Compressibility Effect on the Rayleigh–Taylor Instability with Sheared Magnetic Fields

We study the effect of plasma compressibility on the Rayleigh–Taylor instability of a magnetic interface with a sheared magnetic field. We assume that the plasma is ideal and the equilibrium quantities are constant above and below the interface. We derive the dispersion equation. Written in dimensionless variables, it contains seven dimensionless parameters: the ratio of plasma densities above and below the interface ζζ, the ratio of magnetic field magnitude squared χχ, the shear angle αα, the plasma beta above and below the interface, β2β2 and β1β1, the angle between the perturbation wave number and the magnetic field direction above the interface ϕϕ, and the dimensionless wave number κκ. Only six of these parameters are independent because χχ, β1β1, and β2β2 are related by the condition of total pressure continuity at the interface. Only perturbations with the wave number smaller than the critical wave number are unstable. The critical wave number depends on ϕϕ, but it is independent of β1β1 and β2β2, and is the same as that in the incompressible plasma approximation. The dispersion equation is solved numerically with ζ=100ζ=100, χ=1χ=1, and β1=β2=ββ1=β2=β. We obtain the following results. When ββ decreases, so does the maximum instability increment. However, the effect is very moderate. It is more pronounced for high values of αα. We also calculate the dependence on ϕϕ of the maximum instability increment with respect to κκ. The instability increment takes its maximum at ϕ=ϕmϕ=ϕm. Again, the decrease of ββ results in the reduction of the instability increment. This reduction is more pronounced for high values of |ϕ−ϕm||ϕ−ϕm|. When both αα and |ϕ−ϕm||ϕ−ϕm| are small, the reduction effect is practically negligible. The theoretical results are applied to the magnetic Rayleigh–Taylor instability of prominence threads in the solar atmosphere

Magnetic Field Generation in Stars

Enormous progress has been made on observing stellar magnetism in stars from the main sequence through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence,in the generation and stability of neutron star fields. Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Thus we maybe at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window. We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field generation in stars to appear in Space Science Reviews, Springe

Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results

The chromosphere is a thin layer of the solar atmosphere that bridges the relatively cool photosphere and the intensely heated transition region and corona. Compressible and incompressible waves propagating through the chromosphere can supply significant amounts of energy to the interface region and corona. In recent years an abundance of high-resolution observations from state-of-the-art facilities have provided new and exciting ways of disentangling the characteristics of oscillatory phenomena propagating through the dynamic chromosphere. Coupled with rapid advancements in magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly investigate the role waves play in supplying energy to sustain chromospheric and coronal heating. Here, we review the recent progress made in characterising, categorising and interpreting oscillations manifesting in the solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review