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Astron. Astrophys. 357, 1093-1104 (2000)

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1. Introduction

The chromospheric bright network has long been observed in narrowband spectroheliograms taken in the H and K cores of the Ca II resonance lines. The Ca II network typically shows H and K profiles with high double peaks and enhanced line wings that persist for extended periods of time (longer than 10 minutes, see, e.g., Rutten & Uitenbroek 1991). The chromospheric network emission pattern is cospatial with small-scale magnetic field concentrations, and defines the supergranular network boundaries. It is this atmospheric component that produces the correlation between H and K excess line-core flux and magnetic activity of cool stars (Schrijver et al. 1989).

The dynamics of the network elements, compared with the internetwork or quiet chromosphere, has been extensively studied (especially from the observational point of view) since these small-scale structures can be important in channeling the energy from photospheric layers to the transition region and corona (Kneer & von Uexküll 1986, 1993; Deubner & Fleck 1990; Kulaczewski 1992; Al et al. 1998).

An assessment of the spectral characteristic properties of Network Bright Points (NBPs) at different layers in the atmosphere has been provided by Lites et al. (1993) using spectral observations in the range of the Ca II H line. In their work, these authors analyzed spectrographic observations of a single network bright patch and of several internetwork points. The wavelength shifts of photospheric and chromospheric lines allowed them to perform a compared analysis between the dynamics of the two atmospheric components. One of the relevant characteristics they describe is that at chromospheric levels (Ca II H3) the NBPs show long period oscillations ([FORMULA] 3 mHz) not correlated with oscillations in the lower atmosphere, while they lack power at higher temporal frequencies. The internetwork regions display instead enhanced power at higher frequencies, well correlated with photospheric oscillations. The presence of these low frequency oscillations in the network has been confirmed by Lites (1994) also for the chromospheric He I 10830 line, in contrast to Bocchialini et al. (1994) which observe, for the same line, oscillations only in the 5 minutes range.

An enhanced power in the low frequency range for network points with respect to the internetwork has also been observed by Kneer & von Uexküll (1986) in the center of the chromospheric H[FORMULA] line. These authors however interpret this feature as not due to oscillations, but of mainly stochastic origin, and attribute it to erratic motions of the corresponding photospheric footpoints.

The problem is still open, and further observations to better address this issue are required (Lites 1994). In particular one would need observations: 1- on a larger number of NBPs, to improve on the statistics; 2- at different heights in the atmosphere, since the analysis of the coherence between fluctuations at different levels can help exploring the nature of oscillations. To this end, a reliable method for the identification of the same physical structure at different atmospheric levels is mandatory, since the inclination of the magnetic field could displace the chromospheric network points with respect to the corresponding photospheric ones.

In this paper, we address some of these issues, and present observational results on the NBPs and internetwork characteristics as derived from a multiwavelength analysis. The observations were obtained in August 1996, during a coordinated observing program between ground-based observatories and the Solar and Heliospheric Observatory (SOHO). For the ground-based observations we used the cluster of instruments at the NSO/Sacramento Peak R.B. Dunn Solar Telescope (NSO/SP-DST), that could provide a complete coverage at lower atmospheric levels. The dataset used is described in Sect. 2. General properties of a sample of NBPs, followed from the photosphere up to the chromosphere and including their relationship with the magnetic structures, are given in Sect. 3. The temporal development of the NBPs is described in Sect. 4. Sections 5 and 6 provide an analysis of the power, phase difference and coherence spectra for the fluctuations observed separately within the NBPs and the surrounding internetwork. Finally, discussion and conclusions are given in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000