Network and internetwork: a compared multiwavelength analysis
Received 24 December 1999 / Accepted 16 March 2000
We analyze the temporal behavior of Network Bright Points (NBPs) using a set of data acquired during coordinated observations between ground-based observatories (mainly at the NSO/Sacramento Peak) and the Michelson Doppler Interferometer onboard SOHO.
We find that, at any time during the observational sequence, all the NBPs visible in the NaD2 images are co-spatial within with locations of enhanced magnetic field. The "excess" of NaD2 intensity in NBPs, i.e. the emission over the average value of quiet regions, is directly related to the magnetic flux density. This property implies that, in analogy with the Ca II K line, the NaD2 line center emission can be used as a proxy for magnetic structures.
We also compare the oscillation properties of NBPs and internetwork areas. At photospheric levels no differences between the two structures are found in power spectra, but analysis of phase and coherence spectra suggests the presence of downward propagating waves in the internetwork. At chromospheric levels some differences are evident in the power spectrum between NBPs and internetwork. At levels contributing to the NaD2 emission the NBPs show a strongly reduced amplitude of oscillations at the p - mode frequencies. At levels contributing to the H core emission, the amplitude of network oscillations is higher than the internetwork ones. The power spectrum of NBPs at this wavelength shows an important peak at 2.2 mHz (7 minutes), not present in the internetwork areas. Its coherence spectrum with H wings shows very low coherence at this frequency, implying that the oscillations at these chromospheric levels are not directly coupled with those present in lower layers.
Key words: Sun: photosphere Sun: chromosphere Sun: magnetic fields Sun: oscillations
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Online publication: June 5, 2000