3  Spectral line profiles

Line shifts and broadenings occurr all the times at all temperatures. They give information about the dynamic nature of the solar atmosphere. Systematic redshifts in transition region lines have been observed in both solar spectra and stellar spectra of late type stars. On the Sun, outflows of coronal material have been correlated with coronal holes.

If the optically thin spectral lines can be fitted with gaussian profiles, the intensity per unit wavelength I_l is defined as:

Il =  I Ö   2 p   s exp[-(l- l0)2 / 2 s2]

(1)

where I=òI_l dl is the integrated intensity and s is the guassian width given by:

s2 =  l2 2 c2 (  2 k T M + x2) +sI2

(2)

for a Maxwellian velocity distribution of temperature T, usually assumed to be the temperature corresponding to peak abundance of the ion. Here M is the ion mass, s_I is the gaussian instrumental width and x is the most probable non-thermal velocity for a Maxwellian velocity distribution.

The root-mean-square velocity < v² > ^1/2 = (3/2)^1/2 x

Figure 10: [from Dere & Mason 1993] Non-thermal velocities from HRTS spectra.