Intensity oscillations at the feet of coronal holes

Abstract

We investigate the regime of chromospheric oscillations at the bases of coronal holes and compare them with the oscillations in the quiet chromosphere outside coronal holes using time series of spectrograms taken at different times in eight quiet regions on the Sun. As the oscillation parameter being studied, we have chosen the central intensity of the chromospheric Ca II K and H and 849.8-nm lines. The intensity measurements at all spatial points (along the spectrograph slit) have been subjected to a standard Fourier analysis. For the identified areas of the networks, cells, and network boundaries, we have calculated the integrated oscillation powers in several frequency bands. For all frequency bands, the powers of the intensity oscillations at the formation level of the Ca II resonance doublet line cores have been found to be enhanced at the bases of coronal holes approximately by a factor of 1.5. For the “three-minute” band, this enhancement is more pronounced in the network than in the cell, while the opposite is true for the “five-minute” band. The power in the five-minute band is higher than that in the three-minute one both at the bases of coronal holes and outside them, but this ratio in the network for a coronal hole is higher (1.40 ± 0.25 and 1.30 ± 0.10). We interpret this fact and the fact that the power of the three-minute oscillations for nonmagnetic regions changes with height differently at the base of a coronal hole and outside it as an increase in the importance of magnetoacoustic portals at the chromospheric base of the coronal hole.