Dan Winske

Various systems can be modeled as a point-like explosion of ionized debris into a magnetized, collisionless background plasma—including astrophysical examples, active experiments in space, and laser-driven laboratory experiments. Debris streaming from the explosion parallel to the magnetic field may drive multiple resonant and non-resonant ion–ion beam instabilities, some of which can efficiently couple the debris energy to the background and may even support the formation of shocks. We present a large-scale hybrid (kinetic ions + fluid electrons) particle-in-cell simulation, extending hundreds of ion inertial lengths from a 3D explosion, that resolves these instabilities. We show that the character of these instabilities differs notably from the 1D equivalent by the presence of unique transverse structure. Additional 2D simulations explore how the debris beam length, width, density, and speed affect debris–background coupling, with implications for the generation of quasi-parallel shocks.

The coupling between a rapidly expanding cloud of ionized debris and an ambient magnetized plasma is revisited with a hybrid (kinetic ion/fluid electron) simulation code that allows a study over a wide range of plasma parameters. Over a specified range of hypothetical conditions, simple scaling laws in terms of the total debris mass and explosion speed are derived and verified for the maximal size of the debris cloud and the fraction of debris that free‐streams from the burst along the magnetic field. The amount of debris that escapes from the burst with minimal coupling to the background magnetic field increases with the debris gyroradius. Test cases with two different debris species—including a heavy minority species with a relatively large gyroradius—highlight how the collisionless coupling of the debris depends on the single particle trajectories as well as the overall conservation of energy and momentum.

Chorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two‐band structure with a quasi‐parallel lower band and an upper band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low‐energy beam‐like electron component or by temperature anisotropy of keV electrons in the presence of a low‐energy plateau‐like electron component. This paper, however, presents an alternative mechanism for generation of this highly oblique lower band chorus. Through a nonlinear three‐wave resonance, a quasi‐parallel lower band chorus wave can interact with a mildly oblique upper band chorus wave, producing a highly oblique quasi‐electrostatic lower band chorus wave. This theoretical analysis is confirmed by 2‐D electromagnetic particle‐in‐cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low‐energy electrons to form a plateau‐like structure in the parallel velocity distribution. The three‐wave resonance mechanism may also explain the generation of quasi‐parallel upper band chorus which has also been observed in the magnetosphere.

The Alfvén‐cyclotron instability is driven by an ion temperature anisotropy such that T_⊥/T_|| >1 where ⊥ and || denote directions perpendicular and parallel to a uniform background magnetic field B_o, respectively. The computations presented here consider a model of a magnetized, homogeneous, collisionless plasma. Two representations of the proton velocity distribution are considered: a single bi‐Maxwellian and a magnetospheric‐like configuration of two components, a more dense, relatively cool, isotropic component and a less dense, relatively hot, bi‐Maxwellian component which drives the instability. Only wave propagation parallel to B_o is considered. Using numerical solutions of the full kinetic linear dispersion equation, concise analytic expressions for the scaling of the dimensionless maximum instability growth rate and the corresponding dimensionless real frequency are derived as functions of three dimensionless variables: the hot proton temperature anisotropy, the relative hot proton density, and the hot proton β_||. Furthermore, using one‐dimensional hybrid particle‐in‐cell simulations of this same instability, a third relation for the scaling of the maximum amplitude of the dimensionless fluctuating magnetic field energy density is derived.

Electromagnetic ion cyclotron (EMIC) waves in the Earth's inner magnetosphere are enhanced fluctuations driven unstable by ring current ion temperature anisotropy. EMIC waves can resonate with relativistic electrons and play an important role in precipitation of MeV radiation belt electrons. In this paper, we investigate the excitation and saturation of EMIC instability in a homogeneous plasma using both linear theory and nonlinear hybrid simulations. We have explored a four‐dimensional parameter space, carried out a large number of simulations, and derived a scaling formula that relates the saturation EMIC wave amplitude to initial plasma conditions. Such scaling can be used in conjunction with ring current models like ring current‐atmosphere interactions model with self‐consistent magnetic field to provide global dynamic EMIC wave maps that will be more accurate inputs for radiation belt modeling than statistical models.

The Los Alamos hybrid simulation code is used to examine kinetic properties of pickup ions at the heliospheric termination shock and in the downstream heliosheath. All simulations are one‐dimensional in spatial variations, represent the electrons as a zero‐mass fluid, and address only perpendicular shocks. Interpretation of measurements from the IBEX and Voyager spacecraft depend sensitively on the properties of the suprathermal ions downstream of the termination shock, so this research addresses three topics concerning such ions. First, a careful examination of pickup ion trajectories shows that their initial acceleration does not require specular reflection at the shock, as is sometimes assumed, but is the consequence of gyromotion by selected ions at the shock. The primary factor in this energy gain is a gyro‐phase‐dependent interaction with the motional electric field upstream of, and the magnetic field at, the shock. Second, shock simulations are carried out in which the upstream pickup ions are assumed to have four different types of velocity distributions. The downstream ion perpendicular velocity distributions f(v_⊥) are similar in each of the runs and may be approximately characterized as a thermal Maxwellian and a suprathermal distribution. The only significant difference among the four downstream distributions is in the tails of the suprathermal component. Third, simulations are carried out for three different upstream Mach numbers; the results show that faster solar wind flows lead to increased fluxes of ions in the tails of the suprathermal component and are generally consistent with energetic neutral atom observations by the IBEX spacecraft.

As a result of cyclotron interaction with Alfvén waves propagating from the sun, pitch angle diffusion of resonant particles takes place and a shell-like distribution function of resonant ions is formed at each distance from the sun. Stability of the solar wind ion shell-like distribution function with respect to excitation of waves at larger distances is addressed. It is shown in linear approximation, that in the case when the phase velocity of Alfvén waves decreases with distance, ions with shell distribution excite outward propagating Alfvén waves with smaller phase velocities when they advance to larger distances. The nonlinear dynamics of the wave spectrum as well as the evolution of the ion distribution function are studied. The characteristic spectrum at the high-frequency edge of the magnetohydrodynamic fluctuations is explained.

The wave and dispersion equations for perturbations propagating parallel to an ambient magnetic field in magnetoplasmas with nongyrotropic ion populations show, in general, the occurrence of coupling between the parallel (left‐ and right‐hand circularly polarized electromagnetic and longitudinal electrostatic) eigenmodes of the associated gyrotropic medium. These interactions provide a means to driving linearly one mode with free‐energy sources of other modes in homogeneous media. Different types of nongyrotropy bring about distinct classes of coupling. The stability of a hydrogen magnetoplasma with anisotropic, nongyrotropic protons that only couple the electromagnetic modes to each other is investigated analytically (via solution of the derived dispersion equation) and numerically (via simulation with a hybrid code). Nongyrotropy enhances growth and enlarges the unstable spectral range relative to the corresponding gyrotropic situation. The relevance of the properties of nongyrotropic populations to space plasma environments is also discussed.