetic electron populations capable of damaging or disabling satellite operations. Their propagation is typically analyzed using either ray-tracing, based on the WKB ansatz, or full-wave solutions employing an anisotropic dielectric tensor—the former being approximate, the latter computationally intensive.

The near-Earth space environment is growing ever more cluttered and has led to an exponential growth of orbital debris. Collisions with these objects traveling at orbital speeds has resulted in serious malfunctions or the disabling of satellite systems. Orbital debris down to a cm are catalogued and tracked to aid in collision avoidance. However, NASA reports that a collision with an object as small as a millimeter traveling at orbital speeds can be mission ending. Consequently, there has been a concerted effort to develop innovative detection techniques that can track the multitude of sub-centimeter sized debris and provide enough warning time for a satellite to react.

Non-gyrotropic distribution functions are often observed in thin current sheets prior to magnetic reconnection. This study uses NASA's Magnetospheric Multiscale mission data to confirm a novel source of agyrotropy in compressed current sheets and highlights its significance in reconnection. Data analysis reveals a strong correlation between agyrotropy at the current sheet center and the perpendicular ambipolar electric field, which develops to maintain quasi-neutrality as the current sheet is compressed to sub-ion gyro-radius scales. This agyrotropy is consistent with theory that includes the effect of a localized transverse electric field on the distribution function. The electric field affects the gyro-plane asymmetrically through the term , where is the spatial gradient of the velocity and is the cyclotron frequency. This asymmetry causes the agyrotropy, which is confirmed by data analysis. For compression such that , the electron distribution function is stretched in the direction of the drift. Conventional methods for quantifying agyrotropy, based on pressure tensor contributions that are uncorrelated to the electric field, are inadequate in the current sheet center where reconnection is likely to be initiated. The perpendicular ambipolar electric field provides a measure of agyrotropy in distribution functions, which may be the most relevant indicator of reconnection.

This abstract presents a supervised deep learning framework, centered on a Convolutional Neural Network (CNN), trained to identify and classify simulated orbital debris using electron saturation Langmuir probe data obtained from the Space Physics Simulation Chamber (SPSC) [1]. The CNN is capable of analyzing both spatial (from moving the probe) and temporal information (from leaving the probe at one place), the model achieved a detection accuracy of over 99% able to determine the presence of debris generated signatures or the absence (determined by a low probe voltage) on a training set of 1550784 samples (80%) and a test set of 387696 (20%) events.

This study reexamines the excitation of ion-acoustic precursor solitons by a supersonically moving charged debris object, incorporating two previously overlooked physical factors: the dynamic charging of the debris and the impermeable nature of its surface. The influence of charging dynamics is explored using an enhanced one-dimensional fluid-Poisson model, where the source charge is treated as a dynamical variable and solved self-consistently alongside the core plasma equations. By comparing these results with prior fixed-charge models, we evaluate the effects on soliton onset and propagation, finding that charging dynamics does not hinder soliton generation or evolution. To assess the impact of the impermeability of debris surface, a two-dimensional fluid model simulates the interaction between an electrostatically biased, impenetrable object and a flowing plasma. Modeling the object as an infinite wall disconnects the upstream and downstream plasma regions, forming a sheath without solitons-consistent with earlier fluid and particle-in-cell simulations. However, replacing the wall with a finite object enables plasma flow around it, restoring upstream-downstream connectivity and naturally generating precursor solitons.

We investigate ion-acoustic soliton propagation in plasmas with thermal ions using Particle-In-Cell simulations with temperature ratios $T_{i} / T_{e}=0.01-1.0$. Despite linear kinetic theory predicting complete decay through ion Landau damping, solitons reach steady states at $\sim 1 \%$ above background density—above the $0.1 \%$ detection threshold. Phase space analysis reveals nonlinear electron trapping drives this behavior: trapped electrons transfer the majority of their energy to ions through an adiabatic expansion-like process, saturating the damping. This electron-mediated mechanism enables soliton persistence, supporting the feasibility of soliton-based debris detection.

We present detailed observations of bursting wave behavior at f≃fpe/2 driven by an electron beam in a laboratory plasma, including high-time-resolution measurements of the wave bursts' interaction with the electron beam. A burst of wave activity is observed when a threshold electron beam density relative to the background plasma density is exceeded. Wave bursts varying in their time duration are observed, but the fundamental structure of the bursts appears to be structures with a symmetric time envelope. Wave bursts with amplitudes large enough to substantially heat the electron beam, disrupt the beam, and eventually trap beam electrons at the phase speed of the waves are observed. These behaviors observed in the laboratory are able to be reproduced via numerical simulations. The laboratory results are applicable to a variety of conditions in space plasmas.

We investigate ion acoustic solitary waves (solitons) of varying amplitudes in a one-dimensional plasma using fully kinetic particle-in-cell simulations. The initial soliton conditions are based on the Korteweg–de Vries (KdV) equation, treating ions as a cold species and electrons with finite temperature. Our findings reveal that KdV solitons evolve nonlinearly to a saturated state at higher amplitude, deviating from KdV predictions for ion density and electric potential, and from the Boltzmann relation for electron density. At this saturated state, the KdV model cannot accurately describe the soliton behavior. For small amplitudes, Sagdeev's model describes the saturated state, but not the soliton width; for larger amplitudes, it models the width accurately, but not the amplitude. These discrepancies arise from assuming a Boltzmann relation for electron density, while electron trapping creates non-Boltzmann densities—a deviation that increases with soliton amplitude. Additionally, we observe that the soliton amplitude oscillates roughly at the electron bounce frequency. The soliton is better described by Schamel's electron density formulation and a modified KdV equation incorporating electron trapping. The soliton velocity matches best with predictions from Sagdeev's and Schamel's models. Moreover, the soliton speed–amplitude relationship differs from existing theoretical predictions. Finally, we find minimal ion and electron Landau damping effects.

Topological phase refers to the notion that in a uniform system the eigenfunction in wave vector space can be characterized by an integer-valued topological invariant that describes a global property of an eigenfunction. A topological mode refers to an edge state, or topological wave, that arises at the interface between two systems with different values of a topological invariant. These ideas have had a profound impact on condensed matter theory with many important applications e.g. topological insulators, topological superconductors, and topological quantum computers. All of the condensed matter systems rely on the quantum properties, but recently these ideas have found their way into classical systems. One of the first applications was to the shallow water equations and the analysis was used to show why equatorial waves predominantly head east. Recently, these ideas have made their way into plasma physics where the quantum/classical analogy has had a long history of development. Plasmas offer a unique playground for these ideas because of their non-equilibrium natures and non-Hermitian linear operators. In this presentation, we consider a simplified model of the Inhomogeneous Energy Density Driven Instability (IEDDI) and the Electron-Ion Hybrid (EIH) instability based on the electrostatic limit of cold fluid equations. These instabilities are driven by an inhomogeneous flow perpendicular to the magnetic field and may be characterized as occurring in the boundary layer between two flowing plasmas. They have extensive applications in both space and laboratory plasmas. For example, these instabilities are known to occur in compressed plasmas such as dipolarization fronts, magnetotail current sheets, plasma sheet boundary layers, etc. and are important for anomalous dissipation, particle energization and transport. These instabilities are also both inherently non-local requiring the solution to a 1D eigenmode equation to accurately understand the driving conditions and mode properties. These instabilities are notable from a theoretical perspective because the source of free energy is contained in the inhomogeneous flow, meaning that the instability exists because there is no single frame that one is able to transform into that can globally remove the flow. This physical interpretation can also be understood through the fact that in a homogenous flowing plasma the linearized dynamics of the cold electrostatic fluid model can be described by a linear operator that can be put into diagonal form. This operator exhibits parity-time (PT) symmetry which has implications on the eigenvalues of the operator. We analyze the topological modes of this simplified linear operator in the limits of the IEDDI instability and the EIH instability and use the symmetries of the linear operator to describe the unique properties of this instability. This analysis shows the power of theoretical technique through its ability to make general statements about the instability without having to obtain the eigenmode, a task which requires assumptions about the initial conditions, and often, extensive and difficult numerical analysis.

The generation mechanism for ion acoustic solitons due to speeding orbital debris in warm isothermal ionospheric plasma with the background magnetic field oriented at an arbitrary angle to the debris trajectory is analyzed. It is found that the fluctuations in the floating potential, which the debris acquires due to charging, can be amplified into growing ion acoustic waves by plasma streaming onto the debris. Normally, the ion acoustic fluctuations are ion Landau damped in the ionosphere because their phase speed matches the acoustic speed for equal ion and electron temperatures. However, in the debris frame, the plasma streams with an inhomogeneous velocity profile. The velocity shear in the streaming ions can overcome Landau damping by effectively increasing the wave phase speed by a factor proportional to the product of the shear and the wave normal angle, causing the Landau resonance to match the velocities of the tail of the distribution rather than the core. Consequently, the fluctuations can grow to sufficiently large amplitudes even in an isothermal plasma and trigger nonlinear effects resulting in ion acoustic solitons. For debris motion at an angle to the magnetic field, unique signatures are generated by the combination of coherent and incoherent processes—both along and across the magnetic field directions. These may be exploited for distinguishing between debris-generated soliton signatures and those arising due to natural causes and thereby facilitate positive identification of the orbital debris.

The topic of precursor nonlinear structures (solitons) excited by a charged object traveling through a plasma has attracted much recent attention due to its potential application in the detection and tracking of small sized space debris objects. The basic principle of such excitations in a plasma medium was expounded in a simple one dimensional fluid model calculation [1] leading to a paradigmatic nonlinear partial differential equation, the forced Korteweg de Vries (fKdV) equation, that yielded a variety of driven nonlinear solutions in the form of precursor solitons, “pinned” solitons, wakes and dispersive shocks. The scale sizes and amplitudes of these nonlinear structures for ionospheric conditions are within the detection capabilities of ground based radars or in-situ sensors. Hence they can provide an indirect means of detecting debris objects. Subsequent studies have established the existence of such solitons in a variety of theoretical models [2, 3] as well as in laboratory experiments in a dusty plasma medium [4], While the basic qualitative features of this novel phenomenon are similar to those appearing in analogous hydrodynamic excitations, there are important physical differences arising from plasma effects that are not well explored yet. In particular, the complex dynamics of the plasma sheath region surrounding the moving charged object that is responsible for the creation of the soliton needs to be better understood. In the present study we address this issue with the help of detailed fluid simulations of the sheath and presheath regions in front of the moving charged object. We investigate the evolution of driven nonlinear ion acoustic waves through numerical investigations of the full set of two fluid equations that go beyond the usual weak nonlinearity approximation of the fKdV or forced Kadomtsev-Petviashvilli (fKP) equations. The simulations are done for both planar and cylindrical objects in plasma conditions specific to the LEO and GEO regions of the ionosphere that are heavily populated by orbital debris. Realistic charging models are used to establish the surface potential and charge on the moving object and stochastic fluctuations of the charge are taken into account. The threshold condition for the excitation of the solitons is explored as a function of the ratio of the object speed to the ion thermal speed, the surface charge of the object, the shape of the object and the sheath parameters in order to identify the physical origins of the “pinned” and precursor solitons. It is found that the presheath region plays a dominant role in the excitation of the solitons and its dynamics needs to be investigated in greater depth. This can be done using a kinetic approach or particle-in-cell simulations both of which can benefit from the basic findings of the present extended fluid simulations.

The mechanisms that control magnetospheric whistler wave generation have been investigated by ground based VLF ($3-30 \mathrm{kHz}$) wave-injection experiments such as at Siple Station and HAARP. Coherent signals were shown to routinely trigger the generation of new coherent waves known as “triggered emissions”. VLF triggered emissions are long-lived, many times the length of the triggering signal, and exhibit spectral characteristics that closely resemble natural magnetospheric chorus waves. These controlled injection experiments have produced a wealth of data, but at the same time, comparison of this dataset to theory is difficult since the ground observations are at the end of the interaction region, giving access only to cumulative effects of the wave-particle interaction.

Satellite data analysis of a compressed gyro-scale current sheet prior to magnetic reconnection in the magnetotail shows that electrostatic lower hybrid waves localized to the region of a transverse ambipolar electric field at the centre of the current sheet are driven by $\boldsymbol{E} \times \boldsymbol{B}$ velocity shear and result from compression. The presence and location of shear-driven waves around the centre of the current sheet, where the magnetic field reverses and the density gradient is minimal, is consistent with our model. This is notable because the free energy source is the curvature of the electron $\boldsymbol{E} \times \boldsymbol{B}$ flow and not the density gradient. Laboratory experiments and particle-in-cell (PIC) simulations have shown that shear-driven lower hybrid fluctuations are capable of producing anomalous cross-field transport (viscosity) and resistivity, which can trigger magnetic reconnection. We estimate the terms in the generalized Ohm's Law directly from MMS data as the spacecraft cross a gyro-scale current sheet. Our analysis shows that the wave effects (resistivity, diffusion and viscosity) and pressure anisotropy effects are comparable. We also find that the quasi-static electric field gradient is correlated with a non-gyrotropic electron distribution function, which is consistent with our model. Furthermore, theoretical arguments suggest agyrotropy is an indicator of the possibility for magnetic reconnection to occur.

Current sheets are important to space and laboratory plasmas, and particularly to the Earth’s magnetotail, where during active periods the solar wind compresses the magnetosphere creating thin current sheets. As a current sheet with a reversed magnetic field configuration is compressed, magnetic reconnection can occur. This process can affect the plasma state of the magnetotail, driving space weather and injecting energetic particles into the radiation belts, which impacts Earth orbiting satellites.

Orbital debris has become a problem of national importance as our dependence on space-based resources has increased and constellations of satellites providing importance services are proliferating. Debris that are too small to be tracked individually, but large enough to damage, disable, or disrupt a satellite are particularly dangerous and thus there are now underway several large-scale efforts to improve our ability to track and detect debris with sizes below 10 cm. Recently, a new approach was suggested [1], in which it was recognized that orbital debris is immersed in a plasma and thus obtains an electrical charge. The linear response of a moving charged particle in a plasma is well known, but it was recently theorized [1], experimentally demonstrated [2], and numerically simulated [3] that a nonlinear response could produce a large amplitude soliton in the background plasma and that such a soliton could perhaps be detected more easily than the orbital debris. In follow up works this theory was extended to include electromagnetic solitons on the ion skin depth scale [4]. In this talk we review the theory, extend the theory to account for the nonuniform flow of the plasma around the debris, and extend the theory to include electromagnetic fluctuations on the electron skin depths scale. We also address the problem of how to determine from the observations if you have indeed observed a soliton. We develop a Bayesian Spectral analysis technique that provides a framework for identifying solitons in the data and a framework for determining the probability that a given event is a soliton and not some other wave-packet. We discuss how these results can enhance our ability to detect and track orbital debris.

We investigate kinetic-scale current sheets with a magnetic field reversal that are formed by compressing a larger scale fluid current sheet down to kinetic scales. The compression forces the current sheet to generate an intense localized ambipolar electric field in the Hall direction. As a result, the ions do not fully respond to the electric field, while the electrons develop an intense sheared ExB flow, which carries a current and alters the structure of the current sheet, giving it multiple spatial scales and enabling bifurcated or embedded current sheets to form depending on the level of compression. The localized electric field also modifies the particle orbits and the equilibrium distribution functions leading to non-gyrotropic distributions by renormalizing the gyromotion. We model this physics via an exact Vlasov equilibrium [1] which captures the essential physics of compression and through asymptotic perturbation theory. This equilibrium is shown, through a non-local linear analysis, to be unstable to lower-hybrid waves due to the electron-ion hybrid (EIH) instability [2]. The analysis shows that the electrostatic EIH develops eigenmodes which are confined to the central portion of the current sheet near the magnetic field reversal point and can lead to substantial anomalous dissipation, both resistivity and viscosity, which can impact reconnection. We model this physics through rigorous eigenmode analysis and through first principles particle-in-cell simulations. With the resulting models and simulations, we are able to make direct comparisons with observations from NASA’s MMS mission [3, 4].

The technique of collective Thomson scatter (also known as incoherent scatter) has been a mainstay of the ionospheric remote sensing community for several decades, as it uniquely provides full altitude profiles of the ionospheric state, including plasma temperatures and composition. The forward model necessary to analyze observed spectra from large aperture, high power radar systems was developed beginning in the late 1950s, with the ability to sample altitude dependent changes stemming from the very weak nature of the scatter which well satisfies the Born approximation. A number of parallel approaches have been developed for the forward model, including a Nyquist fluctuation-dissipation theorem based framework (e.g. [1]), statistical dressed particle approaches [2], and equivalent parallel circuit models [3] in which the source currents are generated by thermal motions of the electrons and ions in the plasma and complex impedances are presented by each species which must obey electrostatic forcing from a common electric field.

In solar flares, simultaneous acceleration of both electrons and ions is observed despite the large mass difference between these species. In a flaring loop, ion velocity distributions can be formed with a ring-like structure either through shock formation, or via loss cone depletion [1]. These distributions are unstable to Lower-hybrid (LH) waves [2]. LH waves can simultaneously accelerate perpendicular-propagating ions and parallel-propagating electrons [1] because on LH timescales the ions are unmagnetized and the electrons are magnetized allowing for simultaneous resonance. However, these early studies did not look into the production of whistler waves in the presence of LH wave turbulence. As we show in this presentation, whistler waves are almost inevitable in the excitation of LH waves in low beta turbulent plasmas [3]. The generation mechanism of whistler waves in the solar wind (SW) is also an active area of research. The Parker Solar Probe (PSP) mission has enabled measurements of the SW at distances very close to the Sun. For example, [4] has confirm the excitation of whistler waves in the quiet SW at ~ 36 solar radii. And more recently, [5] shows the existence of broadband LH turbulence in the near solar wind (0.074 AU). However, the mechanisms of excitation of these waves are still debated and the causal linkage between LH waves and whistlers in the solar wind has not been studied. Here, we present results of 2-dimensional (2D) PIC simulations of nonlinear whistler wave generation by LH turbulence scattering by plasma particles. This nonlinear mechanism is sometimes known as either: induced scattering or nonlinear Landau damping. In our study, we generate the LH waves through an energetic ion ring velocity distribution, as in a solar flare, and study the impact of nonlinear scattering on the evolution of the instability. We show that the instability is saturated by the scattering of LH to whistler (W) waves. We present detailed analysis for the evolution of the LH and W waves. We also study the electron and ion velocity distribution functions (VDFs) and show their evolution in time. The simulation data analysis shows agreement with the whistlers being generated by the nonlinear induced scattering mechanism. Specifically, we identify the quasimodes (which are low frequency density perturbations driven by the ponderomotive force due to the beating of LH and W waves) and the individual constituent waves involved in the nonlinear induced scattering. * This work was supported by the NRL Base Program.