In recent papers (Kerr et al. 2004, 2005), we have undertaken calculations of radiative transfer effects for the Fe XV line emission from a solar coronal plasma. We predict that the 284.16A resonance transition will, under certain optically thick conditions, show intensity enhancements over values calculated for an optically thin plasma. This is in stark contrast to the normal expectation that opacity effects will lead to a reduction in line intensity. The exact mechanism by which the intensity increase occurs depends on an enhancement of the overall plasma source function, relative to the optically thin case, by the radiation which has the largest pathlengths to traverse in the plasma (Kerr et al. 2004). As a consequence, the degree of line intensity enhancement not only depends on the plasma properties usually associated with opacity (e.g. temperature, column density), but also the plasma geometry and angle of observation. For example, predictions of the line intensity enhancement differ significantly if the plasma is a plane-parallel slab, a cylinder, or a sphere, and also depend critically on the direction from which the plasma is viewed (Kerr et al. 2005).
To test our predictions, we wish to obtain time-series EIS spectra of the Fe XIII 200.03 + 202.04A, Fe XIV 264.78 + 274.20A and Fe XVI 262.98A, Fe XV 284.16A emission features in an active region loop as it rotates from the solar limb to disk centre (or vice-versa). Simultaneous XRT images will provide detailed information on the loop morphology, and in particular will allow us to construct a 3-dimensional picture of its geometry as it rotates from the limb to disk centre.
The Fe XIII I(200.03 .A) and Fe XIV I(264.78 .A) intensity ratios will yield the electron density in the active region loop (Keenan et al. 2007; Storey et al. 2000), while the electron temperature will be determined from ratios which include lines from different ionization stages (Brosius et al. 1994). Once we know the plasma parameters, theoretical intensity ratios involving the 284.16A feature of Fe XV may be accurately calculated. A comparison of these theoretical ratios with the EIS observations will then allow us to reliably detect intensity enhancements in the 284.16A line. In addition, the derived electron density, in conjunction with the active region loop pathlength (available from its known geometry), will provide an estimate of the Fe XV column density. We will then be able to compare the observed 284.16A line enhancement with the predictions of our radiative transfer models for the measured column density and plasma geometry conditions, and investigate the accuracy of our theory.
Should we detect line intensity enhancements and con.rm our theoretical predictions, this will have 2 important implications. Firstly, as the line enhancement effect depends critically on plasma geometry, it may be possible to develop this technique as a diagnostic to determine the structure of distant astronomical sources where coronal opacity has been detected, such as clusters of galaxies (e.g. Sanders & Fabian 2006). Secondly, the detection of line enhancement could explain why coronal opacity effects in stellar sources are proving difficult to .nd. Several authors have searched for coronal opacity in late-type stars using e.g. the Fe XVII 15.01A line in Chandra and XMM-Newton satellite observations (e.g. Phillips et al. 2001; Ness et al. 2003). However, only Matranga et al. (2005) have convincingly detected opacity. This could be due to line intensity reductions in one part of the stellar corona being cancelled out by enhancements in another region (Kerr et al. 2004). |
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