Stability of three-dimensional relativistic jets: implications for jet collimation

The stable propagation of jets in FRII sources is remarkable if one takes into account that large-scale jets are subjected to potentially highly disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical simulations can address this problem and help clarify the causes of this remarkable stability. Following previous studies of the stability of relativistic flows in two dimensions (2D), it is our aim to test and extend the conclusions of such works to three dimensions. We present numerical simulations for the study of the stability properties of 3D, sheared, relativistic flows. This work uses a fully parallelized code Ratpenat that solves equations of relativistic hydrodynamics in 3D. The results of the present simulations confirm those in 2D. We conclude that the growth of resonant modes in sheared relativistic flows could be important in explaining the long-term collimation of extragalactic jets.Comment: Accepted for publication in A&

Charged Particles on Surfaces: Coexistence of Dilute Phases and Periodic Structures on Membranes

We consider a mixture of one neutral and two oppositely charged types of molecules confined to a surface. Using analytical techniques and molecular dynamics simulations, we construct the phase diagram of the system and exhibit the coexistence between a patterned solid phase and a charge-dilute phase. The patterns in the solid phase arise from competition between short-range immiscibility and long-range electrostatic attractions between the charged species. The coexistence between phases leads to observations of stable patterned domains immersed in a neutral matrix background.Comment: 5 pages, 3 figure

Electrostatic attraction between cationic-anionic assemblies with surface compositional heterogeneities

Electrostatics plays a key role in biomolecular assembly. Oppositely charged biomolecules, for instance, can co-assembled into functional units, such as DNA and histone proteins into nucleosomes and actin-binding protein complexes into cytoskeleton components, at appropriate ionic conditions. These cationic-anionic co-assemblies often have surface charge heterogeneities that result from the delicate balance between electrostatics and packing constraints. Despite their importance, the precise role of surface charge heterogeneities in the organization of cationic-anionic co-assemblies is not well understood. We show here that co-assemblies with charge heterogeneities strongly interact through polarization of the domains. We find that this leads to symmetry breaking, which is important for functional capabilities, and structural changes, which is crucial in the organization of co-assemblies. We determine the range and strength of the attraction as a function of the competition between the steric and hydrophobic constraints and electrostatic interactions.Comment: JCP June/200

Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method

The role of magnetic fields in the chromospheric heating problem remains greatly unconstrained. Most theoretical predictions from numerical models rely on a magnetic configuration, field strength and connectivity whose details have not been well established with observational studies. High-resolution studies of chromospheric magnetic fields in plage are very scarce or non-existent in general. Our aim is to study the stratification of the magnetic field vector in plage regions. We use high-spatial resolution full-Stokes observations acquired with CRISP instrument at the Swedish 1-m Solar Telescope in the Mg I $\lambda$5173, Na I $\lambda$5896 and Ca II $\lambda$8542 lines. We have developed a spatially-regularized weak-field approximation (WFA) method based on the idea of spatial regularization. This method allows for a fast computation of magnetic field maps for an extended field of view. The fidelity of this new technique has been assessed using a snapshot from a realistic 3D magnetohydrodynamics simulation. We have derived the depth-stratification of the line-of-sight component of the magnetic field from the photosphere to the chromosphere in a plage region. The magnetic fields are concentrated in the intergranular lanes in the photosphere and expand horizontally toward the chromosphere, filling all the space and forming a canopy. Our results suggest that the lower boundary of this canopy must be located around 400-600 km from the photosphere. The mean canopy total magnetic field strength in the lower chromosphere ($z\approx760$ km) is 658 G. At $z=1160$ km we estimate $\approx 417$ G. We propose a modification to the WFA that improves its applicability to data with worse signal-to-noise ratio. These methods provide a quick and reliable way of studying multi-layer magnetic field observations without the many difficulties inherent to other inversion methods.Comment: Accepted for publication on 2020-08-2

Partially disordered state and spin-lattice coupling in an S=3/2 triangular lattice antiferromagnet Ag2CrO2

Ag2CrO2 is an S=3/2 frustrated triangular lattice antiferromagnet without orbital degree of freedom. With decreasing temperature, a 4-sublatice spin state develops. However, a long-range partially disordered state with 5 sublattices abruptly appears at TN=24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic.Comment: 5 pages, 5 figure

Pattern formation on the surface of cationic-anionic cylindrical aggregates

Charged pattern formation on the surfaces of self--assembled cylindrical micelles formed from oppositely charged heterogeneous molecules such as cationic and anionic peptide amphiphiles is investigated. The net incompatibility $\chi$ among different components results in the formation of segregated domains, whose growth is inhibited by electrostatics. The transition to striped phases proceeds through an intermediate structure governed by fluctuations, followed by states with various lamellar orientations, which depend on cylinder radius $R_c$ and $\chi$. We analyze the specific heat, susceptibility $S(q^*)$, domain size $\Lambda=2\pi/q^*$ and morphology as a function of $R_c$ and $\chi$.Comment: Sent to PRL 11Jan05 Transferred from PRL to PRE 10Jun0