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

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2. Observations and data reduction

A description of the general data acquisition has been presented in Cauzzi et al. (1997, 1999). Table 1 gives a summary of the observing setup, and we recall here only some short information on the data used in this paper. Monochromatic intensity images were obtained with the tunable Universal Birefringent Filter (UBF) and the Zeiss filter, at high spatial and temporal resolution. Several spectra were obtained around the chromospheric CaII K line with the Horizontal Spectrograph (HSG). The spectra have been acquired setting the spectrograph slit on different bright points at different times; the field of view in the HSG row of Table 1 hence refers to a single slit exposure. Onboard SOHO, the Michelson Doppler Imager (MDI, Scherrer et al. 1995) acquired data in high resolution mode, i.e. with an image scale of 0.605"/pixel. Maps of pseudo-continuum intensity, line-of-sight velocity, and longitudinal magnetic flux were obtained in the NiI 6768 Å line at a rate of one per minute for several hours. The line-of-sight velocity images were available in a binned 2x2 format, i.e. with an effective spatial scale of 1.2"/pixel.


Table 1. Summary of the observations

We observed the small Active Region (AR) NOAA 7984 over 5 consecutive days (Aug. 15-19, 1996). Its activity was very low and although some stronger magnetic structures (a small spot and some pores) were present in the field of view (FOV), no major eruptions of magnetic flux were recorded. The situation was then appropriate to study and characterize the properties of NBPs visible within and around the AR. We remark that our set of data allows us to directly compare the NBPs as visible at different wavelengths with the corresponding magnetic field structure, and to follow them from the deep photosphere to the higher chromosphere. In this paper we analyze the data obtained on August 15th, 1996, since for that day we had the best uniformity in time coverage for NSO and MDI data. Fig. 1 shows the FOV at several wavelengths. The period of best seeing for ground-based observations ran from 15:15 to 16:05 UT. This interval is adequate for the study of network points, since it allows an analysis of their (possible) periodical properties, while they still maintain their identity (Lites et al. 1993; von Uexküll & Kneer 1995). MDI data were available for many hours around this interval; we consider in this work the period 14:00-17:00 UT.

The data were re-scaled to the 0.605"/pixel of the MDI maps, and the co-alignment of the whole dataset was obtained comparing the position of the prominent solar features, i.e. the little spot and pores (see Fig. 1). At each given time the alignment among the images acquired with different instruments was better than about [FORMULA], the mean spatial resolution limit of the ground-based frames.

[FIGURE] Fig. 1a-d. AR NOAA 7984 observed on Aug. 15, 1996, N29E16. a) NaD2 image averaged over the time interval 15:15-16:05 UT. The black squares indicate the position of the NBPs analyzed in this work. b , c and d Same FOV as a , observed in H[FORMULA], NiI pseudo-continuum and longitudinal magnetic flux

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000