Spectrum synthesis of chromospheric lines of Si II and Si III

Profiles of emission lines of Si II and Si III in the EUV spectrum near the solar limb and near disk center are synthesized using a range of temperature and microturbulence models. Reasonably good agreement between computed and observed profiles near disk center is obtained with the Vernazza, Avertt, and Loeser (VAL) temperature model but with a modified microturbulence model. The adopted microturbulent velocity is equal to the mean thermal velocity of hydrogen temperatures above 7000 K and, at lower temperatures, decreases linearly to a value of 1 km s-1 in the low chromosphere. The 20,000 K temperature plateau in the VAL model is found to be of importance in determining the central intensities in the Si II lines at lambda1816, lambda1265, and lambda1533 and in determining the total intensity of the Si III line at lambda1206.The VAL temperature model with modified microturbulence does not give good agreement with the observed profiles near the solar limb. This failure is attributed to departures from spherical symmetry in the temperature and density model