5.1 Global acoustic mode The SOHO/SUMER spectral instrument has recently discovered
quasi-periodic oscillations of intensity and a Doppler shift in the coronal emission lines Fe XIX and Fe XXI
(Kliem et al., 2002; Wang et al., 2002, 2003b,a, and references therein). Figure 14 gives an illustration of
such oscillations. These spectral lines are associated with a temperature of about , corresponding to
the sound speed of about . The observed periods are in the range , with
decay times , and show an initial large Doppler shift pulse with peak velocities
up to . The intensity fluctuation lags the Doppler shifts by period. In a
statistical study of oscillation cases, Wang et al. (2003a) found that except for a few
cases, the presence of definite periodicity in intensity fluctuations is not certain. Moreover,
oscillations are not seen in other emission lines observed simultaneously with the oscillating
lines. In the initial stage of all cases analysed by Wang et al. (2003a), there is a rapid
increase in the line intensity and a large Doppler shift, indicating that the oscillations are excited
Ofman and Wang (2002) suggested that these oscillations are produced by the global standing acoustic
where is the longitudinal velocity, is the speed of sound, is the loop length,
and is a distance along the loop with the zero at the loop top. More strictly, the phase
speed of the longitudinal mode of a coronal loop should, in the long wave length limit, be equal
to the tube speed inside the tube, however in the low plasma- plasma of the solar
corona this value is very close to the sound speed . From Equation (48), the oscillation
period is given by the expression . According to the thermally conductive, viscous,
nonlinear one-dimensional MHD simulations, the short decay time is connected with the dissipation
because of high thermal conductivity of the hot plasma filling the loop. Mendoza-Briceño
et al. (2004) has recently developed this study, taking into account effects of stratification. It
was found that stratification would lead to insignificant changes in the decay times (maximum
||Doppler oscillation events in the Fe XIX line observed with the SUMER instrument on
9 March 2001. a) Doppler shift time series. The redshift is represented with the bright colour, and
the blueshift with the dark colour. b) Average time profiles of Doppler shifts along cuts AC and BD.
The thick solid curves are the best fit functions of the form .
c) Line-integrated intensity time series. d) Average time profiles of line-integrated intensities along
cuts AC and BD. For a clear comparison, the intensity profile for BD has been stretched by a factor
of . e) Line width (measured Gaussian width) time series. f) Average time profiles of line width
along cuts AC and BD (from Wang et al., 2003a).
There are still several open questions in both the theory and the observations: how are the oscillations
triggered and excited; why are intensity oscillations not always seen, whether the occurrence rate of
oscillations is temperature dependent (in major cases in Fe XIX, i.e., in hot plasma), what is the role of
non-adiabatic effects (e.g., thermal instability)?
Also, it is not clear how the SUMER oscillations are related with other coronal oscillations, observed in
the radio (e.g., Aschwanden, 2003) and X-ray bands.