Theoretical astrophysics and plasma physics at RPC

Abstract:

We study the global bending modes of a thin annular disc subject to both an internally generated magnetic field and a magnetic field due to a dipole embedded in the central star with axis aligned with the disc rotation axis. When there is a significant inner region of the disc corotating with the star, we find spectra of unstable bending modes. These may lead to elevation of the disc above the original symmetry plane facilitating accretion along the magnetospheric field lines. The resulting non-axisymmetric disc configuration may result in the creation of hotspots on the stellar surface and the periodic photometric variations observed in many classical T Tauri stars (CTTSs). Time-dependent behaviour may occur including the shadowing of the central source in magnetic accretors even when the dipole and rotation axes are aligned. © 1997 RAS.

Abstract:

We extend a previous study of the global stability of a stratified differentially rotating disc containing a toroidal magnetic field to include the effect of a non-zero resistivity η. We consider the situation when the disc is stable to convection in the absence of the magnetic field. The most robust buoyancy driven unstable modes, which occur when the field is strong enough, have low azimuthal mode number m. They grow exponentially, apparently belonging to a discrete spectrum. They exist for the dimensionless ratio η/(H2Ω) smaller than ∼ 10-2, where Ω is the angular velocity and H is the disc semithickness. In contrast the magnetorotational modes develop arbitrarily small radial scale and show transient amplification as expected from a shearing sheet analysis. The most robust modes of this type are local in all directions. Because of their more global character, the buoyancy driven modes may be important for the generation of large-scale fields and outflows. © 1997 RAS.

Abstract:

Previous studies (Jordan 1975, 1980) have found that the resonance lines of He I and He II appear to have higher fluxes than expected from models made using other transition region lines formed at similar temperatures. The enhancement factor was shown to be less in coronal holes than in the quiet Sun. These results referred to observations made with low spatial resolution. Using the higher spatial resolution provided by the CDS instrument onboard SOHO, we have started a systematic re-examination of the behaviour of the He I and He II line fluxes by examining several quiet Sun regions, from Sun-centre to near the limb, including a coronal hole. While the helium line fluxes are overall relatively lower in the coronal hole, their behaviour in the quiet Sun differs between the supergranulation cell boundaries and cell interiors. New simultaneous observations with the SUMER instrument are being used to investigate possible correlations between the electron density, non-thermal velocities and the helium line fluxes. We comment in passing on tests of the accuracy of the calibration of the CDS/NIS2 spectra.