Astron. Astrophys. 336, 743-752 (1998)
5. Summary and outlook
To provide a better overview, the results discussed in the previous
section are summarized here:
The power spectra obtained separately from the intranetwork on the
one hand and from the network on the other differ conspicuously
little, while other observers using different chromospheric lines
found different oscillation periods in their respective power spectra.
Here, both regions show enhanced power in a frequency range between 2
and about 6 - 6.5 mHz. This result was derived from Doppler
The corresponding power spectra derived from the fluctuation of the
line-centre intensity also show increased power in roughly the same
range, but here, the network regions produced a much higher power
maximum around 0.15 mHz which is mostly due to the formation of
new network regions and also to the fading of some parts of the
The temporal development of the fluctuation of the line-centre
brightness was monitored and clearly supports the explanation given
The phase jump at about 8 mHz found by some observers in
one-dimensional V-I phase spectra could not be confirmed here. The
missing phase jump might be due to aliasing as the Nyquist frequency
is only 8.9 mHz here, yet from these Na D2 data, it
cannot be excluded that this jump really does not occur.
Neither the two-dimensional power spectrum computed from the
Doppler shifts of the line cores nor the corresponding spectrum from
the intensity fluctuation of the line minimum gives a clue to a
chromospheric eigenmode. In case of existence, it should have produced
a horizontal bright streak between 5 and 5.5 mHz.
Below 2 mHz and bordered by the Lamb and the f-mode, a plateau
of phase values around 70o was found in the two-dimensional
V-I phase spectrum which can be explained by the dispersion relation
for evanescent waves with upward propagating energy.
The two-dimensional V-I phase spectrum possesses an isolated region
of high coherence for less than 0.5 mHz and between about 1.5 and
2.5 Mm-1, to a high degree of probability representing
the signature of gravity waves in the line formation height of
The decrease of phase differences seen in the two-dimensional phase
spectrum from about for the f-mode to about
for higher modes is still puzzling. A tentative
explanation is given in Sect. 4.2, a better supported one may finally
follow from model calculations of wave propagation with due
consideration of radiative transport.
The investigation of the oscillatory behaviour of the solar
chromosphere will continue to profit from further observations. In
principle, data from only one spectral line do not suffice for a
proper study of e.g. the propagation of energy. More detailed
knowledge would rather be obtainable from V-V or I-I spectra of two
different spectral lines forming in different chromospheric heights.
Major steps towards a better understanding of chromospheric
oscillations can therefore be expected from simultaneous
two-dimensional observations in such lines. For some time to come,
this will remain a challenge to the development of even more powerful
and versatile instruments than presently available.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998