7. Discussion and conclusions
The observations presented in this paper allowed us to define the characteristics of network bright points at different atmospheric heights, and to compare them with those of the surrounding internetwork areas. We improved on the existing statistics using a good-sized sample of NBPs, and the same number of "test" internetwork areas, defined in a comparable way. The method we adopted to study the temporal evolution of NBPs insures that each bright structure is properly followed in time and position at each height. In fact, the evaluation of the light curves and their properties after a spatial averaging over a well defined area guarantees that we are studying the same NBP at all heights, and avoids the problem (first pointed out by Lites 1994) of a possible structure displacement due to the magnetic field inclination. Given the characteristic horizontal size of the NBPs, the analysis and the comparison of power spectra and phase differences concern the propagation of waves pertaining to a horizontal wavenumber of about 3 Mm-1.
The quasi-simultaneous series of NaD2 images and of MDI maps allowed us to establish for the first time a correspondence between NaD2 bright network and magnetic network at high spatial and temporal resolution. A correspondence between bright chromospheric structures (Ca II, Ly, Mg I and UV continuum) and magnetic structures had been observed before, but not at this high temporal resolution. We also established for the NBPs a quantitative relationship between the Na excess and the corresponding absolute value of magnetic flux density. This relationship is best expressed by a power law with an exponent very close to the one found by Schrijver et al. (1989, 1996) for the Ca II - K excess, and indicates that the emission in NaD2 may be used as a proxy for the magnetic flux density.
The NBPs considered in this work have the following properties: - are bright in the Ca II wings and in the Ca II K2 peaks; - are visible in the NaD2 images for about 1 hr; - coincide spatially with the magnetic structures; - are nearby or within a lower activity region. The general characteristics found for these NBPs do not differ from the ones derived in absolutely quiet regions (Deubner & Fleck 1990, Lites et al. 1993).
Our results referring to photospheric and chromospheric properties are so summarized:
At photospheric levels: No difference is detected between network and internetwork power spectra, either in intensity or in velocity, within the limits of sensitivity and accuracy of the instruments used for this work. The phase difference spectra between photospheric signatures in general do not show different characteristics for network or internetwork. However, when analyzing the phase difference between H red wing and white light images( km), we find in the frequency window 1.5-2.5 mHz and in the internetwork. (The value is more uncertain in the network, due to a lower coherence value). A phase lag of this amplitude and sign is usually considered a signature of gravity waves directed radially inward. A possible explanation for their origin might be sought in recent models of convection, described as a non-local process driven by cooling at the solar surface rather than by heating from the lower layers (Spruit, 1997). One can imagine that the downward flowing cooled plasma can trigger some inward directed waves, and hence justify the fact that the external layers "lead" the deeper layers. The general inhibition of convection in magnetic structures might be the reason for the lack of this signature in network points.
The power spectrum of the magnetic flux variations in NBPs shows a small but significant peak around 3 mHz, that could be related to a "transformation" of acoustic waves into MHD waves. However, the phase difference and coherence spectra between magnetic flux and velocity (B-V) for the NBPs indicate a very low correlation between the two signals so we cannot conclude anything on the presence of MHD waves within the network points.
At chromospheric levels: Network and internetwork areas have a rather different behaviour in the power spectra. We do not see any evidence for the typical chromospheric period of 3 minutes (but it must be reminded that they are best seen in velocity variations rather than intensity). In the low chromospheric levels, where NaD2 originates, the NBPs power spectrum is compressed at all frequencies if compared to the internetwork, while in the high chromosphere, where H originates, the power of NBPs is higher than the one of internetwork. This opposite effect may be an indication that the magnetic field disturbs and reduces the amplitude of oscillations already present in the low chromosphere while it assumes a leading rôle in the high chromosphere.
In the layers contributing to the NaD2 emission it seems that the oscillations present in network points change regime with respect to both the photosphere and the high chromosphere and we think that it would be important to perform observations of NBPs in tha Na line, with high spectral resolution. Unfortunately we cannot analyze the phase difference spectrum for NaD2 intensity fluctuations with respect to others formed at different layers, since the NaD2 intensity fluctuations, measured with the UBF filter (FWHM Å), are more related to velocity than to temperature fluctuations (see Sect. 5.2).
The power spectrum of H intensity in NBPs has the more relevant peak at 2.2 mHz, but this signal is not correlated with the photospheric fluctuations, as indicated by the very low coherence measured at all frequencies between the H core and the blue and red wings. We can then confirm, using a larger sample of NBPs, the presence of the peak found by Lites et al. (1993) around 2 mHz in the power spectrum of K3 velocity fluctuations for one network point. Kalkofen (1997) and Hasan & Kalkofen (1999) proposed an explanation for this peak in terms of transverse magneto-acoustic waves in magnetic flux tubes, excited by granular buffeting in the solar photosphere. In their model the low coherence between photospheric and chromospheric signatures could be explained by a partial conversion of the transverse waves to longitudinal modes in the higher chromosphere.
A general result of our analysis, valid from the low photosphere to the high chromosphere, is that the NBPs always show a coherence lower than the internetwork, pointing out that the presence of the magnetic field changes the propagation regime of waves with respect to the non-magnetic regions.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000