Particle-in-Cell Simulations of Astrophysical Relativistic Jets

Astrophysical relativistic jets in active galactic nuclei, gamma-ray bursts, and pulsars is the main key subject of study in the field of high-energy astrophysics, especially regarding the jet interaction with the interstellar or intergalactic environment. In this work, we review studies of particle-in-cell simulations of relativistic electron–proton (e−−p+) and electron–positron (e±) jets, and we compare simulations that we have conducted with the relativistic 3D TRISTAN-MPI code for unmagnetized and magnetized jets. We focus on how the magnetic fields affect the evolution of relativistic jets of different compositions, how the jets interact with the ambient media, how the kinetic instabilities such as the Weibel instability, the kinetic Kelvin–Helmholtz instability and the mushroom instability develop, and we discuss possible particle acceleration mechanisms at reconnection sites.

Hybrid-Vlasov modelling of three-dimensional dayside magnetopause reconnection

<p>Dayside magnetic reconnection at the magnetopause, which is a major driver of space weather, is studied for the first time in a three-dimensional (3D) realistic setup using the Vlasiator hybrid-Vlasov kinetic model. A noon–midnight meridional plane simulation is extended in the dawn–dusk direction to cover 7 Earth radii. The southward interplanetary magnetic field causes magnetic reconnection to occur at the subsolar magnetopause. Perturbations arising from kinetic instabilities in the magnetosheath appear to modulate the reconnection. Its characteristics are consistent with multiple, bursty, and patchy magnetopause reconnection. It is shown that the kinetic behavior of the plasma, as simulated by the model, has consequences on the applicability of methods such as the four-field junction to identify and analyse magnetic reconnection in 3D kinetic simulations.</p>

Cumulative instabilities of anisotropic protons and electrons in the solar wind: New insights from a quasilinear approach

<p>In collision-poor space plasmas the main physical processes are governed by fluctuations and their interactions with plasma particles. An important <span>source of waves and coherent fluctuations are kinetic instabilities driven </span>by, e.g., protons and electrons exhibiting temperature anisotropies. Unfortunately, such instabilities are generally investigated independently of each other, thereby ignoring their interplay and preventing a realistic treatment of their implications. Here we present the first results of an extended quasilinear approach, which not only confirms linear predictions but also unveils new regimes triggered by cumulative effects of the proton <span>and electron instabilities (e.g., electromagnetic cyclotron, firehose). By </span>comparison to individual excitations combined proton- and electron-induced fluctuations grow and saturate at different intensities as well as different temporal scales in the quasilinear phase. Moreover, the enhanced wave fluctuations can markedly stimulate or inhibit the relaxation of temperature anisotropies, this way highly conditioning the evolution and saturation of instabilities.</p>

A two-step role for plasma expansion in solar wind heat flux regulation

<p>Already several decades ago, it was suggested that kinetic instabilities play a fundamental role in heat flux regulation at relatively large distances from the Sun, R> 1 AU [Scime et al, 1994]. Now, Parker Solar Probe observations have established that this is the case also closer to it [Halekas et al, 2020].</p><p>Electron scale instabilities in the solar wind are driven and affected in their evolution by the slow, large scale process of solar wind expansion, as demonstrated observationally [Stverak et al, 2008; Bercic et al, 2020], and via fully kinetic Expanding Box Model simulations [Innocenti et al, 2019b].</p><p>Now, connecting the dots, we examine an indirect role of plasma expansion in heat flux regulation in the solar wind. We show, as a proof of principle, that plasma expansion can modify heat flux evolution as a function of heliocentric distance, with respect to what is expected within an adiabatic framework, due to the onset of kinetic instabilities, in this case, an oblique firehose instability developing self consistently in the presence of a core and suprathermal electron population [Innocenti et al, 2020].</p><p>This result highlights, once again, the deeply multi scale nature of the heliospheric environment, that calls for advanced simulation techniques. In this work, the simulations are done with the fully kinetic, semi-implicit [Markidis et al, 2010], Expanding Box Model [Velli et al, 1992] code EB-iPic3D [Innocenti et al, 2019a].</p>

Study of the relationship between the observations of electron distributions in the solar wind and interplanetary magnetic field fluctuations

<p>The space between the Sun and our planet is not empty. It is filled with the expanding plasma of the solar corona called the Solar Wind, which is a tenuous weakly collisional plasma composed mainly by protons and electrons. Due to the lack of sufficient collisions, the electron velocity distribution function in the Solar Wind usually exhibits a variety of non-thermal characteristics that deviate from the thermodynamic equilibrium. These deviations from equilibrium provide a local source for electromagnetic fluctuations, intimately related to the shape of the distribution function, and associated with the commonly observed kinetic instabilities such as the whistler-cyclotron for T<sub>⊥</sub>/ T<sub>∥</sub>>1, and firehose for T<sub>⊥</sub>/ T<sub>∥</sub><1 and large enough plasma beta. In this work we carry out systematic statistical study of correlations of various plasma moments and interplanetary magnetic fluctuations as a function of time, in order to describe the role and evolution of these parameters in the solar plasma through the solar cycle. We consider a large time interval during solar cycle 23, ranging from solar minimum (1995-1996) to solar maximum (2000-2001). Using NASA's Wind space mission and its SWE and High-Resolution MFI instruments with resolutions of 6-15 sec and 11 vectors/sec, respectively, we show that collisionless kinetic instabilities can regulate the electron distribution as the whistler-cyclotron and firehose instability thresholds bound the temperature and plasma beta electron distributions, and such regulation is more effective during solar minimum. Subsequently, the magnetic fluctuations level increases as the electron VDF acquires a configuration close to the thresholds. In addition, we note that there is a high difference between the fast and slow wind regimes given a greater tendency towards larger collisionallity and isotropization for low speeds streams, and magnetic fluctuations amplitude decreases as collisional age increases. In summary, our results indicate that collisionless plasma processes and Coulomb collisions effects coexist and both seem to play relevant roles in shaping the observed electron distributions.</p>

Noise-induced magnetic field saturation in kinetic simulations

Monte Carlo methods are often employed to numerically integrate kinetic equations, such as the particle-in-cell method for the plasma kinetic equation, but these methods suffer from the introduction of counting noise to the solution. We report on a cautionary tale of counting noise modifying the nonlinear saturation of kinetic instabilities driven by unstable beams of plasma. We find a saturated magnetic field in under-resolved particle-in-cell simulations due to the sampling error in the current density. The noise-induced magnetic field is anomalous, as the magnetic field damps away in continuum kinetic and increased particle count particle-in-cell simulations. This modification of the saturated state has implications for a broad array of astrophysical phenomena beyond the simple plasma system considered here, and it stresses the care that must be taken when using particle methods for kinetic equations.

Rapid particle acceleration due to recollimation shocks and turbulent magnetic fields in injected jets with helical magnetic fields

ABSTRACT One of the key questions in the study of relativistic jets is how magnetic reconnection occurs and whether it can effectively accelerate electrons in the jet. We performed 3D particle-in-cell (PIC) simulations of a relativistic electron–proton jet of relatively large radius that carries a helical magnetic field. We focused our investigation on the interaction between the jet and the ambient plasma and explore how the helical magnetic field affects the excitation of kinetic instabilities such as the Weibel instability (WI), the kinetic Kelvin–Helmholtz instability (kKHI), and the mushroom instability (MI). In our simulations these kinetic instabilities are indeed excited, and particles are accelerated. At the linear stage we observe recollimation shocks near the centre of the jet. As the electron–proton jet evolves into the deep non-linear stage, the helical magnetic field becomes untangled due to reconnection-like phenomena, and electrons are repeatedly accelerated as they encounter magnetic-reconnection events in the turbulent magnetic field.