Abstract
Firehose-like instabilities (FIs) are cited in multiple astrophysical applications. Of particular interest are the kinetic manifestations in weakly collisional or even collisionless plasmas, ...where these instabilities are expected to contribute to the evolution of macroscopic parameters. Relatively recent studies have initiated a realistic description of FIs, as induced by the interplay of both species, electrons and protons, dominant in the solar wind plasma. This work complements the current knowledge with new insights from linear theory and the first disclosures from 2D-PIC simulations, identifying the fastest growing modes near the instability thresholds and their long-run consequences on the anisotropic distributions. Thus, unlike previous setups, these conditions are favorable to those aperiodic branches that propagate obliquely to the uniform magnetic field, with (maximum) growth rates higher than periodic, quasi-parallel modes. Theoretical predictions are, in general, confirmed by the simulations. The aperiodic electron FI (a-EFI) remains unaffected by the proton anisotropy, and saturates rapidly at low-level fluctuations. Regarding the FI at proton scales, we see a stronger competition between the periodic and aperiodic branches. For the parameters chosen in our analysis, the aperiodic proton FI (a-PFI) is excited before than the periodic proton FI (p-PFI), with the latter reaching a significantly higher fluctuation power. However, both branches are significantly enhanced by the presence of anisotropic electrons. The interplay between EFIs and PFIs also produces a more pronounced proton isotropization.
Abstract
We demonstrate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially nonmagnetized ...plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor–Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave–particle interaction regions, the seed magnetic field grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magnetogenesis in a variety of astrophysical plasmas, wherever turbulence is present.
In situ observations of the solar wind show a limited level of particle temperature anisotropy with respect to the interplanetary magnetic field direction. Kinetic electromagnetic instabilities are ...efficient to prevent the excessive growth of the anisotropy of particle velocity distribution functions. Among them, the firehose instabilities are often considered to prevent the increase of the parallel temperature and hence to shape the velocity distribution functions of electrons and protons in the solar wind. We present a nonlinear modeling of the parallel firehose instability, retaining a kinetic description for both the electrons and protons. One-dimensional (1D) fully kinetic particle-in-cell simulations using the energy conserving semi-implicit method (ECsim) are performed to clarify the role of the electron temperature anisotropy in the development of the parallel proton firehose instability. We found that in the presence of an electron temperature anisotropy, such that the temperature parallel to the background magnetic field is higher than the temperature in the perpendicular direction, the onset of the parallel proton firehose instability occurs earlier and its growth rate is faster. The enhanced wave fluctuations contribute to the particle scattering reducing the temperature anisotropy to a stable, nearly isotropic state. The simulation results compare well with linear theory. A test case of 1D simulations at oblique angles with respect to the magnetic field is also considered, as a first step to study the cumulative effect of protons and electrons on the full spectrum of instabilities.
Dipolarization fronts (DFs) are often associated with the leading edge of earthward bursty bulk flows in the magnetotail plasma sheet. Here multispacecraft Time History of Events and Macroscale ...Interactions during Substorms (THEMIS) observations are used to show that a spatially limited region of counterpropagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low‐density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle‐in‐cell simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self‐consistent back tracing shows that the ion beams originate from the thermal component of the preexisting high‐density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high‐density preexisting plasma sheet with an order‐of‐magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the preexisting plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.
Key Points
Spacecraft data/ PIC simulation used to study dipolarization front ion physics
DFs entrain in the jet ions from the pre‐existing plasma sheet as they propagate
Accelerated ions may overtake the DF, shedding new light on reflection processes
ABSTRACT The long-term evolution of large domain particle-in-cell simulations of collisionless magnetic reconnection is investigated following observations that show two possible outcomes for ...collisionless reconnection: toward a Petschek-like configuration or toward multiple X points. In the present simulation, a mixed scenario develops. At earlier time, plasmoids are emitted, disrupting the formation of Petschek-like structures. Later, an almost stationary monster plasmoid forms, preventing the emission of other plasmoids. A situation reminiscent of Petschek's switch-off then ensues. Switch-off is obtained through a slow shock/rotational discontinuity compound structure. Two external slow shocks (SS) located at the separatrices reduce the in-plane tangential component of the magnetic field, but not to zero. Two transitions reminiscent of rotational discontinuities (RD) in the internal part of the exhaust then perform the final switch-off. Both the SS and the RD are characterized through analysis of their Rankine-Hugoniot jump conditions. A moderate guide field is used to suppress the development of the firehose instability in the exhaust.
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