On the resilience of helical magnetic fields to turbulent diffusion and the astrophysical implications

The extent to which large scale magnetic fields are susceptible to turbulent diffusion is important for interpreting the need for in situ large scale dynamos in astrophysics and for observationally inferring field strengths compared to kinetic energy. By solving coupled equations for magnetic energy and magnetic helicity in a system initiated with isotropic turbulence and an arbitrarily helical large scale field, we quantify the decay rate of the latter for a bounded or periodic system. The energy associated with the non-helical magnetic field rapidly decays by turbulent diffusion, but the decay rate of the helical component depends on whether the ratio of its magnetic energy to the turbulent kinetic energy exceeds a critical value given by M_{1,c} =(k_1/k_2)^2, where k_1 and k_2 are the wave numbers of the large and forcing scales. Turbulently diffusing helical fields to small scales while conserving magnetic helicity requires a rapid increase in total magnetic energy. As such, only when the helical fields are sub-critical can they so diffuse. When super-critical, the large scale helical field decays slowly, at a rate determined by microphysical dissipation even when macroscopic turbulence is present. Amplification of small scale magnetic helicity abates the turbulent diffusion. Two implications are that: (1) Standard arguments supporting the need for in situ large scale dynamos based on the otherwise rapid turbulent diffusion of large scale fields require re-thinking since only the non-helical field is so diffused in a closed system. Boundary terms could however provide potential pathways for rapid change of the large scale helical field. (2) Since M_{1,c} <<1 for k_1 << k_2, the presence of long-lived ordered large scale helical fields, as in extragalactic jets, does not guarantee that the magnetic field dominates the kinetic energy.Comment: published in MNRAS (in this replacement, the missing .bbl file has been added

Nonlocal conductance reveals helical superconductors

Helical superconductors form a two dimensional, time-reversal invariant topological phase characterized by a Kramers pair of Majorana edge modes (helical Majorana modes). Existing detection schemes to identify this phase rely either on spin transport properties, which are quite difficult to measure, or on local charge transport, which allows only a partial identification. Here we show that the presence of helical Majorana modes can be unambiguously revealed by measuring the nonlocal charge conductance. Focusing on a superconducting ring, we suggest two experiments that provide unique and robust signatures to detect the helical superconductor phase.Comment: 4 pages, 2 figure

Силовий портрет зміни радіального розміру пружно-гвинтового хону

The article discloses the research principle of the deformation of helical spring deformed surface of a helical spring hone taking into consideration the results of theoretical, experimental and computer studies. As a result we got the system of equations which defines linear and angle loads in case deformation which appear on the helical spring surface. The scheme of definition of the torque performance while loading the helical spring deformed surface was suggested. The research enabled to build the force depiction of loads and spring deformation of helical spring deformed surface. We also suggested the scheme of definition of deformation force of the helical spring hone and its metering. В статі розглядається принцип дослідження деформації пружно-деформуємої оболонки пружно-гвинтового хона, враховуючі результати теоретичних, експериментальних та комп’ютерних досліджень. В результаті чого було отримана система рівнянь, яка визначає лінійні і кутові навантаження при деформації, котрі виникають в пружно-деформуємій оболонці. Запропонована схема визначення дії крутного моменту при дії навантажень на пружно-деформуєму оболонку. Проведені дослідження дали змогу побудувати силовий портрет навантажень та пружних деформації пружно-деформуємої оболонки. Також представлена схема визначення сили деформації пружно-гвинтового хону та його заміру

Helical spin textures in dipolar Bose-Einstein condensates

We numerically study elongated helical spin textures in ferromagnetic spin-1 Bose-Einstein condensates subject to dipolar interparticle forces. Stationary states of the Gross-Pitaevskii equation are solved and analyzed for various values of the helical wave vector and dipolar coupling strength. We find two helical spin textures which differ by the nature of their topological defects. The spin structure hosting a pair of Mermin-Ho vortices with opposite mass flows and aligned spin currents is stabilized for a nonzero value of the helical wave vector.Comment: 7 pages, 6 figure