Session YM9: Mini-conference: Shocks and Particle Energization in Collisionless Plasma

Pickup ions in the heliosphere

Charge exchange between the solar wind (SW) protons and H atoms in the local interstellar medium (LISM) gives birth to non-thermal, pickup ions (PUIs). The distribution function of PUIs created this way quickly becomes isotropic, but Maxwellian equilibrium is not reached. This raises questions about the proper description of PUIs crossing collisionless shocks, which are abundant in the space plasma. We discuss numerical approaches to modeling the flow of the mixture of thermal and non-thermal ions which constitute the SW. In particular, the observational data sets suitable for the validation of numerical simulations are discussed. We also compare solutions where PUIs are treated as a separate component of plasma with those where the plasma is assumed to be in Maxwellian equilibrium. Finally, we analyze the boundary conditions on the distribution functions and bulk properties of the SW flow and heliospheric magnetic field at collisionelss shocks, e.g., the heliospheric termination shock. Numerical simulations are compared with observations.   

Acceleration of pickup ions in interplanetary space

Pickup ions are created in the solar wind through charge exchanges or photoionization with the interstellar neutrals. Their initial speed in the plasma frame is equal to the solar wind speed $V_{sw}$. So pickup ions can build up its internal energy quickly. In the spacecraft frame, they have a cutoff speed of $2 V_{sw}$. Observations returned from the SWICS instrument on Ulysses show that the cutoff energy can go higher than $2 V_{sw}$. Furthermore, there is always a tail distribution of pickup ions much beyond $2 V_{sw}$. All these observations suggest that pickup ions are accelerated in the solar wind. Since interplanetary shocks do not occur more frequently than a few times per month, the pickup ions may have to rely on small-scale plasma turbulence prevalent in the solar wind. In this paper, we will discuss the roles of Alfv\'{e}nic turbulence and compressive plasma turbulence in the acceleration of pickup ions. We will show that the Alfv\'{e}nic turbulence is ineffective. Although compressive turbulence has a much smaller amplitude than the Alfv\’{e}nic turbulence in the solar wind, the acceleration is much faster. If its initial amplitude is high enough, a fully evolved compressive turbulence will tend to yield a $v^{-5}$ speed spectrum of high-energy pickup ions.   

Voyager 1 and 2 Measurements of Particle Acceleration at the Termination Shock and in the Heliosheath

We discuss energetic charged particle intensities, energy spectra, composition, and angular distributions measured at Voyager 1 and 2 near the termination shock (TS), in the heliosheath (HSH), and in the local interstellar medium (LISM) just outside the heliopause (HP). Voyager 1 (Voyager 2) crossed the TS in late 2004 at 94 AU (mid-2007 at 84 AU), explored a 28 AU (35 AU) wide HSH for 8 years (11 years), and crossed the HP in mid-2012 at 122 AU (late 2018 at 119 AU). Before the TS crossings, both Voyagers saw beam-like energetic ion intensities moving away from the TS along the solar wind magnetic field. These data implied a TS blunted near its nose region, with pre-TS ions produced by acceleration and reflection at the TS and by escape from the HSH. The lower energy portion of the TS/HSH ion spectrum, called TS particles (TSPs), is mainly interstellar pickup ions heated and accelerated at the TS and by other HSH processes. It was expected that higher energy portion of this ion spectrum, anomalous cosmic rays (ACRs), was accelerated at the TS. But, in situ data from both Voyagers showed this not to be the case, at least near the TS nose region. It is now believed ACRs are accelerated along the TS flank. We also describe details of TSP and ACR ions measured in the near-HP LISM.   

Particle Acceleration at Shocks Moving Through Turbulent Plasmas

I will discuss the physics of charged-particle acceleration at astrophysical shocks moving through turbulent plasmas, from the perspective of lessons learned from \textit{in-situ} observations in the Heliosphere. Diffusive shock acceleration theory is widely accepted and used routinely to model cosmic-ray acceleration at astrophysical shocks, but \textit{in-situ} Heliospheric observations present challenges to our understanding of the basic physics involved. The observations also provide significant insight on important unsolved aspects of this problem such as the injection problem, the maximum energy, and the role of the magnetic-field geometry. The presence of pre-existing plasma and field turbulence plays a critical role, particularly with regards the acceleration of low energy ions. When pre-existing large-scale turbulence is included in the theory and numerical models, it is found that shocks are efficient accelerators of low-energy ions, even thermal plasma, and the acceleration efficiency has no obvious dependence on the magnetic obliquity of the shock. I will show a few examples from theory, modeling and observations, and discuss their implications.   

Energy Partition at Collisionless Shocks in Plasmas With Super-Thermal Pickup Ion Populations

Collisionless shocks are ubiquitous in many space physics, astrophysics, and laboratory settings. One of the basic unsolved theoretical issues is quantitative prediction of the downstream plasma state for given upstream parameters. This problem becomes even more significant in situation where super-thermal population of ions are present upstream from the shock, as is the case for many shocks encountered in heliosphere, e.g. the termination shock. In this contribution, we discuss the problem, present overview of the previous investigations, and introduce recent advancements obtained with the aid of a semi-analytical theoretical model, which is compared against results of fully kinetic particle-in-cell simulations.   

Experimental Studies of High-Mach-Number Magnetized Collisionless Shocks in Laboratory Plasmas

We review experiments undertaken on the Omega laser facility that study high-Mach-number magnetized collisionless shocks through the interaction of a laser-driven piston plasma and ambient plasma in an external magnetic field. Through time-resolved, 2-D refractive imaging and proton radiography we observe the formation of shocks with an Alfv\'{e}nic Mach number $M_A\sim15$ that occur over ion kinetic length scales. Additional measurements with optical Thomson scattering directly demonstrate the shock formation process through the evolution of the piston and ambient ion velocity distributions and their coupling through electric and magnetic fields. We also show particle-in-cell simulations constrained by experimental data that further detail how laser-driven piston plasmas generate magnetized shocks in laboratory plasmas. The results provide key features of the structure of piston-driven shocks and several criteria by which to determine when shocks have formed under laboratory conditions. The development of this experimental platform opens the way for controlled laboratory investigations of high-Mach-number collisionless shocks, including mechanisms of shock heating and particle acceleration.   

Particle acceleration in astrophysical and laboratory plasmas.

The fast progress in high-energy-density (HED) experimental facilities and computational capabilities is creating a unique window of opportunity to push the boundaries of our understanding of particle acceleration in astrophysics and laboratory plasmas. I will discuss recent results from fully-kinetic particle-in-cell simulations and HED experiments on particle acceleration in jets and collisionless shocks.   

Diagnosing Energy Dissipation in Fully Kinetic Continuum Vlasov-Maxwell Plasmas

We present a novel algorithm for the direct discretization of the Vlasov-Maxwell system using the Gkeyll simulation framework that employs high order discontinuous Galerkin finite elements on an up to 3D-3V phase space grid, including the implementation of a Dougherty collision operator. We leverage the pristine phase space representation made possible by direct discretization to examine energy dissipation in a variety of systems relevant to space and astrophysical plasmas. Specifically, we employ the field-particle correlation technique in phase space to directly diagnose the exchange of energy between fields and particles. We present results from a variety of simple systems, including magnetic pumping and resonant wave damping, and we also apply the field-particle correlation technique to 2D-3V Vlasov-Maxwell simulations of reconnection and turbulence.   

Particle Energization in Different Turbulent Environments with Applications to Solar Corona, Accretion Flows and Astrophysical Jets

Plasma turbulence and magnetic reconnection are ubiquitous in astrophysical environments. Particle energization often arises in such environments, such as solar coronal heating, flares from accretion disk coronae and astrophysical jets. Recent theoretical and numerical studies have highlighted the role of dynamic current sheets, including their formation and associated reconnection processes, which in turn are postulated to strongly impact the turbulence dynamics as well as particle energization processes. Here, we present 3D MHD and kinetic simulations to study the interplay between turbulence and the pre-existing current sheets. These systems are large enough that turbulence can be fully developed. Whereas some turbulence properties are similar between these two cases, we highlight the differences in available free energy that can lead to different particle energization efficiency and processes. We will discuss the implications for solar and accretion disk corona and astrophysical jets.