The Parametric Decay Instability of Alfven waves in Turbulent Plasmas and the Applications in the Solar Wind

We perform three dimensional (3D) ideal magnetohydrodynamic (MHD) simulations to study the parametric decay instability of Alfven waves in turbulent plasmas and explore its possible applications in the solar wind. We find that, over a broad range of parameters in background turbulence amplitudes, the parametric decay instability of an Alfven wave with various amplitudes can still occur, though its growth rate in turbulent plasmas tends to be lower than both the theoretical linear theory prediction and that in the non-turbulent situations. Spatial - temporal FFT analyses of density fluctuations produced by the parametric decay instability match well with the dispersion relation of the slow MHD waves. This result may provide an explanation of the generation mechanism of slow waves in the solar wind observed at 1 AU. It further highlights the need to explore the effects of density variations in modifying the turbulence properties as well as in heating the solar wind plasmas.Comment: Accepted for publication in The Astrophysical Journa

Influence of fine structures on gyrosynchrotron emission of flare loops modulated by sausage modes

Sausage modes are one leading mechanism for interpreting short period quasi-periodic pulsations (QPPs) of solar flares. Forward modeling their radio emission is crucial for identifying sausage modes observationally and for understanding their connections with QPPs. Using the numerical output from three-dimensional magnetohydrodynamic (MHD) simulations, we forward model the gyrosynchrotron (GS) emission of flare loops modulated by sausage modes and examine the influence of loop fine structures. The temporal evolution of the emission intensity is analyzed for an oblique line of sight crossing the loop center. We find that the low- and high-frequency intensities oscillate in-phase at the period of sausage modes for models with or without fine structures. For low-frequency emissions where the optically thick regime arises, the modulation magnitude of the intensity is dramatically reduced by the fine structures at some viewing angles. On the contrary, for high-frequency emissions where the optically thin regime holds, the effect of fine structures or viewing angle is marginal. Our results show that the periodic intensity variations of sausage modes are not wiped out by the fine structures, and sausage modes remains a promising candidate mechanism for QPPs even when flare loops are fine-structured.Comment: Accepted for publication in ApJ Letter

Damped kink motions in a system of two solar coronal tubes with elliptic cross-sections

This study is motivated by observations of coordinated transverse displacements in neighboring solar active region loops, addressing specifically how the behavior of kink motions in straight two-tube equilibria is impacted by tube interactions and tube cross-sectional shapes.We work with linear, ideal, pressureless magnetohydrodynamics. Axially standing kink motions are examined as an initial value problem for transversely structured equilibria involving two identical, field-aligned, density-enhanced tubes with elliptic cross-sections (elliptic tubes). Continuously nonuniform layers are implemented around both tube boundaries. We numerically follow the system response to external velocity drivers, largely focusing on the quasi-mode stage of internal flows to derive the pertinent periods and damping times. The periods and damping times we derive for two-circular-tube setups justify available modal results found with the T-matrix approach. Regardless of cross-sectional shapes, our nonuniform layers feature the development of small-scale shears and energy accumulation around Alf\'ven resonances, indicative of resonant absorption and phase-mixing. As with two-circular-tube systems, our configurational symmetries make it still possible to classify lower-order kink motions by the polarization and symmetric properties of the internal flows; hence such mode labels as $S_x$ and $A_x$. However, the periods and damping times for two-elliptic-tube setups further depend on cross-sectional aspect ratios, with $A_x$ motions occasionally damped less rapidly than $S_x$ motions. We find uncertainties up to $\sim 20\%$ ($\sim 50\%$) for the axial Alfven time (the inhomogeneity lengthscale) if the periods (damping times) computed for two-elliptic-tube setups are seismologically inverted with canonical theories for isolated circular tubes.Comment: Accepted for publication in A&

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth's Magnetosheath: Observation Matches Theory

Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales and affects energetic particle transport. Recent advances in the understanding of compressible magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping in shaping energy distributions on small scales, yet its observational evidence is still lacking. This study provides the first observational evidence of substantial collisionless damping (CD) modulation on small-amplitude compressible MHD turbulence cascade in Earth's magnetosheath using four Cluster spacecraft. Based on an improved compressible MHD decomposition algorithm, turbulence is decomposed into three eigenmodes: incompressible Alfv\'en modes, and compressible slow and fast (magnetosonic) modes. Our observations demonstrate that CD enhances the anisotropy of compressible MHD modes because CD has a strong dependence on wave propagation angle. The wavenumber distributions of slow modes are mainly stretched perpendicular to the background magnetic field ($\mathbf{B_0}$) and weakly modulated by CD. In contrast, fast modes are subjected to a more significant CD modulation. Fast modes exhibit a weak, scale-independent anisotropy above the CD truncation scale. Below the CD truncation scale, the anisotropy of fast modes enhances as wavenumbers increase. As a result, fast mode fractions in the total energy of compressible modes decrease with the increase of perpendicular wavenumber (to $\mathbf{B_0}$) or wave propagation angle. Our findings reveal how the turbulence cascade is shaped by CD and its consequences to anisotropies in the space environment.Comment: Main text: 5 pages, 6 figures. Accepted by ApJ on Dec. 5, 2023. Published on Feb. 08, 202