Astron. Astrophys. 336, 743-752 (1998)

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 shifts.
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 existing network.
The temporal development of the fluctuation of the line-centre brightness was monitored and clearly supports the explanation given above.
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 70^o 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 Na D₂.
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.

Online publication: July 20, 1998
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