Astron. Astrophys. 337, 287-293 (1998)

3. Results

Gaussian profiles were fitted to each spectral line (for some ions we had two or more lines at different wavelengths although in many instances these additional lines were weak). As found previously all of the measured line widths are in excess of their thermal width for all solar locations. This excess width may be associated with velocity fields. When the scale of these motions is small compared with the instrumental resolution, they are often accounted for by adding a component () to the Doppler width

where is the most probable non-thermal velocity, and the ion mass. This, of course, assumes that the non-thermal velocities also have a Gaussian distribution. Furthermore, additional broadening due to the instrument must first be removed. The temperatures used are the ionisation equilibrium temperatures of Arnaud & Rothenflug (1985) and Arnaud & Raymond (1992).

A summary of the derived line widths for the strongest lines are given in Table 1. The difference in the derived non-thermal velocities at disk center compared to those just inside the limb and those outside the limb are given in Fig. 2. Here we see a clear difference in two different temperature regimes, 20,000 K and 250,000 K. Both of these temperature regimes indicate excess line broadening at the limb compared to disk center. It should also be noted that the data taken at 800 arc sec from disk center also shows additional broadening, although the number of lines observed is smaller. In deriving , an instrumental width of 77 mÅ was used. Several of the low temperature lines measured on-disk (e.g., Fe II , Cl I ) have measured FWHM close to 110 mÅ which translates to a FWHM of 73 mÅ (or a Doppler width of 44 mÅ) when the instrumental correction is applied.

Fig. 2. The excess non-thermal velocities in lines from the chromosphere to the corona for Limb A (top panel, i.e., just inside the limb) and Limb B (bottom panel, just outside the limb) compared to that at disk center. The estimated error in the derived is 1-2 km s-1

Table 1. A summary of the measured FWHM in mÅ, plus the derived Doppler width corrected for instrumental broadening for lines formed in the solar chromosphere to the corona

Table 2. The derived non-thermal velocities () for the different regions based on the measured line width given in Table 1. The estimated error in is 1-2 km s^-1.

The range of non-thermal velocities in Table 1 are in very good agreement with values obtained previously by other authors, e.g., Hassler et al. (1990) obtain 26 km s^-1 for Mg X at disk center, Cheng et al. (1979) obtained for the solar limb values ranging from 10 to 25 km s^-1 for Fe XII while Doyle et al. (1997) obtained 12-30 km s^-1. For O VI at disk center, Warren et al. (1997) obtained 33 km s^-1, while Doschek et al. (1981) obtained 27 km s^-1 for N V .

© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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