Photon scattering in the solar ultraviolet lines of He I and He II
Monthly Notices of the Royal Astronomical Society 362:2 (2005) 411-423
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.Modelling the chromosphere and transition region of epsilon Eri (K2 V)
Monthly Notices of the Royal Astronomical Society 361 (2005) 1102-1120
Emission measures for the single giant beta Ceti
ESA SP PUBL 560 (2005) 931-934
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.The value of density measurements in stellar coronae
AIP CONF PROC 774 (2005) 187-189
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.The structure and dynamics of the outer atmosphere of epsilon Eri
IAU SYMP 2004:219 (2004) 254-258