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

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3. Characteristics of NBPs

In order to identify suitable network points, we first selected on the spectra bright features showing strong K2 peaks and enhanced wings emission in CaII K. Since the NBPs lifetime is typically longer than 10 minutes (Rutten & Uitenbroek 1991), we further required that the points be visible for the entire observing period in the NaD2 images as bright structures with intensity above the average. A total of 11 NBPs with the required characteristics were selected. They are distributed over the FOV both near the center of the AR and away from it, as can be seen in Fig. 1a. Several more network points (or structures) are visible on the longitudinal magnetic flux maps, but in this work we limit our analysis to only these 11 points for which corresponding CaII K spectra are available.

3.1. Intensity and velocity

Within the areas enclosing the NBPs defined above, we tried to identify the bright points at each wavelength by choosing an intensity (or other signature) threshold that could clearly separate them from their surroundings.

On white light and NiI pseudo-continuum images, an intensity threshold cannot clearly discriminate between NBPs and other areas. The contrast averaged over the spatial locations corresponding to the NBPs is of the order of 1%, i.e. smaller than the rms noise calculated in quiet areas (about 3.5% and 2% for the white light and pseudo-continuum images, respectively). This is consistent with the observations of Topka et al. (1997), that find low continuum intensity contrast in network points with magnetic flux density smaller than [FORMULA] 300 G (as typical of our points, see Sect. 3.2).

The line of sight velocities averaged over the NBP areas are small, about 80 m s-1 downward with respect to the average values over the quiet areas of the FOV. Since the standard deviation of the measurements is about 200 m s-1, for both NPBs and quiet areas, it is not possible to define a velocity threshold that allows to discriminate the NBPs within the FOV. The small average red-shift is however in agreement with recent observations by Solanki (1993) and Martínez-Pillet et al. (1994).

The NBPs are instead well visible in the images acquired in the H[FORMULA] far wings and NaD2 center, as sharp and isolated bright structures of comparable size (3"-4" wide). The bright points, as seen at these wavelengths, spatially coincide within the overlapping error of 1". In the H[FORMULA] center images the morphology is less clear. The presence of contiguous features, at times brighter than the selected NBPs, made more uncertain their identification. In total, however, 10 of the 11 considered NBPs were unambiguosly identified in the H[FORMULA] center images.

The characteristics of the NBPs for different signatures, including their typical contrast with respect to quiet areas, are summarized in Table 2. It must be remarked that the formation height of these signatures has been computed in a mean quiet atmosphere, i.e. that it represents only a generic indication for magnetic structures such as the NBPs. In particular, due to the Wilson depression, the radiation coming from photospheric magnetic structures is believed to be formed in deeper geometric layers with respect to non-magnetic ones. Since the spatial resolution for our observations is not sufficient to resolve the (supposedly) elementary magnetic fluxtubes (with dimensions smaller than 0.3", as seen for example in G-band images), the signals we analyze are a non-linear combination of magnetic and non-magnetic ones.


Table 2. Characteristics of the NBPs at different atmospheric heights. The first column gives the formation height for each wavelength, computed in the quiet Sun VALC model (Vernazza et al. 1981), taking into account the different widths of the filters. The second column gives the typical diameter of the structures when they are clearly visible. The third column gives the intensity contrast values, averaged over time. A range of values is reported when variations are large over the sample.

3.2. Magnetic structures

The selected NBPs are clearly recognizable on the MDI magnetic maps as sharp and isolated structures, 3"-4" wide. Each one corresponds to a patch of definite polarity, with magnetic flux densities ranging from a minimum of 30 G, to a maximum of about 250 G (for comparison, the spot in the FOV has a maximum flux density of 1100 G). The error on a single pixel in each image is given at about 15 G (Schrijver et al. 1997). Since the network fields are mostly vertical (see, e.g., Lites et al. 1999), the flux measure is only slightly affected by the position of the FOV on the solar disk ([FORMULA]).

The magnetic evolution of the NBPs is quite varied. Some of the points maintain stable positions and flux values, while others experience a steady increase during the observing period. In one single case we see a weak magnetic structure appearing during the observing sequence, simultaneously with the appearance of a network bright point.

The spatial correspondence between the NaD2 network points and the magnetic structures is very good, and will be analyzed in more detail in the next section. The same correspondence is noticeable also for the network points as seen in the H[FORMULA] wings, although this is less evident than for NaD2 due to their lower contrast.

3.3. Magnetic structure and NaD2 emission

We checked the spatial correspondence between the network points as visible in NaD2 and in magnetic maps. To this end we first removed from both signatures, by means of appropriate smoothing techniques, short term variations due to noise and oscillations (especially in the 5-minutes range). We then subtracted a threshold, chosen as the average quiet area value for the NaD2 images, and as the nominal data noise (15 G) for the magnetic ones. Finally, within the 11 selected areas, a NBP was identified in both signatures as the locus of the pixels exceeding 50% of the local maximum. This allowed its clear separation from the surroundings.

We find that, at any given time, the NaD2 network points are coincident in position, size and shape with the corresponding magnetic patches, within 1" (the overlapping error). Any change in the characteristics of the NaD2 NBPs reflect almost perfectly those of the magnetic features, within the temporal resolution of the MDI data. This is well exemplified in Fig. 2, where we show NaD2 contours overlaying magnetic flux maps of several NBPs at two different times. To our knowledge, this is the first time that such a correspondence is reported at high spatial and temporal resolution. A good agreement between the chromospheric network emission pattern and the locations of enhanced magnetic flux had been noted in earlier works (Skumanich et al. 1975; Schrijver et al. 1989; Nindos & Zirin 1998) but mostly for the CaII K emission, with lower spatial and temporal resolution. No temporal resolution was available in the observations of Beckers (1976) or in those of Daras-Papamargaritis & Koutchmy (1983), that established a correlation between magnetic structures and facular structures in the wings of the Mgb lines.

[FIGURE] Fig. 2. Longitudinal magnetic flux maps (scaled between +100 and -200 G) for two different NBPs at two different times. The contours represent the NaD2 threshold defining a network bright point. One can see the change in shape and position of the magnetic structures during this interval, and the corresponding changes in NaD2 emission. The arrow indicates a new NBP appeared simultaneously with a weak magnetic structure

In analogy with the CaII H and K case, we could establish a quantitative relationship between the NaD2 excess and the magnetic flux density for the network points. This property implies that also the emission in the center of the NaD2 line can be used as a proxy for the magnetic field structures. For sake of simplicity, the details of the determination of this relationship are given in Appendix A.

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Online publication: June 5, 2000