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Nonlinear dynamics of runaway electron induced wave instabilities can significantly modify the runaway distribution critical to tokamak operations. Here we present the first-ever fully kinetic simulations of runaway-driven instabilities towards nonlinear saturation in a warm plasma as in tokamak start up. It is found that the slow-X modes grow an order of magnitude faster than the whistler modes, and they parametrically decay to produce whistlers much faster than those directly driven by runaways. These parent-daughter waves, as well as secondary and tertiary wave instabilities, initiate a chain of wave-particle resonances that strongly diffuse runaways to the backward direction. This reduces almost half of the current carried by high-energy runaways, over a time scale orders of magnitude faster than experimental shot duration. These results beyond quasilinear analysis may impact anisotropic energetic electrons broadly in laboratory, space and astrophysics.

Primordial magnetic fields (PMFs) play a pivotal role in influencing small-scale fluctuations within the primordial density field, thereby enhancing the matter power spectrum within the context of the $\Lambda$CDM model at small scales. These amplified fluctuations accelerate the early formation of galactic halos and stars, which can be observed through advanced high-redshift observational techniques. Therefore, Stellar Mass Density (SMD) observations, which provide significant opportunities for detailed studies of galaxies at small scales and high redshifts, offer a novel perspective on small-scale cosmic phenomena and constrain the characteristics of PMFs. In this study, we compile 14 SMD data points at redshifts $z > 6$ and derive stringent constraints on the parameters of PMFs, which include the amplitude of the magnetic field at a characteristic scale of $\lambda=1\,{\rm Mpc}$, denoted as $B_0$, and the spectral index of the magnetic field power spectrum, $n_{\rm B}$. At 95\% confidence level, we establish upper limits of $B_0 < 4.44$ nG and $n_{\rm B} < -2.24$, along with a star formation efficiency of approximately $f_*^0 \sim 0.1$. If we fix $n_{\rm B}$ at specific values, such as $-2.85$, $-2.9$, and $-2.95$, the 95\% upper limits for the amplitude of the magnetic field can be constrained to 1.33 nG, 2.21 nG, and 3.90 nG, respectively. Finally, we attempt to interpret recent early observations provided by James Webb Space Telescope (JWST) using the theory of PMFs, and find that by selecting appropriate PMF parameters, it is possible to explain these results without significantly increasing the star formation efficiency.

Understanding plasma dynamics and nonthermal particle acceleration in 3D magnetic reconnection has been a long-standing challenge. In this paper, we explore these problems by performing large-scale fully kinetic simulations of multi-xline plasmoid reconnection with various parameters in both the weak and strong guide field regimes. In each regime, we have identified its unique 3D dynamics that leads to field-line chaos and efficient acceleration, and we have achieved nonthermal acceleration of both electrons and protons into power-law spectra. The spectral indices agree well with a simple Fermi acceleration theory that includes guide field dependence. In the low-guide-field regime, the flux-rope kink instability governs the 3D dynamics for efficient acceleration. The weak dependence of the spectra on the ion-to-electron mass ratio and $\beta$ ($\ll1$) implies that the particles are sufficiently magnetized for Fermi acceleration in our simulations. While both electrons and protons are injected at reconnection exhausts, protons are primarily injected by perpendicular electric fields through Fermi reflections and electrons are injected by a combination of perpendicular and parallel electric fields. The magnetic power spectra agree with in-situ magnetotail observations, and the spectral index may reflect a reconnection-driven size distribution of plasmoids instead of Goldreich-Sridhar vortex cascade. As the guide field becomes stronger, the oblique flux ropes of large sizes capture the main 3D dynamics for efficient acceleration. Intriguingly, the oblique flux ropes can also run into flux-rope kink instability to drive extra 3D dynamics. This work has broad implications for 3D reconnection dynamics and particle acceleration in heliophysics and astrophysics., Comment: 15 pages, 11 figures

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emissions. Using fully kinetic particle-in-cell simulations, we investigate how guide-field strength and domain size affect characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy $\gamma_{\rm inj}$ and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and $\gamma_{\rm inj}$, which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to large-scale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emissions in high-energy astrophysics, including black hole jets and pulsar wind nebulae., Comment: 27 pages, 15 figures, accepted for publication to The Astrophysical Journal

Magnetic reconnection drives multi-species particle acceleration broadly in space and astrophysics. We perform the first 3D hybrid simulations (fluid electrons, kinetic ions) that contain sufficient scale separation to produce nonthermal heavy-ion acceleration, with fragmented flux ropes critical for accelerating all species. We demonstrate the acceleration of all ion species (up to Fe) into power-law spectra with similar indices, by a common Fermi acceleration mechanism. The upstream ion velocities influence the first Fermi reflection for injection. The subsequent onsets of Fermi acceleration are delayed for ions with lower charge-mass ratios (Q/M), until growing flux ropes magnetize them. This leads to a species-dependent maximum energy/nucleon $\propto(Q/M)^\alpha$. These findings are consistent with in-situ observations in reconnection regions, suggesting Fermi acceleration as the dominant multi-species ion acceleration mechanism., Comment: version accepted by Physical Review Letters

Recently, Sironi (PRL, 128, 145102; S22) reported the correlation between particles accelerated into high energy and their crossings of regions with electric field larger than magnetic field (E>B regions) in kinetic simulations of relativistic magnetic reconnection. They claim that electric fields in E>B regions (for a vanishing guide field) dominate in accelerating particles to the injection energy. S22 presented test-particle simulations showing that if particle energies are reset to low energies in E>B regions, efficient injection is suppressed. This Comment re-examines these claims by analyzing a simulation resembling the reference case in S22. We show that during crossings E>B acceleration only contributes a small fraction to the injection energy as E>B regions only host particles for a short duration. The energization before any E>B crossings has a comparable contribution, indicating E>B regions are not unique in pre-accelerating particles. A new test-particle simulation shows that zero-outing electric fields in E>B regions does not strongly influence the injection. We suggest that the procedure used in S22 to exclude E>B acceleration partly removes acceleration outside E>B regions, leading to a false conclusion., Comment: 2 pages, 1 figure

We investigate the cosmological evolution of the power law K-essence dark energy (DE) model $F(X)= -\sqrt{X} + X$ with a new interaction $Q = \alpha\rho _m\rho _{\phi }H^{-1}$ in FRWL spacetime. The evolution behavior of dark energy under this interaction is analyzed by using dynamical systems method, and ten critical points are obtained. Among those critical points, a new stable point, which we called Scaling-like dark energy(DE) solution, is very important and interesting. The cosmological meaning of this attractor is different from the Scaling solution and dark energy dominated solution. For some value of model parameters, the universe will evolve to the attractor solution with the dark energy density parameter $\Omega_{\phi}=0.682946$ and the the equation of state $w_{\phi}=-0.99$, which can be in good agreement with the observed data, and alleviate the Coincidence Problem.

We propose a model for interpreting highly variable ion composition ratios in solar energetic particles (SEP) events recently observed by Parker Solar Probe (PSP) at $0.3 - 0.45$ astronomical unit. We use numerical simulations to calculate SEP propagation in a turbulent interplanetary magnetic field with a Kolmogorov power spectrum from large scale down to the gyration scale of energetic particles. We show that when the source regions of different species are offset by a distance comparable to the size of the source regions, the observed energetic particle composition He/H can be strongly variable over more than two orders of magnitude, even if the source ratio is at the nominal value. Assuming a $^3$He/$^4$He source ratio of $10 \%$ in impulsive $^3$He-rich events and the same spatial offset of the source regions, the $^3$He/$^4$He ratio at observation sites also vary considerably. The variability of the ion composition ratios depends on the radial distance, which can be tested by observations made at different radial locations. We discuss the implication of these results on the variability of ion composition of impulsive events and on further PSP and Solar Orbiter observations close to the Sun., Comment: 10 pages, 5 figures, ApJ accepted

The formation, development and impact of slow shocks in the upstream region of reconnecting current layers are explored. Slow shocks have been documented in the upstream region of magnetohydrodynamic (MHD) simulations of magnetic reconnection as well as in similar simulations with the {\it kglobal} kinetic macroscale simulation model. They are therefore a candidate mechanism for preheating the plasma that is injected into the current layers that facilitate magnetic energy release in solar flares. Of particular interest is their potential role in producing the hot thermal component of electrons in flares. During multi-island reconnection, the formation and merging of flux ropes in the reconnecting current layer drives plasma flows and pressure disturbances in the upstream region. These pressure disturbances steepen into slow shocks that propagate along the reconnecting component of the magnetic field and satisfy the expected Rankine-Hugoniot jump conditions. Plasma heating arises from both compression across the shock and the parallel electric field that develops to maintain charge neutrality in a kinetic system. Shocks are weaker at lower plasma $\beta $, where shock steepening is slow. While these upstream slow shocks are intrinsic to the dynamics of multi-island reconnection, their contribution to electron heating remains relatively minor compared with that from Fermi reflection and the parallel electric fields that bound the reconnection outflow., Comment: 9 pages and 5 figures

The relaxation of field-line tension during magnetic reconnection gives rise to a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux-ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to strong field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux-ropes and undergo further acceleration. As a consequence, both ions and electrons develop clear power-law energy spectra which contain a significant fraction of the released energy. The low-energy bounds are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. These results have strong relevance to observations of nonthermal particle acceleration in space and astrophysics., Comment: 8 pages, 3 figures. Accepted by Physical Review Letters

In strongly magnetized astrophysical plasma systems, magnetic reconnection is believed to be a primary process during which explosive energy release and particle acceleration occur, leading to significant high-energy emission. Past years have witnessed active development of kinetic modeling of relativistic magnetic reconnection, supporting this magnetically dominated scenario. A much less explored issue is the consequence of 3D dynamics, where turbulent structures are naturally generated as various types of instabilities develop. This paper presents a series of 3D, fully-kinetic simulations of relativistic turbulent magnetic reconnection (RTMR) in positron-electron plasmas with system domains much larger than kinetic scales. Our simulations start from a force-free current sheet with several different modes of long wavelength magnetic field perturbations, which drive additional turbulence in the reconnection region. Because of this, the current layer breaks up and the reconnection region quickly evolves into a turbulent layer filled with coherent structures such as flux ropes and current sheets. We find that plasma dynamics in RTMR is vastly different from their 2D counterparts in many aspects. The flux ropes evolve rapidly after their generation, and can be completely disrupted due to the secondary kink instability. This turbulent evolution leads to superdiffusion behavior of magnetic field lines as seen in MHD studies of turbulent reconnection. Meanwhile, nonthermal particle acceleration and energy-release time scale can be very fast and do not strongly depend on the turbulence amplitude. The main acceleration mechanism is a Fermi-like acceleration process supported by the motional electric field, whereas the non-ideal electric field acceleration plays a subdominant role. We discuss possible observational implications of 3D RTMR in high-energy astrophysics., Comment: 26 pages, 22 figures, ApJ in press

The role of turbulence in low-$\beta$, guide-field reconnection exhausts is explored in 2D reconnection and 2D and 3D Riemann simulations. The structure of the exhaust and associated turbulence is controlled by a pair of rotational discontinuities (RDs) at the exhaust boundary and a pair of slow shocks (SSs) that are generated by counterstreaming ions beams. In 2D the exhaust develops large-amplitude striations at the ion Larmor radius scale that are produced by electron-beam-driven ion cyclotron waves. The electron beams driving the instability are injected into the exhaust from one of the RDs. However, in 3D Riemann simulations, the additional dimension (in the out-of-plane direction) results in strong Buneman and electron-electron streaming instabilities at the RD which suppress electron beam formation and therefore the striations in the exhaust. The strength of the streaming instabilities at the RD are controlled by the ratio of the electron thermal speed to Alfv\'en speed, lower thermal speed being more unstable. In the 3D simulations an ion-ion streaming instability acts to partially thermalize the counterstreaming ion beams at the SSs. This instability is controlled by the ratio of the sound speed to Alfv\'en speed and is expected to be stable in the low $\beta$ solar corona. The results suggest that in a guide field reconnection exhaust with $1 \gg \beta > m_e/m_i$, the kinetic-scale turbulence that develops will be too weak to play a significant role in energy conversion and particle acceleration. Therefore, the energy conversion will be mostly controlled by laminar physics or multi-x-line reconnection.

Kinetic Riemann simulations have been completed to explore particle heating during guide field reconnection in the low-$\beta$ environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RD) and two slow shocks (SS) form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The SSs differ greatly from those in the MHD model. The turbulence at the shock fronts and both upstream and downstream is weak so the shocks are laminar and produce little dissipation. Downstream of the SSs the counterstreaming ion beams lead to higher density, which leads to a positive potential between the SSs that acts to confine the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to downstream region and traps a small fraction but only modestly increases the downstream electron temperature above the upstream value. In the low-$\beta$ limit the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons. That the model does not produce strong electron heating nor an energetic electron component suggests that other mechanisms, such as multiple x-line reconnection, are required to explain energetic electron production in large flares. The model can be tested with the expected data from the Parker Solar Probe., Comment: Some figures' quality is downgraded due to the size limit of arXiv