The observational data presented here (see Fig. 2 or Table 2) suggests a center-to-limb variation, namely broadening , for the upper chromospheric and transition region lines. Calculated excess line widths assuming magneto-acoustic waves display the opposite effect, while those calculated assuming Alfvén waves have the correct order of magnitude (see last columns in Tables 4 & 5). The values used for the electron density, energy flux, etc. were average values taken from, e.g., McClements et al. (1991), Mariska (1992), and Bray et al. (1991). Changes of less than a factor of two in these values could reduce the difference between theory and observation.
However, we do not want to `arrange' these input constants to `fit' the observed line broadening. The uncertainty in the energy flux of the different MHD waves is large. The same is true for the density and topology of the magnetic field (effect of curvature, etc.). Besides these arguments we are more concerned about the errors introduced by the linear approximation of the calculations. Erdélyi & Goossens (1995), Ruderman et al. (1997) and Ballai et al. (1998) showed that nonlinear effects are very important when studying wave propagation and the related heating of the solar corona. The observations presented here (and elsewhere) clearly show excess broadening even at disk center while the linear Alfvén assumption assumes a thermally broaden line at disk center only. Consideration of such effects are, however, outside of the scope of the present study. Nonlinear relations can change the characteristics of the travelling waves which, in turn, could have an important effect on the line broadening.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998