摘要：Aims. Our aim is to improve on previous radiative transfer calculations
in illuminated cylindrical threads to better understand the physical conditions in cool
solar chromospheric and coronal structures commonly observed in hydrogen and helium
lines.
Methods. We solved the radiative transfer and statistical equilibrium
equations in a two-dimensional cross-section of a cylindrical structure oriented
horizontally and lying above the solar surface. The cylinder is filled with a mixture of
hydrogen and helium and is illuminated at a given altitude from the solar disc. We
constructed simple models made from a single thread or from an ensemble of several threads
along the line of sight. This first use of two-dimensional, multi-thread fine structure
modelling combining hydrogen and helium radiative transfer allowed us to compute synthetic
emergent spectra from cylindrical structures and to study the effect of line-of-sight
integration of an ensemble of threads under a range of physical conditions. We analysed
the effects of variations in temperature distribution and in gas pressure. We considered
the effect of multi-thread structures within a given field of view and the effect of
peculiar velocities between the structures in a multi-thread model. We compared these new
models to the single thread model and tested them with varying parameters.
Results. The presence of a temperature gradient, with temperature
increasing towards the edge of the cylindrical thread, reduces the relative importance of
the incident radiation coming from the solar disc on the emergent intensities of most
hydrogen and helium lines. We also find that when assuming randomly displaced threads in a
given field of view, the integrated intensities of optically thick and thin transitions
behave considerably differently. In optically thin lines, the emergent intensity increases
proportionally with the number of threads, and the spatial variation of the intensity
becomes increasingly homogeneous. Optically thick lines, however, saturate after only a
few threads. As a consequence, the spatial variation of the intensity retains much
similarity with that of the first few threads. The multi-thread model produces complex
line profiles with significant asymmetries if randomly generated line-of-sight velocities
are added for each thread.
Conclusions. These new computations show, for the first time, the effect
of integrating the radiation emitted in H and He lines by several cylindrical threads that
are static or moving along the line of sight. They can be used to interpret high-spatial
and spectral resolutions of cylindrical structures found in the solar atmosphere, such as
cool coronal loops or prominence threads.
