Carole Jordan

Abstract:

Observations made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO) are used to investigate the behaviour of the intensities of the emission lines of He I, He II and O III at the quiet Sun-centre and at θ = 60° towards the equatorial limb. The aim is to examine the possible effects of photon scattering on the spatial variation of the optically thick helium lines. At the quiet Sun-centre, we find that, in agreement with previous work, the ratios of the intensities of the He I 584-Å and He II 304-Å lines to those of the O III 600-Å line decrease systematically as the intensity of the O III line increases. However, we find that the dependence of these ratios on the O III intensity is not unique, but differs between the individual regions studied. Similar results are found at θ = 60°. We have also used line intensities and intensity ratios to investigate limb-to-disc effects and variations across a sample of supergranulation cell boundaries and adjacent cell interiors at both locations. The results do not exclude photon scattering as the cause of the larger observed ratios in cell interiors. The differences between the apparent widths of boundaries in O III at Sun-centre and 60° show that the emitting material is extended in height, which will aid the process of scattering into cell interiors. Photon scattering could also account for the lack of oscillations in the He I intensities in a cell interior studied by Pietarila & Judge. Three-dimensional radiative transfer calculations in chosen geometries are now needed to account for the observations in detail. © 2005 RAS.

Abstract:

We present emission measures of the single giant beta Ceti derived from line fluxes obtained with the XMM-Newton instruments RGS1 and RGS2. Photospheric and coronal element abundances are discussed, as well as the FIP effect. The RGS fluxes of 1(st) and 2(nd) order were measured with CORA in order to pay attention to Poissonian statistics which apply at low count rates. The results for beta Ceti are compared to those obtained from line fluxes published for Capella.

Abstract:

The grating instruments on board Chandra and XMM-Newton now allow measurements of electron densities. These rely on the ratios of fluxes in emission lines, where one line depends on both collisional and radiative decay rates. The electron density is required to constrain the physical extent of the emitting region, and large samples of measurements are of interest in the context of trends in coronal activity. Here we discuss the important He I-like ions and the differences in densities that result when different current data bases are used.

Abstract:

We present results from our study of the active dwarf is an element of Eri (K2 V) based on ultraviolet spectra recorded with the Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer. A combination of simple theoretical arguments and observational constraints derived from measured line fluxes are used to deduce new information about the structure of the upper transition region/corona. The area filling factor of emitting material is determined in the upper atmosphere as a function of temperature. This provides new constraints on how the magnetic field might spread out in the atmosphere of an active main sequence star. Measured emission line widths are used, together with a new semi-empirical model of the atmosphere, to place limits on the energy fluxes carried by MHD waves. These are compared with estimates of the energy input required to support the combined radiative/conductive losses in the upper atmosphere. It is shown that, in principle, waves which propagate at the Alfven speed could provide sufficient energy to heat the corona.